1992 W124 300D 2.5 Turbo – AC compressor speed sensor - PeachParts Mercedes-Benz Forum

SW · #1 03-31-2008, 01:36 PM

Background info: My AC compressor only engages when the engine is cold. It will work for about 5 to 10 minutes and then the compressor clutch disengages as the engine starts to warm up.

Testing/diagnosis: I followed the procedure in the MB climate control manual and found that the ac compressor speed sensor or wiring may be faulty. I get zero voltage (measuring ac voltage, terminals 5 and 7 jumped) when measuring across pins 9 and 11 of the KLIMA socket. The measured resistance is around 900 ohms. The spec should be 530-650 ohms.

Questions: How does one replace the speed sensor? Does the compressor have to come out? I tried searching the topic but did not find any posts on the topic. Any thoughts and advice?

mbdoc · #2 03-31-2008, 02:12 PM

Number 1 question is whether or not you have the command signal(ground) to pin # 10 of the klima relay connector, when the a/c compressor isn't working.

SW · #3 03-31-2008, 02:29 PM

Yes. I have ground.

SW · #4 03-31-2008, 08:16 PM

Anybody else want to chime in? I see from the “Does You’re A/C Work Or Not?” poll that 41% of you have working A/C’s but had to repair them to proper working order.

Help a brother out.

jcyuhn · #5 04-01-2008, 10:11 AM

You have to remove the Klima relay and jumper the compressor on to test the voltage generated by the speed sensor. It's expected to have 0v when the compressor is not turning.

There are a bunch of causes of premature compressor shutoff in the 124 cars. I don't recall seeing failed speed sensor as very common. Have you checked the air gap on the compressor clutch? It increases over time as the clutch material wears. The specification is 0.50mm, plus or minus a tolerance that I cannot recall at the moment. Suffice it to say that once the gap hits 0.80mm or so compressor engagement becomes a dicey proposition.

Sometimes simply cleaning the compressor clutch air gap to remove grease & grime is sufficient to restore normal operation. Use brake cleaner or just simple green and water.

Have you used the search function? There are a ton of old threads on troubleshooting compressor cutout. Grab your beverage of choice and settle down for an evening of reading...

- JimY

SW · #6 04-01-2008, 01:41 PM

Jcyuhn, thanks for the response. Yes, I used the search function. I always search before I ask. That’s probably why I only have 52X posts since joining in August of 2000.

Anyway, to restate and elaborate what I mentioned in my 1st post, I followed the “Checking Compressor Cutoff and Remedial Measures” procedure in the MB climate control manual for the w124 models.

KLIMA removed, engine running, temp wheel at “MIN”, middle button pushed in, blah, blah…..

Step 1: Volt meter at terminals 5 and 1 on the KLIMA socket = ground (12V)
Step 2: Volt meter at terminals 5 and 10 = 12V
Step 3: Bridged terminals 5 and 7 and the compressor clutch engages
Step 4: Check compressor speed sensor. Bridge 5 and 7 and measure AC voltage across terminals 9 and 11. I get ZERO voltage. I checked the resistance between 9 and 11. The result was 900 Ohms which is outside of the 530-650 Ohms spec.
Step 5: Check speed sensor. Bridge 5 and 7 and measure AC voltage across terminals 1 and 2. I get 3.8 to 4.2 volts.

I also found some of your posts in the Tech Section about the clutch. I sprayed brake parts cleaner into the clutch area and it helped a little, but the compressor still cuts out after 5 to 10 minutes. I just want to know if the compressor speed sensor can be replaced while the compressor is in the car. If not, I’ll probably do the “ice cube relay” bypass and carry a spare serpentine belt with me.

jcyuhn · #7 04-01-2008, 04:04 PM

Hey, you have been reading. Good job!

It does seem you a problem with either the speed sensor or perhaps the wiring harness between compressor and Klima.

Here is a question. Why does the a/c operate for 5 or 10 minutes? There must be a speed signal for the compressor to engage for any detectable period of time. It seems an odd failure to have the speed sensor consistently quit after a period of time. Have you tested the speed sensor immediately after verifying normal a/c operation with the Klima?

To answer your original question, the speed sensor cannot be replaced with the compressor in the car. Replacement requires removing and partially disassembling the compressor. I don't have the procedure at hand, but don't recall that it appeared overly complex. But it does require discharging the a/c refrigerant.

Good luck.

