need help diagnostic A/C compressor clutch....

[Ertech]

Hi guys.
I came back from a one month trip and the A/c does not work.have a full charge still clutch does not engage .

1-checked fuse 7 and all fuses in BM module.
2-applied power direct to clutch and it does engage < so compressor and clutch works >.
3-I bypassed the pressure switch <jumper ,clutch should engage even if no freon in system ,>, the is no power to any of the 2 leads to switch .once bypassed there is no power to the plug at the compressor .
so my understanding is it could be the BM module?anything other test to do before I purchase a BM?
thanks guys .
ps:I did read all I could from the search function

 

[gsxr]

Sounds like the BM/GM. I recommend having a good spare of EVERY module for exactly this reason... fortunately they are not expensive on eBay, or you can shell out $500+ for a new one since they are still available from MB.

You can study the schematics at the link below.

24 Electrical Troubleshooting

 

[Ertech]

thanks Dave .
Hi happened to have a extra BM , tried it and still no go.
when I check power and the switch terminal I get 12v with my power probe but it does not beep. like a shadow voltage it that makes sense.
Are there any other sensors that would make the compressor not engage ?
Is there a way to test the climate control switch ?
Thanks

 

[emerydc8]

Have you taken the belt off and tried turning the compressor by hand? According to a recent post by @gsxr, you should be able to rotate the face plate by hand with moderate force.

Also. Have you checked the codes to see if you're getting any compressor lockout messages?. Maybe clear all the codes and see what returns.

 

[Ertech]

I Put power direct on the clutch with the engine on and the a/c worked .for some reason my mercedes code reader computer was never able to communicate with the BM, the problem is my computer since it works using my friends computer .I swapped with a working BM and still the same problem.is there another sensor that can block the function of the clutch ?those ambiant temp sensors?
thanks

 

[gsxr]

Check the schematics, and also read the climate control manual. The only items in the circuit AFAIK are the pushbutton unit in the console, the BM/GM, compressor, and pressure switch. If you bypass the pressure switch and there's no change, and the compressor runs normally when manually energized with +12v, it appears to be something with the BM/GM or the pushbutton unit. Are you certain the spare BM/GM is good? And there were no codes on it when trying to run the AC?

 

[Ertech]

I will scan the BM this week when I go to my friends place .
I did a few tests at the wiring to the pressure switch .

1-from one wire I get 12V ,the other wire no + or - < this does not change if A/C push button is pressed or not>
2-when I ground the one that had 12V the clutch engages .
3-put the 2 wires together and nothing.
can this info be helpful in the diagnostic?
thanks guys for your patience .

 

[gsxr]

#1 - this sounds normal. A switch typically only has voltage on one side, not both.

#2 - do you mean the system works when you ground the "12v" wire? That sounds like a bad BM/GM.

#3 - Shorting the wires should have the same effect as #2.

Did you trace the schematics to see where the 2 wires from the switch go?

 

[Ertech]

# 2 yes. , When I put that 12v wire to ground The church engages
#3 shorting the 2 wires does nothing

it shown one wire to BM the other to the push button via X26
toke to BM out and applied 12v to pin 20 and the compressor engages
I ordered a BM on Ebay now
could the speed sensor be at fault and cause this?
thanks Dave for your help

 

[Kridre]

There is also a speed sensor at the back of the compressor. Wiring for it is made from the same eco junk like the rest of the wiring. Check the plug at the top of the compressor to see if insulation is intact and the wires are not touching each other and giving not correct reading, preventing the clutch from engaging.

Here is how it looked like in my case.

 

[Ertech]

I will take to plug out again and check
thanks

 

[gsxr]

Schematic indicates the "compressor on" signal comes from the pushbutton unit, through the switch, out to the BM/GM. But it doesn't specify which side provides +12v, and these German engineers often switch the ground rather than the positive, which can be confusing. And, the schematic shows both wires as the same color, so you can't tell which goes to which module without using an ohm meter to trace each wire. It's possible the pushbutton unit is faulty, I don't know how to test this without sourcing another pushbutton unit to try out...

 

[Ertech]

anyway to test the speed sensor?
thanks

 

[emerydc8]

If you do trace it down to a speed sensor, I looked into this a few years ago and IIRC the sensor was nearly as expensive as the whole compressor unit (with sensor).

I had intermittent compressor engagement and lived with it while troubleshooting. If I started the car and the air didn't work (no compressor) or if the air stopped working, I would drop it into neutral while driving, restart the engine, and it would usually allow the compressor to work again. Sometimes it took two or three tries.

My troubleshooting process included replacing the compressor, expansion valve, pressure switches, left cooling fan, pushbutton unit, cockpit air temp sensor, belt and belt tensioner. It turned out to be a bad Base Module.

Were you able to clear the codes and see if any returned? Just one more thing to rule out if you don't see any slip/lockout-related codes.

 

[gsxr]

I was thinking about this some more. If one of the 2 wires at the switch has +12v, AND the compressor turns on when you connect that wire to ground... turns off when you remove from ground... turns back on when you connect to ground... that wire must be connected to the BM/GM. Because the other wire goes to the pushbutton unit (PBU), if you shorted that to ground, nothing would happen. I think this indicates the BM/GM is ok and something is up with your PBU...

 

[Ertech]

So when I press the PBU I should get a ground from one of the pressure switch wires ?

 

[gsxr]

So when I press the PBU I should get a ground from one of the pressure switch wires ?

The PBU may connect that wire to ground, yes... but I don't know how it looks on a meter. I assume you tried both buttons that should turn on AC.

