[SOLVED] A/C Compressor not working -- tracking down the cause

[gerryvz]

Following up on my Top-End Rebuild thread, I have driven the car around 1,700 miles since completing the job last September-ish.

I have only had one problem that I've noted since I completed the job -- the A/C compressor through the fall/winter/spring has been sort of hesitant to come on. It comes on either after I drive the car a little bit, or I "shock" the A/C into running by pressing the HI fan setting + Max A/C. Then it will come on, and it works just fine after it does.

I checked the A/C refrigerant & system pressure last year, and it was just fine on both the high and low sides, so I know it was not a leak / lack of refrigerant causing the system to not generate enough pressure to activate the compressor. The only thing I could think of was that I somehow disturbed the compressor or the lines going to it during my top-end work, perhaps by hosing down the area with brake cleaner or somehow being over-zealous when I was removing and installing the tandem pump. Note that I did not disturb the A/C compressor itself, or the lines (other than perhaps slightly moving them when accessing the tandem pump and lines going to it).

I had a clue today -- I checked all of the codes on the car's systems, and everything came up negative (the DI and EA-CC-ISC systems had old codes from before the top-end job, which I think I deleted all of) except the BM (Base Module) system, which threw a Code 6 and a Code 7. These correspond to the A/C compressor clutch somehow being stuck, and code 7 is the poly belt (serpentine belt) slipping. So this is a clue that perhaps I got some oil or other slippery stuff on the A/C compressor belt pulley, which perhaps is causing it to slip (or caused it to slip) and throw the codes.

I also noted that the last time I drove the car, very recently, and in warmer ambient temperatures (high 80s), that the A/C system went on right away and acted normally.

So I am going to see if I can't remove the serpentine belt and really clean off the A/C compressor pulley, in case there is any residual oil or grease on the pulley, and see if that solves the problem once and for all. It also may just be that time and engine bay heat from driving the 1,700 miles may have baked or dried up any slippery stuff on the pulley so that it is working correctly.

I will also check the fuses in the BM, and the BM itself, as I have 4-5 spare units. I am wondering if the speed sensor on the back of the compressor, or the compressor wiring, somehow got damaged or moved when I was working on things down there last year.

Anyhow, it was REALLY nice to have exactly ZERO codes thrown, with the exception of these two A/C compressor related codes. Especially after driving so many miles. That means that everything is operating 100% as it should !!!!

 

[kegmankipp]

I had similar random intermittent ac issues for many years that no shop could seem to track down. Same dtc. New compressor, new belt, swapped climate panels, fuses etc. all will no change.

I had read about a bad solder joint in the Base Module that would go bad, pin 21 IIRC. I had a backup Base Module and swapped it in and the problem has not happened again in over a year.

Whrn I inspected the suspect BM the solder joint was in fact wonky and loose. Probably from the high current going through it feeding the ac compressor directly partially desoldered it? I resoldered the joint and tested the unit and problem went away.

May not be your issue but it’s worth a shot Jerry since you have extras on hand.

Oh, and welcome back!

 

[gerryvz]

Thanks for the points. I can easily swap out a BM/GM and see if that helps. I should probably do a visual inspection of the compressor wiring and such, as well.

One question for the experts here -- is the compressor speed sensor available as a separate part, or it is integrated/integral with the compressor?

Slowly getting through all of the unread threads here on the forum. I'd say I'm about 50% through them. It's amazing that since about March 15, there are hundreds of unread threads.

Also, we're going to hit 10,000 members before the annual July 4 "State of the 500Eboard" post that I'll be making. That's a big milestone. I think when we made the transition to XenForo back in March 2019, we were around 8,200 members. And we just crossed 16,000 threads -- again we were around 12,000? when we transitioned two years ago. So we've had quite a lot join on over the past two-plus years. Another milestone will be 250,000 total posts, which we'll hit in the next year or so. That is a milestone that marks the transition of a forum from a "small" niche forum to official "medium-sized" forum status.

Forums that are considered "large" tend to have 1 million-plus posts.

 

[gsxr]

The compressor speed sensor is not available as a separate part. It is integrated with the compressor, although I think it can be removed & swapped? Never done this so I'm not certain.

 

[gerryvz]

To be honest I think this is just a situation where I got some power steering fluid or SLS fluid that dropped down onto the A/C pulley, and didn't see it / clean it off before I put the serp belt back on. There was also a LOT of leaked/seeped fluid from my short hose, as well as engine oil flung from the crank seal, also in that pulley area. I flushed it as good as I could with plenty of brake cleaner, but I'm sure there was plenty of residue left.

I'm glad that it threw the codes, though. I just don't think it is a problem with the compressor itself.

 

[gerryvz]

Today I drove the car a little bit, down into Annapolis onto the Naval Academy campus to pick up our midshipman to bring him back to the house, before he ships out in the morning to Connecticut for his mandatory summer cruise.

Before I left, I replaced my stock Base Module with a spare unit (one of two I have) with the same part number. I checked all of the red 10-Amp fuses in both the spare unit and my original unit as it came out of the car.

