No Radiator Cooling Fans (High or Low) SOLVED: BAD PRESSURE SWITCH

[emerydc8]

I noticed my low-speed fans had stopped working so I checked the usual suspects: I jumpered the wires to the receiver/dryer to rule out the pressure switch (no fix), replaced the big resistor behind the headlight, rechecked the wires and cleaned the terminals going to the resistor (no fix), and replaced both relays (high and low) in the relay box behind the fuse box (no fix).

This is a '95 so it is the face-lifted model where Mercedes moved the 30 amp strip fuse from the driver's side fender to the relay box. I didn't get any voltage to the low-speed resistor with the air on, engine running and receiver/dryer wires jumpered, although the compressor was running and it was blowing ice-cold air. Also, I didn't hear the relay click when I jumpered the two wires going to the receiver/dryer with the key on, engine off and air selected on (this is why I suspected a relay and replaced them both during the troubleshooting process).

Just before closing the hood for the night, I decided to pull the plug connectors to all three temp sensors on the intake manifold with the engine running and was surprised that not even the high-speed fans activated. I think pulling the round connector should activate the high-speed fans but don't recall which one so I just pulled all three. I should have tried this step first but I’ve never had a situation where the high-speed fans went inoperative. The fans are just not running in low or high. Any thoughts on what else I could be missing? One thing I haven't done yet is check the plug going to the actual cooling fans in front of the radiator. I guess it's possible that both fans failed and I never noticed, but unlikely since I'm not getting any voltage to the low-speed resistor.

 

[nocfn]

Maybe I missed it but the push button unit, are you getting voltage to the harness when you press buttons?

 

[emerydc8]

It is the cooling fans in front of the radiator that are inoperative. I need to edit the header of my original post. I guess I wasn’t too clear on that.

 

[gsxr]

Jon, pulling the 2-pin temp sensor (for the HVAC pushbutton unit) will usually, but not always, trigger the fans on high speed.

Based on your current troubleshooting, I think the next step is following the ETM schematics for the relay control voltage. Seems like neither relay is getting triggered, which is odd. Wouldn't hurt to check all other fuses as well.

Note the schematic is different for -->93 and 94-->, make sure to follow the correct section for the facelift car:

https://www.500eboard.co/forums/misc/Manuals/Servicev3.2/W124/w124CD1/Program/ETM/2483600.pdf

 

[emerydc8]

Thanks, Dave. It is strange if power is not getting to either high or low relay. In Arizona corrosion of wires and grounds is usually a non~issue and it’s usually a component failure causing the problem. I’ll use the schematic to see which components, if failed, could cause this. It could be a bad ignition switch. That has been replaced in the past year or two and It concerns me about the worsening quality of Mercedes parts since COVID happened. I checked the fuses already but I’ll check again. Thanks for the diagram.

 

[gsxr]

Jon, are the relays not receiving +12v main power (from X4/10, circuit 30)... or not receiving the signal to energize the relay coil?

If the former... maybe check the ignition switch. I sure hope a recent one hasn't failed already!

If the latter... check the ground wire that goes to terminal 86 on both relays. And also the signal wire to terminal 85 on each relay.

 

[emerydc8]

Thanks Dave. I'll have to get back to the relay box with a multi-meter later tonight and check.

 

[emerydc8]

I'm not too good with schematics but it looks like the "AIR relay module" feeds into the first stage of the Aux fan relay module. Maybe this is nothing, but last week I replaced the smog/air pump and alternator. Is the AIR relay module in the schematic referring to the smog system? The smog pump works fine now (much quieter too), as does the alternator. This was the last work I performed on the car.

 

[emerydc8]

EDIT: SOLVED! I initially used a homemade jumper wire with small (cheap) alligator clips to jumper the wires going to the red pressure switch on the receiver/dryer and the fans would not run. Turns out there is a short in my homemade jumper wire at the clip. Sometimes it works but I discovered that if it is held in different positions it cuts out (nothing like having a troubleshooting tool that is intermittent). I just discovered this because when I tried jumpering again with a paperclip the low-speed fans activated. It is a bad pressure switch.