- JimY

SW · #8 04-01-2008, 08:21 PM

Quote:

Originally Posted by jcyuhn

Here is a question. Why does the a/c operate for 5 or 10 minutes? There must be a speed signal for the compressor to engage for any detectable period of time. It seems an odd failure to have the speed sensor consistently quit after a period of time. Have you tested the speed sensor immediately after verifying normal a/c operation with the Klima?

Good question/thought. I need to investigate this a little more. Thanks for the input on the sensor. I was afraid of having to remove the compressor. On the brigth side, it does have the factory 134a system, so a recharge won't be too costly. Thanks again bud. I'll report back on what I find.

It's scotch and soda time

oldiesel · #9 04-02-2008, 10:31 PM

i had intermittent compressor drop on mine ,didnt seem to be temp related,but on occasion the compressor would just quit.All i had to do was turn the key off and back on and it would engage again.I replaced the serp belt and it has never happened since,maybe the belt slipped enough so the compressor speed and the engine speed did not agree so it shut down the compressor,worth a try,by the way the belt did not look bad but the tensioner was jumping a littlee so i put a new belt,had no thought that it would cure the a/c problem. My $.02 Don

compress ignite · #10 04-09-2008, 01:36 AM

Check #40-44
(Man This guy Can Post!)

saumil saumil is offline
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Join Date: Aug 2001
Location: Houston, Texas
Posts: 373
If this car has a Klima relay, there could be any number of possibilities. If your relay is the same as my 87, 260E then here are steps to test the relay:

1. Turn Car Off
2. Pull Klima Relay Out
3. Put your multimeter on DC Volts setting
4. Connect the ground terminal of the meter (black color usually) to the battery's negative terminal
5. Connect the positive terminal of the meter to pin number 5 in the socket (Hopefully, you know which pin is 5)
6. You should get close to 12V, if not, the fuse is blown (#7 I think). Replace fuse. Put the Klima relay back in and check a/c again.
7. If the fuse is o.k. and the compressor clutch still does not run, then lets move on
8. Remove the Klima relay. Now connect the negative terminal of the meter to pin number 10 in the socket. (Hopefully you know which is pin 10). Put the positive terminal of the meter on the positive terminal of the battery.
9. Turn on the car and press one of the a/c switches on the push button unit.
10. You should get close to 12V when the a/c switch is on.
11. If you dont get close to 12V then there are one of two possibilities:
a. the push button unit is not sending the correct control signal
b. the pressure switch is not closed
For now, I am ignoring the trivial possibilities that wires or connectors may be broken. We can dive into these once we have some answers to the basic tests.
12. Turn the car off.
13. If you dont get the 12V, disconnect the two cables from the pressure sensor (should be at the drier), and jumper them.
14. Repeat steps 9 and 10.
15. If you still dont get the 12V, the push-button unit is not sending the signal.
16. Turn car off, dont keep the car running for a long time with the pressure switch cables jumpered.
17. If at step 14, you did get 12V, then the pressure switch is not closed
18. If the pressure switch is not closed there could be three possibilities:
a. refrigerant pressure is too low (lower than 2 bar)
b. refrigerant pressure is too high (higher than 30 bar)
c. the pressure switch is bad
you will need pressure gauges to seperate these possibilities.
16. If at step 11, if you did get 12V, then the push-button unit is sending the control signal and the pressure sensor is o.k. and we will move on.
17. If push-button unit is sending a signal and the pressure sensor is o.k. but the compressor clutch will still not run, then need to check the compressor clutch coil resistance.
18. Connect the positive terminal of the meter to pin number 7 in the socket and negative terminal of the meter to negative terminal of the battery.
19. Change the setting of the meter from volts to ohms.
20. You should get a low resistance about a few ohms.
21. If you don’t get a low resistance, need one last test.
22. Get to the compressor and disconnect the connector for the compressor clutch coil (there is a speed sensor cable that connects to the compressor connector also, you have to trace the clutch wire to the compressor connector to pick the right pin on the connector).
23. Put one of meter’s terminals on battery ground and the other terminal on the compressor’s clutch coil pin (polarity does not matter).
24. You should get a low resistance, about a few ohms. If you do get the low resistance at the compressor then compressor clutch coil is o.k.
25. If you get low resistance in step 23 but do not get a low resistance in steps 18-19, then the cable from the Klima relay to the compressor is bad, replace it.
25. If you don’t get a low resistance in step 23, then the compressor coil is open, normally this would require a new compressor.
26. If the compressor clutch coil is o.k. and the cable from the Klima relay to the compressor is o.k. then let us move on.
27. At this point, if the fuse is o.k., and the compressor coil’s resistance and cable are o.k., then jumper pin number 5 and 7 in the socket.
28. Turn on the car and your compressor should come on.
29. Turn car off
29. If the compressor does not come on, there are two possibilities:
a. the fuse is blown
b. the compressor has an internal mechanical problem, is likely seized.
30. No matter whether it is 29a (shorted coil) or 29b (seized), compressor is bad, replace it.
31. If the compressor comes on at step 28, but does not come on when the Klima relay is put back in and the push button switches are on a/c, then the Klima relay is not producing the 12V signal at its pin 7 to drive the compressor.
32. If Klima relay is not driving the compressor at step 31, then there are five possibilities:
a. the temperature sensor is bad
b. faulty throttle cutout microswitch (only for diesel automatic)
c. bad compressor speed sensor
d. bad engine speed sensor
e. bad Klima relay
33. Put the meter back on voltmeter setting. Do this test with engine cooler than 105 deg-C, better when just cold.
34. Put the positive terminal of the meter on the positive terminal of the battery. Put the negative terminal of the meter in pin 12 of the socket.
35. If you get about 12V, then the temperature sensor is bad. I am not sure about the physical location of this sensor but on my 87 260E, it is a 3 pin sensor on the engine I think third from last from the windshield side. You can ask your parts vendor about it.
36. If the temperature sensor is bad, then disconnect the sensor cables, put the Klima relay back, and try you’re a/c. If this was the only bad component, your compressor should start working. But again, don’t keep running like this, replace the sensor as soon as you can. If disconnecting the temp sensor cable does not work, there is still another problem. Remove the Klima relay and move on.
37. Put the positive terminal of the meter on the positive terminal of the battery. Put the negative terminal of the meter in pin 4 of the socket.
38. If you get about 12V, then the throttle cut-off switch is bad. This switch is supposed to be open and only close when the throttle is wide open. I am not sure about the physical location of this switch but you can ask your parts vendor about it. Once again, if you can find this part, you can disconnect the wire, put the Klima relay back and check a/c. If it still does not come on, then there is some other problem and move on.
39. Change the meter setting to measure resistance.
40. Put the positive terminal of the meter on pin 9 of the socket. Put the negative terminal of the meter in pin 11 of the socket.
41. You should get about 350 – 450 ohms. If you don’t get the low resistance but get very high resistance, either the compressor speed sensor is open circuit or the wire is broken somewhere. Do the same test as close as possible to the speed sensor (i.e. on the compressor connector) to rule out a broken wire. Replace sensor if it is bad.
41. If the resistance is o.k., then, jumper pins 5 and 7 on the socket.
42. Change the meter setting to ac volts.
43. Turn on the car.
42. You should get about 0.3V ac at idle of about 750 rpm. If you don’t get the ac voltage, and the resistance was kind of o.k., replace the sensor.
43. Turn off the car.
43. To check engine speed signal, put the meter setting to ac volts
44. Connect one terminal of the meter to pin 1 of the socket and the other to pin 2 of the socket.
45. Turn the car on.
46. For diesel: At idle of about 750 rpm, you should get a voltage greater than about 4V ac. And the voltage should increase with increase in RPM. For gasoline: at idle the voltage should be about 9V.
47. Stop the car
48. If you don’t get the voltage, switch the meter to resistance setting and readout the resistance between pins 1 and 2 of the socket.
49. You should get about 2 Kohms. If you get a very high resistance, check the resistance closer to the sensor to rule out a cut in the cable. Otherwise replace the sensor. Ask the parts person about its location.
50. If in step 15, you determine that the push-button unit is not sending the control signal, replace the push-button unit. This has happened to me once, the electronics driver inside the push-button unit is blown and it will not send the control signal to the Klima relay. What I did is bypassed the electronics and just used the mechanical switches to send the control signals. The downside is that there is no more auto modes, so when I feel like it is too cold, I would press the economy switch and I designated that switch to send a signal to cut-off the compressor. So my brain does what the box would otherwise do, have still not fixed the electronics, just a big headache to test this unit once it is out of the car. If you just bought a new one, you can ask for a replacement. Most likely the problem you are having is not in that unit.

This is awfully long and I may have missed some possibility, but you will have enough info to test this problem in a systematic manner and when you come to a point where you think something is missing, let us know, some one will help.

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Saumil S. Patel
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SW · #11 06-20-2008, 11:45 PM

Well, I gave up and took it to an indy (1st time i've taken any of my cars to a mechanic in about 8 years!). He replaced some sort of switch/relay related to the aux fans. It cools great now. Plus, he replaced my broken accelerator cable and the car actually behaves like a turbo diesel. Best $255 i ever spent.

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