The replacement BM made no difference -- I was still not able to make the A/C come on. I did get it to work a few days ago, but only after driving the car some 10 miles and all of the sudden it came on. When the A/C DOES come on, it is ice cold and works perfectly. But when I park the far for 5-10 minutes, say to go into a store, I am usually back to "square one" with the A/C not working again when I start the car back up (although it will usually kick back on if I drive far enough).

I can definitely note through testing that my pushbutton unit is working fine, and activating all of the pods that it is supposed to. So I do not believe it is a control issue -- but rather an electrical/signal issue that triggers the compressor. As mentioned, once the compressor is initially triggered, it cycles fine and operates perfectly until the car is shut off.

Seeing as this situation started ONLY after I did the Top-End job last year, I MUST have disturbed something in the engine compartment associated with the compressor wiring, or pressure sensor, or something along those lines. Since I have eliminated the Base Module and (with 95% certainty) the PBU, and refrigerant pressure is within spec, I need to focus on the base electrical issues.

The next step in this diagnosis is going to be to remove the serpentine belt, to see if there are any anomalies visually with the compressor pulley (oil on the pulley surface, etc.). Next, I will do a visual inspection with the car up on the ramps as to if there is anything wrong with the external wiring on the front of the compressor.

Other than the refrigerant pressure switch, and the compressor speed sensor on the back of the compressor, and the PBU, what other electrical controls or inputs exist in the A/C compressor control circuit? What controls activation of the A/C compressor clutch?

Is it possible to "jump start" the compressor by shorting the pressure switch? Does anyone have any troubleshooting short-cuts or tips relating to the electrical control of the compressor/clutch assembly?

Cheers,
Gerry

 

[nocfn]

May need to check that strip fuse for hairline cracks. You can check volts coming off the plug from the pbu to the resistor too. Also be sure electrical switch connections to the drier etc are clean and tight. Low gauge wires on the compressor really fragile at that white block

 

[emerydc8]

Is it possible to "jump start" the compressor by shorting the pressure switch? Does anyone have any troubleshooting short-cuts or tips relating to the electrical control of the compressor/clutch assembly?

Cheers,
Gerry

I chased the same problem as Kegmankipp for over a year (intermittent a/c compressor engagement) and after changing the control head, overhead cockpit sensor, compressor, expansion valve, v-belt and two belt tensioners, it finally turned out to be a bad base module.

I think I would be thinking the same as you, Gerry, and bypassing the pressure switch. Have you put a set of gauges on the system just to see what the pressures are when it actually does run?

BTW, when I had the intermittent compressor, if I turned the engine off and restarted, it would usually work. I got good at slipping into N and doing a restart while driving at 45 MPH.

 

[gerryvz]

I chased the same problem as Kegmankipp for over a year (intermittent a/c compressor engagement) and after changing the control head, overhead cockpit sensor, compressor, expansion valve, v-belt and two belt tensioners, it finally turned out to be a bad base module.

I think I would be thinking the same as you, Gerry, and bypassing the pressure switch. Have you put a set of gauges on the system just to see what the pressures are when it actually does run?

BTW, when I had the intermittent compressor, if I turned the engine off and restarted, it would usually work. I got good at slipping into N and doing a restart while driving at 45 MPH.

Hi Jon,

I did put gauges on the system late last year and refrigerant pressure was right on, on both the high and low sides, right where it needed to be. So was all good from that standpoint. My compressor will not engage when I turn the engine off and on again. I did find last week that when it ran, and I stopped and turned off the car briefly, it did start again. But 90% of the time, when I park the car and go into a store, and come back out say 5-10 minutes later, it will not run, and the problem starts all over again.

I may install the second spare BM today just to completely eliminate that as a factor, but the fact that my original and first spare are providing equivalent results, I think that I can generally rule that out.

I did a visual inspection from above (as good as I could) last night with an LED flashlight. The pulley is dry and there are no issues with the serpentine belt. I looked at the wiring atop the front of the compressor (which I had hosed down quite well several times with brake cleaner when I did the Top-End job last year) and all of the wiring looked visually fine.

Will remove the belly pan and look at things from below, with the car up on ramps. Not a single drip of oil or any dirt or leaks on the belly pan as viewed from above, so it looks like I got the front crank seal leak, and the weeping from the tandem pump "short hose," taken care of. Hopefully a closer visual inspection of the wiring on the compressor, and the speed sensor, will tell me something.

Will definitely look at the fuses too, @nocfn, though the strip fuse controls the fan, not the compressor. I believe one of the 10A fuses in the top of the BM is what controls voltage to the compressor. I agree that the wiring on top of the front of the compressor at that white block (exactly the area where I hosed it down with brake cleaner and tried to clean things really good last year) could well be the source of the issue, as that are is the ONLY area that I "disturbed" or monkeyed with last year.

Here is the top of the compressor area, in a photo from last year.

I will mess with the pressure switch next to see if I can get it to fire. I DEFINITELY think this is an electrical issue, not a mechanical issue with the compressor.

Question for the @gsxr -- are there any tests I can do via the STAR C3 on the system, to try to force the compressor on?

 

[Eno]

It can be a simple swap of the contacts on top of the pressure switch which do not power the compressor. Check if they are in their correct place.