So I just ordered a red pressure switch, part number 004 820 68 10 ($22.79 @ AutohausAZ for the Mahle/Behr). I also ordered the other switch on the receiver/dryer with the spade connector since it's only $13, plus I have to open the system anyway.

I still don't know why I didn't get high-speed fans when I pulled the sensor on the intake manifold. Maybe that's a different problem.

What happened to the price of R134a? It was $19 a pint yesterday at Walmart! Last month it was only $7. Is R134a the new R-12?

 

[gsxr]

Good to hear you solved the mystery, Jon!

FYI... not sure you want to use the aftermarket Mahle/Behr switches, made in Taiwan. Neil had several fail shortly after installation (leaking refrigerant). Details in this thread. If you don't want to repeat the vacuum / recharge dance, I'd shell out for OE. The red switch is ~$40 (link), the safety switch is around $75 (link), from MB of Laredo.

I'm seeing 30-lb cylinders of USA-made Chemours R-134a at ~$300 or so. Ouch. We're heading into summer, so the price tends to go up with ambient temperatures.. but still, ouch. I haven't been tracking the price, I'm not sure what the low was over the winter.

 

[emerydc8]

Well that was a waste of $50 and it’s too late to cancel. Thanks for the information on the switch quality. I guess I’ll order one from MB Laredo now.

 

[gsxr]

D'OH! Maybe call AHAZ and see if they can stop the package before it ships today? Might be worth returning even if you have to pay shipping. The amount of poor quality parts is getting ridiculous. I'm just glad we have some of the known bad actors documented on the forum, to help avoid repeating the job...

 

[emerydc8]

They claim that once the order is “processed” it cannot be canceled. i tried. It’s hard to believe someone processed it at 8:15 on a Monday morning but I will let them send it anyway because MB Laredo shows the o-ring (140 997 1145) as NLA and I don’t know if the factory sensor comes with an o-ring. It also shows the switch as a special order item so who knows how long it will take.

 

[gsxr]

They claim that once the order is “processed” it cannot be canceled. i tried. It’s hard to believe someone processed it at 8:15 on a Monday morning...

Yeah... that seems fishy to me. I'd tell the person on the phone "well, please start the return process for me now, before you have shipped it" and see if that changes the response at all.

...but I will let them send it anyway because MB Laredo shows the o-ring (140 997 1145) as NLA

This O-ring isn't NLA, yet! Note that the Rev Parts message is "Currently Unavailable" (link), which is not the same as "Discontinued". I have only recently seen this new "Currently Unavailable" description for some items. Best I can tell, these are parts which may have to come out of Germany warehouses. If so, it's possible that some dealers do not want to bother ordering out of Germany, and flag the item incorrectly as "unavailable". The more accurate description is "we can get this, but we don't want to bother." My local dealer was able to obtain parts like this without any issues, other than having to wait a week or two. Very annoying that online dealers are making our lives difficult for no good reason.

...and I don’t know if the factory sensor comes with an o-ring. It also shows the switch as a special order item so who knows how long it will take.

I am 99% certain the factory sensor comes with the O-ring, but not 100% certain. I think "special order" in RevParts lingo means it may have to come out of a USA warehouse, but probably not Germany. So it adds a few days to get to Laredo before they ship out to you.

 

[emerydc8]

All good information Dave. I will probably return the switches when they arrive since the return shipping won't be that much and the parts aren't even something I would want on my shelf as spares. That is sad that the quality of Mahle/Behr is so bad. I guess it just gets worse from here.

 

[emerydc8]

I am 99% certain the factory sensor comes with the O-ring, but not 100% certain.

Got the switch from MB Laredo. As usual, you are right--it comes with an O-ring.

 

[gsxr]

Got the switch from MB Laredo. As usual, you are right--it comes with an O-ring.

Nice! Is there any COO on the switch or label?

 

[emerydc8]

I don’t see any. I see Sensata and >PA66< stamped on it if that helps.