 

[Ertech]

This is getting fun I have a spare PBU and still no go . I pour ground on pin 7 of the plug going to PBU Iand clutch started.
My spare PBU must be bad.

 

[gsxr]

Hmmm. I'm confused too. Next up I'd trace each wire and verify which one is going to BM/GM and which to PBU, and confirm which one engages the AC.

And also double-check all the fuses, just in case...

 

[Ertech]

its the one going to the BM that engages when I put it to ground .
does the under dash evap sensor and the small sensor by the light have anything to do with this ?

 

[gsxr]

its the one going to the BM that engages when I put it to ground .

Cool! That seems to confirm the BM/GM is good (assuming there are no fault codes stored). The problem must be upstream, either the PBU or something related to the PBU, so it never sends the "Turn On" signal to the switch/BM-GM. Wouldn't hurt to verify the wire is intact between pressure switch and PBU, although I expect it's fine.

does the under dash evap sensor and the small sensor by the light have anything to do with this ?

It shouldn't, unless they are reading way off spec. The evap temp sensor will turn off the compressor if it reads too cold, to prevent icing of the evaporator. There are specs in the climate manual, you can test to verify the sensor is accurate.

The sensor by the overhead light is for interior temp, you can test this also but I don't think it has any direct control over the compressor, i.e. even if the overhead sensor was bad it wouldn't prevent the compressor from engaging.

 

[Ertech]

the wire going from the PBU is good since if I put ground to the wire by the pub plug the clutch kicks in. I will hunt for a 3rd PBU.

 

[Ertech]

Stumped
Tried 3 different PBU’s and still no go
Can the rpm sensor be the culpin?or one of the sensors ??
Thanks

 

[Alphasud40]

What year is your car?

 

[gsxr]

RPM sensor on the compressor should trigger a fault code.

Also, the system will normally engage the compressor for 1-2 seconds, then determine if RPM's don't match, then shut off the compressor. It sounded like your car never turned on the compressor at all, however if you have not checked this, watch the compressor while someone else presses the button (start the car with "EC" selected).

Assuming all fuses have been checked (ALL of them) and are good, next would be trying to study the climate control manual to see if there's info about what conditions need to be met for AC engagement.

 

[Ertech]

1994 E500
will try and report back trying switch while watching .anyone have climate control manual?
thanks

 

[Alphasud40]

By grounding pin 7 on the X1 connector what you are doing is testing the pressure switches, the base module and the rotation speed sensor function if that compressor continues to run. You have switched out the PBU so the problem has to lie in the sensor input side of the PBU. To test this locate pin 10 of the X2 connector I’m showing brown/yellow wire And insert one of your test leads in there for all the tests. This is the reference voltage sent from the PBU to all the sensors. Pin 2 gray/yellow is the cabin sensor, pin 4 is the evap. sensor gray/red, pin 7 is the heater core sensor gray/green, and pin 9 is your outside ambient air sensor gray/black. I don’t have a resistance value for the sensors but typically NTC sensors runaround 2.5K ohms at 20 degrees C. The important thing is with the vehicle at ambient with the engine at ambient all sensors should jive or be close to one another.
One other sensor to check is the coolant temp sensor. Test this with your test lead in pin 10 of the X2 connector like the others but the input is found on pin 8 of the X1 connector blue/gray wire. Again probably will read around 2.5K ohms at ambient. Wild ass guess on my part is either the outside air sensor or the coolant temp sensor is going to be your issue because either one failing will harm either the compressor or the engine.

 

[Ertech]

will do these tests this morning , can the engine be off for the tests or running ?
thank you

 

[Alphasud40]

Yes, engine off as you are just measuring resistance values

 

[gsxr]

Climate control manual at link below, scroll down to Group 83. This has data on resistance values for all the sensors:

Mercedes Benz Model 124 - M119 Maintenance Manuals

 

[Ertech]

did all the tests and got a value for pin 10-7 and 10-9. nothing for 10-2 , 10-4 , 10- x1 plug #8.
I am trying my best with electrical wiring lol.
thanks guys

 

[gsxr]

Had the AC been working fine until just recently, and they it quit? And this wasn't right after you did some repair work?

 

[Ertech]

I am getting real close .
I changed the drain from the evap a month ago maybe something there?

Alphasud40 are you sure pin 8 from ect sensor is on plug x1 ? when I look at the wire 8 from X2 its blue grey and I get a reading.​

I hoop you guys can follow me my English is not great .
toke the dome light out to test in car sensor . wire from sensor to pin 2 of X2 of PBU gets continuity < grey/yellow .>between the other pin brown/yellow ------heater core sensor brown/yellow nothing
. when I ground that wire at in car sensor ac works as it should. so I think my problem is between in car sensor brown/yellow------x1----B10/6<evap sensor>----plug in heater core sensor<B10/1>at the plug there is 2 brown/yellow one going to my problem the other to pin 10 .
the green marker line seems to be my problem . where is the evap sensor and x1 connertor< circle in red >.is X1 a ground?
blue marker were tested and fine .
thanks

 

[gsxr]

B10/6 Evaporator temperature sensor has a plug-in connector which appears to be labeled "x1". This is not a ground.

B10/6 is located on the left/driver side of the heater box, near/above the driver footwell heat vent area?

 

[Ertech]

SOLVED
thanks for all your help guys .
I tested the brown wire from evap to over head sensor its good
tested from evap to heater core the brown wire/jellow and no connectivity .
that wire must be cut somewhere. put a jumper between heater core and evap and all good now .