 

[gerryvz]

It can be a simple swap of the contacts on top of the pressure switch which do not power the compressor. Check if they are in their correct place.

I do not recall whether I removed the contacts on the pressure switch last year, but I don't think that I did. That said, I will take a look.

The photo below from last year shows the pressure switch, and the wires look undisturbed.

 

[gsxr]

There's no SDS activations or testing available for the compressor that I know of.

I would try temporarily bypassing the pressure switch, short it out with a piece of wire. If the problem goes away, the switch is faulty.

 

[gerryvz]

Problem has been solved.

After some reading on the forum this morning, and prompting by @Eno's post above, I decided to switch out the leads that connect to the spade terminals atop the pressure switch. These are the two identical blue with pinkish-red stripe wires with the black female terminals on them, as shown in the lower right of the photo I posted immediately above.

Voila! Immediate A/C out the vents. Checked to see whether the compressor was running/engaged. YES it was.

Turned off the ACC system at the push-button unit. Checked compressor status -- OFF.

Turned things on again at the PBU. Compressor ON / clutch engaged, and immediate freezing cold air.

So it turns out that there is an "order" of the two contacts atop the pressure switch. Important to get these correct.

Thank you @Eno and others for your input. I'm going to switch my original BM back into the car now.

Cheers,
Gerry

 

[gerryvz]

Evidently, my original BM may be borked.

I switched the original back in, after testing with the replacement, and I could not get the A/C to work. Then I fitted the replacement BM again into the CAN box, and the A/C worked again.

Went to Home Depot in Annapolis and came back. Was in the store for about 15 minutes; A/C worked immediately when I returned to my car and was ice cold all the way home.

When I got home I checked A/C pressure and all was within spec. No leaks or other problems.

On the drive home, I took a quick shot of my temp gauge. This was taken at freeway speed, hot engine, 95F ambient temp, 60% humidity, and A/C ON. You can see the temp gauge at 102C.

It would probably have been 96-98C if the A/C had not been on.

 

[emerydc8]

Evidently, my original BM may be borked.

I switched the original back in, after testing with the replacement, and I could not get the A/C to work. Then I fitted the replacement BM again into the CAN box, and the A/C worked again.

Was it the base module that was causing the problem or both the BM and the switch polarity? I can't understand how the terminals to the pressure switch would need to be installed in only one particular direction.

 

[Jlaa]

There's no SDS activations or testing available for the compressor that I know of.

I would try temporarily bypassing the pressure switch, short it out with a piece of wire. If the problem goes away, the switch is faulty.

Problem has been solved.

After some reading on the forum this morning, and prompting by @Eno's post above, I decided to switch out the leads that connect to the spade terminals atop the pressure switch. These are the two identical blue with pinkish-red stripe wires with the black female terminals on them, as shown in the lower right of the photo I posted immediately above.
........

So it turns out that there is an "order" of the two contacts atop the pressure switch. Important to get these correct.

Thank you @Eno and others for your input. I'm going to switch my original BM back into the car now.

Cheers,
Gerry

Was it the base module that was causing the problem or both the BM and the switch polarity? I can't understand how the terminals to the pressure switch would need to be installed in only one particular direction.

<going apeshit on electrical diagrams with @kiev's saga.....>

According to the wiring diagrams, the pressure switch is S31/1, which is "Dual function AC compressor pressure switch (OFF 2.0/3.0 bar, ON 2.6/22.0 bar).

One of the BU/RD leads on the pressure switch (pin 2) goes to X26 (a junction) and then on to pin 7 N22, (AC pushbutton control module).
The other BU/RD lead on the pressure switch (pin 1) goes to pin 38 N16/1(base module).






Note that pin 12 of X26 is the junction where the BU/RD from the pressure switch changes to a BU/GN-WT wire before going into the Pushbutton control module.

X26:

Note X26 location here:

 

[gerryvz]

It is not yet fixed.

I just went out after about two hours to test the A/C. When I started the car, the compressor would not fire up. I removed the two leads from the pressure switch, and waited a second, and then replaced them. The compressor started up, but just for a few seconds. I did the same thing again and the same thing -- the compressor fired up, but just for a few seconds.

I removed the two leads from the switch and tried to jump the spade terminals with a 15A fuse. No go -- the compressor didn't fire up. I will do this again with a short length of wire, just to be sure.

I'm starting to believe that the pressure switch may be the issue here, not the compressor, BM or wiring.

Also, I am starting not to believe that there is a polarity associated with the two spade connectors on top of the pressure switch.

Dang, I thought I had this reliably licked, but it seems not.

Unfortunately if I replace the switch, I'm going to have to open up the A/C system. Something I don't have much experience with.

 

[Jlaa]

It is not yet fixed.

I just went out after about two hours to test the A/C. When I started the car, the compressor would not fire up. I removed the two leads from the pressure switch, and waited a second, and then replaced them. The compressor started up, but just for a few seconds. I did the same thing again and the same thing -- the compressor fired up, but just for a few seconds.

I removed the two leads from the switch and tried to jump the spade terminals with a 15A fuse. No go -- the compressor didn't fire up. I will do this again with a short length of wire, just to be sure.

I'm starting to believe that the pressure switch may be the issue here, not the compressor, BM or wiring.

Also, I am starting not to believe that there is a polarity associated with the two spade connectors on top of the pressure switch.

I don't think there's any polarity associated with those two BU/RD connectors but maybe just check X26 for a reliable connection, just for grins. If x26 is loose, it would take out the pressure switch. If x26 is loose, it would also foil any attempt to jump the two connections of the pressure switch as well.

Also maybe just triple check the connector of pin 38 of N16/1(base module). Pin corrosion or anything like that?

Dang, I thought I had this reliably licked, but it seems not.

Unfortunately if I replace the switch, I'm going to have to open up the A/C system. Something I don't have much experience with.

 

[emerydc8]

Opening the system is easy -- even I can do it. I use a vacuum pump 2.5 CFM Vacuum Pump and a set of gauges A/C R134A Manifold Gauge Set to draw it down for about 30 minutes to get any moisture out. Four cans of R134A is cheap from Walmart. You'll also need the adapter that connects to the can and punches a hole in the top to allow the refrigerant out. I think I had to go to Autozone or O'Rielly's for that.

As far as recovering the old R134, who cares. I mean, Japan is dumping billions of gallons of radioactive water into the sea and no one seems to be complaining about that. I don't think a gallon of refrigerant is going to matter.

 

[luckymike]

Is it possible that the pressure switch wires have a break in them and moving them around each time makes them have contact?

 

[gsxr]

I just went out after about two hours to test the A/C. When I started the car, the compressor would not fire up. I removed the two leads from the pressure switch, and waited a second, and then replaced them. The compressor started up, but just for a few seconds. I did the same thing again and the same thing -- the compressor fired up, but just for a few seconds.

Just triple checking - there are NO codes on the BM/GM, correct?

I removed the two leads from the switch and tried to jump the spade terminals with a 15A fuse. No go -- the compressor didn't fire up. I will do this again with a short length of wire, just to be sure.

For now, bypass the switch with a wire. It's only there to prevent the compressor running with the system empty, or with excessively high pressures (due to blocked TX valve or something).

 

[Trae]

I have had identical symptoms in the 93' for more than a year and had suspected a compressor speed sensor on the back of the compressor.

When it works, it will frost your nose. Then, after I park the car and return, can't get the compressor to engage again!

Was going to purchase a new one, but will follow this thread before I pull the trigger.

 

[gsxr]

Trae, if you have a spare BM/GM (and you should!), swap it out!

 

[gerryvz]

Just triple checking - there are NO codes on the BM/GM, correct?



For now, bypass the switch with a wire. It's only there to prevent the compressor running with the system empty, or with excessively high pressures (due to blocked TX valve or something).

I cleared the codes a few days ago, and as previously mentioned had two codes relating to the clutch, and belt slippage. I cleared them and they did clear.

I have not checked in the past few days to see if the codes came back. I will do that today and report back.

I will also try jumping the two pins of the pressure switch with a short length of wire. Using a fuse yesterday did not cause the compressor to activate.

I am starting to think this is a faulty/intermittent switch. I don't believe that I have two faulty BM/GM units (although I do have a third one, plus two W140 four-fuse units I could swap in). It is something electrical that is keeping the compressor from firing, of that I now have no doubt.

@7delta's idea of perhaps a broken connection of one of the two switch wires is something I will investigate. Certainly possible. I can wiggle them and see if I get a compressor activation on demand. Good thinking, @7delta, and thank you.

 

[maw1124]

@gerryvz when I had similar problems, it turned out to be a bad Evaporator Temp Sensor, IIRC (126-830-11-72 - HVAC Evaporator Temperature Switch 1986-1995 Mercedes-Benz | Mercedes-Benz USA Parts). There were other threads on this. Wasn't terribly expensive or hard to replace. I don't know if that's the switch you're playing with here. I'll see if I can dig up the invoice for a part number.

Cheers,

maw

 

[nocfn]

Envirosafe refrigerant... but on that switch, vulnerable wires there from heat and working in the area, they get brittle. Not a bad idea (unrelated to this issue) but to get new O rings in the cap coverings for the schrader valves. Mine were hard and cracked.

 

[nocfn]

@gerryvz when I had similar problems, it turned out to be a bad Evaporator Temp Sensor, IIRC (126-830-11-72 - HVAC Evaporator Temperature Switch 1986-1995 Mercedes-Benz | Mercedes-Benz USA Parts). There were other threads on this. Wasn't terribly expensive or hard to replace. I don't know if that's the switch you're playing with here. I'll see if I can dig up the invoice for a part number.

Cheers,

maw

Without that switch being installed on the C126, the AC would not even turn on. If you omitted to "plug" that sucker in somewhere, you will not get any cool air at all..... ever.

 

[maw1124]

Without that switch being installed on the C126, the AC would not even turn on. If you omitted to "plug" that sucker in somewhere, you will not get any cool air at all..... ever.

My switch was failing intermittently, which was the relevance here.

maw

 

[gerryvz]

I ordered a new refrigerant temp sensor, and a new pressure switch, today. Both of these are the electrical items that screw directly into the receiver/drier. Just in case I need to replace one or both of them.

I need to find my vacuum pump so I can vacuum out the A/C system. I know I had it in my garage at the apartment I lived in in Texas, just before I moved to Maryland. But for the life of me, I can't find the vacuum pump. I know it's down in my basement somewhere. I thought I'd gone through every box, but I still have a bunch of stuff missing, so I guess I need to keep searching.

Will pull codes and post an update later tonight.

Cheers,
Gerry

 

[gerryvz]

I ordered a new refrigerant temp sensor, and a new pressure switch, today. Both of these are the electrical items that screw directly into the receiver/drier. Just in case I need to replace one or both of them.

I need to find my vacuum pump so I can vacuum out the A/C system. I know I had it in my garage at the apartment I lived in in Texas, just before I moved to Maryland. But for the life of me, I can't find the vacuum pump. I know it's down in my basement somewhere. I thought I'd gone through every box, but I still have a bunch of stuff missing, so I guess I need to keep searching.

Will pull codes and post an update later tonight.

Cheers,
Gerry

Well, I drove the car on a few errands around town yesterday evening. The compressor was on and off. It would start up for a few seconds when I first started the car, and then turn off. When out driving (after after a few miles) it came back on again, then turned off, then on again. When I parked at Home Depot and the post office for errands, it would be off when I started up the car after coming out of the store, then came on pretty quickly while driving.

It's very very intermittent. I can't really place my finger on what is triggering the compressor to come on and go off, but generally when I am driving (particularly at higher speeds, say on arterials or highways/freeway) it tends to stay on once the compressor activates.

When I got home, last night, I checked codes. The only relevant code for the BM was code 7, which is the "belt slippage" code. I had cleared that one previously. There were numerous other codes, but those were the Fuse 1-4 codes that were thrown because the electrical connection was broken when I removed the BM to replace it with the spare, then back to my original, then back to my spare BM. All of that swapping I did with the battery still connected, which I believe led to the codes being stored because of the electrical disconnection.

I will remove those codes tonight and see what comes back. As said though, the code 7 "belt slippage" is the one relevant code here.

Also, when home, and with the engine idling in the driveway, and with the compressor RUNNING, I unplugged one and then the other leads on the pressure switch. The compressor immediately stopped upon unplugging either spade connector on the pressure switch, and then immediately started again when I replaced the connector on the switch.

I am almost 100% sure that there are no refrigerant leaks in the system. Checking the system pressures tonight (again) will eliminate that once and for all as an issue.

I am going to drag out my proper A/C gauges tonight and check the system pressure on both the high and low side to make sure that everything is in spec. I will also try to jump the pressure switch to get the compressor going, if it is not running.

As mentioned, I have ordered new refrigerant temp and system pressure switches, for delivery later this week. In any case, these will be good to have on hand for the future if ever needed.

I need to study the wiring diagrams in detail.

The quest continues.....

 

[gerryvz]

I don't think there's any polarity associated with those two BU/RD connectors but maybe just check X26 for a reliable connection, just for grins. If x26 is loose, it would take out the pressure switch. If x26 is loose, it would also foil any attempt to jump the two connections of the pressure switch as well.

Also maybe just triple check the connector of pin 38 of N16/1(base module). Pin corrosion or anything like that?

Hi @Jlaa -- No pin corrosion on the base module. I visually checked both the BM male pins and the female sockets in the CAN. All looks perfect.

I am going to replace my SECOND SPARE Base Module today and see if that makes any difference.....I doubt it will though.

Is it possible that the pressure switch wires have a break in them and moving them around each time makes them have contact?

@luckymike -- this was a great thought given the crap wiring on the E500s. I did wiggle the two wires going to the pressure switch, last night, both with the compressor running and not running, and no difference in compressor running status. So I think these wires are probably OK. Though when I did remove each connector with the compressor running, the compressor immediately stopped, and then started again when I reconnected the terminal to the pressure switch.

Just triple checking - there are NO codes on the BM/GM, correct?



For now, bypass the switch with a wire. It's only there to prevent the compressor running with the system empty, or with excessively high pressures (due to blocked TX valve or something).

Just a code 7 ("belt slipping") that I pulled last night. I will delete all codes and start over and see what comes back (again).

 

[emerydc8]

Hi Gerry,
You don‘t happen to be using a cool harness on your car, do you?

 

[gerryvz]

Hi Gerry,
You don‘t happen to be using a cool harness on your car, do you?

No. No way would I use that.

 

[gsxr]

"Belt slipping" means the engine RPM is higher than compressor RPM.

This could also mean the tach on the compressor is flaky, or the wiring from the tach.

 

[Trae]

I'm going to switch BM modules to see if it makes a difference in mine.

If not, I am semi convinced that it is the speed sensor on the compressor.

 

[kegmankipp]

Maybe you have two bad BM?

I had the same belt slippage code on
mine and totally random, intermittent working status. New belt and new compressor, didn’t change the error.

The pin 21 (iirc) solder connection on BM was not good. Looked ok, but wasn’t. It started working 100% once I resoldered with a quad eutectic solder. I believe the high current along with dissimilar metals and ho-hum solder were the issue.

I used Cardas Audio solder from leftovers from my years of owning a Hi-Fi store. It’s great stuff

Cardas Soldering Wire Quad Eutectic Silver Solder with rosin flux 1/4 lbs (113g) roll Cardas Soldering Wire Quad Eutectic Silver Solder with rosin flux 1/4 lbs (113g) roll - - Amazon.com

 

[Jlaa]

Hi @Jlaa -- No pin corrosion on the base module. I visually checked both the BM male pins and the female sockets in the CAN. All looks perfect.

I am going to replace my SECOND SPARE Base Module today and see if that makes any difference.....I doubt it will though.

There could be cold solder joints in your two Base Modules, but aside from that, maybe do a quick check of X26 (post 16) just to rule it out. Fast and easy to rule X26 out.

The pin 21 (iirc) solder connection on BM was not good. Looked ok, but wasn’t. It started working 100% once I resoldered with a quad eutectic solder. I believe the high current along with dissimilar metals and ho-hum solder were the issue.

I used Cardas Audio solder from leftovers from my years of owning a Hi-Fi store. It’s great stuffCardas Soldering Wire Quad Eutectic Silver Solder with rosin flux 1/4 lbs (113g) roll - - Amazon.com

Oh cool! I had no idea that there is such a thing as "speciality solder for hifi components!" I see that it is a rosin core solder with with "Pb, Cu, Ag. Silver for higher conductivity and stronger solder joints".

Interesting. I use Kester 60/40 solder but maybe I should switch to 63/37.

KESTER SOLDER 24-6040-0027 Wire Solder, 0.031"Dia., Pack of (1),32117: Amazon.com: Tools & Home Improvement

KESTER SOLDER 24-6040-0027 Wire Solder, 0.031"Dia., Pack of (1),32117: Amazon.com: Tools & Home Improvement

www.amazon.com

 

[maw1124]

Well, I drove the car... (snip)

The quest continues.....

Along the way to solve my AC compressor issues was a new idler pulley [at the belt tensioner]. It was a pulley Rex reported seeing failures in, and his question was whether we replace with OE or OEM. We decided to try OEM to confirm his suspicions -- that the OEM were not as robust as OE. Sure enough, the OEM failed within a couple weeks, and he put an OE in (no labor charge, true up on part cost, etc.).

Having ruled out the pulley, that's when we taped the inlets and focused on the temp switch. When my compressor would stop, sometimes turning the AC off then on again would cause it to re-engage, and that's what led us to focus on the switch. For yours, the belt slippage (pulley) might be the only thing wrong. But both the slipping belt and the evaporator temp switch were on the path to resolution.

maw

 

[gsxr]

Gerry's car is a 1994 which has the automatic spring-loaded belt tensioner. It wouldn't hurt to verify the tension feels normal, but otherwise, it's a rare failure. Very different animal compared to the rubber-bearing style 1992 manual tensioner which has a relatively short lifespan, especially if over-tightened. I'm not aware of any OEM manual tensioners sold anymore, everything I've seen is straight aftermarket, not true OEM with the Star logo ground off.

 

[emerydc8]

As a part of my previous troubleshooting process for an intermittent compressor, I changed the belt, tensioner (twice) and then used half a can of belt dressing that flung itself all over my engine compartment. Mine turned out to be a bad Base Module.

 

[alabbasi]

I don't know the W124 well but on the W126, there was a big module that controlled the a/c compressor among other stuff in the fuse box. I've had that go south on me about 12 years ago. Luckily a cash for clunker saved the day

 

[gerryvz]

Maybe you have two bad BM?

I had the same belt slippage code on
mine and totally random, intermittent working status. New belt and new compressor, didn’t change the error.

The pin 21 (iirc) solder connection on BM was not good. Looked ok, but wasn’t. It started working 100% once I resoldered with a quad eutectic solder. I believe the high current along with dissimilar metals and ho-hum solder were the issue.

There could be cold solder joints in your two Base Modules, but aside from that, maybe do a quick check of X26 (post 16) just to rule it out. Fast and easy to rule X26 out.

I have a third (second spare) BM out in the shop. I will swap it in and see if that changes anything. I think it is a pretty low probability that I would have TWO bad BMs.

I did remove the cover on the spare unit (in the car now) and looked closely at the solder joints, and didn't see anything obvious. Would have to get my magnifying glass out to really look it over, and I will do that with all of them.

As a part of my previous troubleshooting process for an intermittent compressor, I changed the belt, tensioner (twice) and then used half a can of belt dressing that flung itself all over my engine compartment. Mine turned out to be a bad Base Module.

I am not ready to blame the compressor yet, but perhaps the speed sensor on the back of it could be an issue. It's on my list of "top 5" issues to check.

I need to pull the car up on the ramps, remove the belly pan, and closely examine all of the wiring all over the compressor. From the top, things looked OK, but I will do a more close check on this.

I don't know the W124 well but on the W126, there was a big module that controlled the a/c compressor among other stuff in the fuse box. I've had that go south on me about 12 years ago. Luckily a cash for clunker saved the day

The V-8 W124 models have a different ACC control system than the late 126s, though the general layout of the actual components is the same (PBU, compressor, etc.). The 126 uses a "Klima" relay, similar to the six-cylinder W124 models, but the V-8 models route the control voltage through the Base Module.

Which reminds me -- I should also visually check the wiring at the battery that supplies direct current to the entire CAN box.

Unfortunately, I wasn't able to do anything last night. I will swap in the second spare BM tonight, and also pull the car up on the ramps for a visual inspection of the compressor wiring. Will also try to "jump" the pressure switch (when the compressor is off) to see if that shocks it back to life.

As a last thing I'll do, I will erase all of the codes before I put the car to bed.




A quick visual inspection of the battery terminals shows everything is right and tight at the battery positive terminal.

 

[alabbasi]

Can it be accessed through the 38 pin?

 

[gsxr]

Can it be accessed through the 38 pin?

The BM live data & fault codes can be accessed via 38-pin diagnostic port.

The A/C wiring does not route directly to the 38-pinner... that is for communication with control modules only, nothing else.

 

[gerryvz]

Due to solid rain the past couple of days here in the DC/Maryland area, I haven't been able to work on the car.

I did receive the two pressure switches today, and I have a full A/C system o-ring and seal kit coming on Tuesday, for if and when I need it (i.e. having to open up the A/C system, to replace a switch or compressor, etc.).

 

[emerydc8]

Due to solid rain the past couple of days here in the DC/Maryland area, I haven't been able to work on the car.

I did receive the two pressure switches today, and I have a full A/C system o-ring and seal kit coming on Tuesday, for if and when I need it (i.e. having to open up the A/C system, to replace a switch or compressor, etc.).

Hi Gerry,

I don't know if you've done this recently, but if you're going to open the a/c system anyway, that's a convenient time to pull the compressor off and change the short S-hose on the p/s system. The only additional work IIRC is to lower/remove the front sway bar to get the compressor out. Also, with a clean compressor on a workbench it will give you a better opportunity to access and examine the wires, just in case it might be a bad speed sensor. I wasn't able to find the speed sensor sold separately when I was going through this issue, so in my case I just got a new compressor. But I did change the S-hose because with the compressor out of the way it's a piece of cake.

 

[gerryvz]

I did the S hose a few years ago, and removed it last year when I removed the tandem pump and bracket. All was good with it and no need for replacement at this time.

 

[J. M. van Swaay]

Hi Gerry, I had a similar problem a while back, here’s the link to the thread:

Intermittent AC | Heating and Cooling (Interior HVAC systems)

94 E500 AC intermittent. Seems to always work on cold start but intermittent when car is hot. When AC not working, compressor is not engaged. Verified function of low speed cooling fans. Suspect faulty relay in base module. With hot car, engine running and AC commanded on but compressor...

www.500eboard.co

 

[gerryvz]

Thanks to all for the notes and input. I have to admit, I haven't made as much progress as I'd hoped to on this in recent days.

However, here is what I have found in the few slices of time I've devoted to this in recent days.

Still had the compressor kicking on at a random interval after driving the car, say a few miles. At times the compressor would shut down and then restart at random intervals, although most of the time once it is running, it would work fine and blow ice cold air. Until I shut the car down, and the process would start all over again.

I deleted all codes, ran the car again, and then checked codes yet again. I came up with a code 6 (again) on the Base Module, which indicates problems with the compressor's clutch not engaging or being jammed.

Two different Base Modules have not made any difference in system operation. With both installed, the operating characteristics have been the same (delayed compressor activation). I have not tried my second spare BM, but at this point I do not believe that the BM is the issue. A visual inspection of the soldered pins/connections in my "original" BM did not show any anomalies.

Last week I pulled the car up on ramps, removed the belly pan, and I visually inspected the wiring on and to the compressor. From everything I could see, the wiring was fine, no insulation missing. WIth the car running, I wiggled all of the wires and not able the make the compressor kick on with the car idling. So I do not believe that the physical wiring was compromised in any way.

I then checked the system pressure yesterday. I used my go-to Snap-On gauge set, as well as a brand new set of Harbor Freight gauges that I bought as a back-up, and a digital gauge that connected to the Low side only to measure system pressure there. This was with the car running, but with the compressor not working.

What I found was that the Low side pressure was around 90 PSI with an 85F ambient temp, when it should have been around 45-55 PSI. So, it was about double the normal pressure on that side. The High side was showing around 100 PSI, when it should have been showing around 200-250 PSI. So, this was a bit less than one-half what it should have been.



I was not able to "jump" the compressor on at any time by shorting/jumping the pressure switch leads.

All of this is leading me to believe, given the high "Low" side and low "High" side pressure symptoms, and the repeated codes having to do with the compressor's clutch, that the compressor itself has developed an intermittent failure. I am going to get the system vacuumed (to ensure there are no leaks) and then evacuated and refrigerant recovered, in the next few days. I will order a new Denso compressor, and will remove the old compressor, temperature switch, and pressure switch, and replace these items. Then I will add new refrigerant and any needed oil at time of installation. Note that the new compressor comes with R134a PAG oil, but I will order some extra oil with dye, just in case it is needed.

ONE NOTE: I realize that I measured system pressures WITHOUT the compressor engaged. I know that accurate pressure measurements require the compressor to be engaged so that things are working, and without the compressor engaged the pressure readings are going to be off (i.e. the low side pressure will be higher with the compressor OFF than with it running). However, I am now unable to get the compressor to activate (it won't jump by jumpering the two leads that fit on top of the pressure switch). So I have no way to get the compressor running......

So, that will be my next course of action. I do not believe that the receiver-drier or expansion valve are comprmised or failed. The compressor is the original one, so after 144K miles and nearly 30 years, I would not be surprised if it had failed.

Looking from underneath the car, it looks like I will definitely have to remove the plastic duct that goes from the front spoiler to the driver's side motor mount to remove the compressor, but hopefully not too much other stuff (i.e. sway bar). If anyone has any tips or tricks they can share as to compressor replacement, I would love to get any info you have. In the meantime, I'll look at the factory docs as well as existing threads from @emerydc8 on what they have done.

Will keep folks in the loop as this continues.

Cheers,
Gerry


P.S. I was very happy to find, when I removed the belly pan, that there was not a SINGLE DROP of oil or other liquid on the top of the belly pan from the engine. So after 2,000+ miles driven since last year's Top-End Refresh, I am super excited that the car has zero leaks of any kind, particularly oil from the front crankshaft seal, tandem pump "short hose" or coolant from the radiator/water pump/hose fittings.

 

[JC220]

Thanks to all for the notes and input. I have to admit, I haven't made as much progress as I'd hoped to on this in recent days.

However, here is what I have found in the few slices of time I've devoted to this in recent days.

Still had the compressor kicking on at a random interval after driving the car, say a few miles. At times the compressor would shut down and then restart at random intervals, although most of the time once it is running, it would work fine and blow ice cold air. Until I shut the car down, and the process would start all over again.

I deleted all codes, ran the car again, and then checked codes yet again. I came up with a code 6 (again) on the Base Module, which indicates problems with the compressor's clutch not engaging or being jammed.

Two different Base Modules have not made any difference in system operation. With both installed, the operating characteristics have been the same (delayed compressor activation). I have not tried my second spare BM, but at this point I do not believe that the BM is the issue. A visual inspection of the soldered pins/connections in my "original" BM did not show any anomalies.

Last week I pulled the car up on ramps, removed the belly pan, and I visually inspected the wiring on and to the compressor. From everything I could see, the wiring was fine, no insulation missing. WIth the car running, I wiggled all of the wires and not able the make the compressor kick on with the car idling. So I do not believe that the physical wiring was compromised in any way.

I then checked the system pressure yesterday. I used my go-to Snap-On gauge set, as well as a brand new set of Harbor Freight gauges that I bought as a back-up, and a digital gauge that connected to the Low side only to measure system pressure there. This was with the car running, but with the compressor not working.

What I found was that the Low side pressure was around 90 PSI with an 85F ambient temp, when it should have been around 45-55 PSI. So, it was about double the normal pressure on that side. The High side was showing around 100 PSI, when it should have been showing around 200-250 PSI. So, this was a bit less than one-half what it should have been.

View attachment 133797

I was not able to "jump" the compressor on at any time by shorting/jumping the pressure switch leads.

All of this is leading me to believe, given the high "Low" side and low "High" side pressure symptoms, and the repeated codes having to do with the compressor's clutch, that the compressor itself has developed an intermittent failure. I am going to get the system vacuumed (to ensure there are no leaks) and then evacuated and refrigerant recovered, in the next few days. I will order a new Denso compressor, and will remove the old compressor, temperature switch, and pressure switch, and replace these items. Then I will add new refrigerant and any needed oil at time of installation. Note that the new compressor comes with R134a PAG oil, but I will order some extra oil with dye, just in case it is needed.

ONE NOTE: I realize that I measured system pressures WITHOUT the compressor engaged. I know that accurate pressure measurements require the compressor to be engaged so that things are working, and without the compressor engaged the pressure readings are going to be off (i.e. the low side pressure will be higher with the compressor OFF than with it running). However, I am now unable to get the compressor to activate (it won't jump by jumpering the two leads that fit on top of the pressure switch). So I have no way to get the compressor running......

So, that will be my next course of action. I do not believe that the receiver-drier or expansion valve are comprmised or failed. The compressor is the original one, so after 144K miles and nearly 30 years, I would not be surprised if it had failed.

Looking from underneath the car, it looks like I will definitely have to remove the plastic duct that goes from the front spoiler to the driver's side motor mount to remove the compressor, but hopefully not too much other stuff (i.e. sway bar). If anyone has any tips or tricks they can share as to compressor replacement, I would love to get any info you have. In the meantime, I'll look at the factory docs as well as existing threads from @emerydc8 on what they have done.

Will keep folks in the loop as this continues.

Cheers,
Gerry


P.S. I was very happy to find, when I removed the belly pan, that there was not a SINGLE DROP of oil or other liquid on the top of the belly pan from the engine. So after 2,000+ miles driven since last year's Top-End Refresh, I am super excited that the car has zero leaks of any kind, particularly oil from the front crankshaft seal, tandem pump "short hose" or coolant from the radiator/water pump/hose fittings.

Is there a way to view the live data in SDS Gerry? And to actuate the compressor clutch using SDS? Agree that your compressor appears defective, just curious if these tests would add any more info