A/C Compressor Doesn't Always Click On
 

So, here's the story:

Yesterday I left work and turned on the A/C. Everything was working fine. I stopped at the store on the way home, and when I started the car, I realized that the A/C compressor would not click on. I messed with the switch (the vertical rolling type), but no luck. After mumbling under my breath, I drove off with the windows down. A few minutes later, I tried it again, and it worked! Excellent.

Then today, I left work, turned on the A/C, and it clicked right on. I drove for a few minutes, stopped, and turned off the car. When I started the car back up, no A/C. So I turned everything off. Then a few minutes later I tried it again and the A/C came right on. :confused:

I don't think it's the switch, because even if I jiggle it, nothing happens. Is there a relay that could be bad? I also jiggled the electrical connection on the top driver side of the compressor, but that didn't make a difference either.

What is going on? Please help! It's hot in Sacramento!

MIKE

If that was my car I would be saying Deja Vue all over again.. my 1981 wagon only considered my pushing on those buttons as vaguely mild suggestions..which were routinely either ignored or followed many minutes later.. with no apparent reason for the later action.
It could be anything... but it is likely in the actual dash unit... The diagnostics in the FSM run to many many pages....and convinced me that I needed a car without that ' control' system...
Good Luck,
Greg

The symptoms are consistent with a bad evaporator temp regulator, although there are other possible causes. The evaporator temp regulator is in line with the compressor relay; it is supposed to open at about 36*F to prevent the evaporator from freezing. A bad regulator can open at a much higher temp, however. You might try jumpering the regulator and seeing if that makes any difference.

Interesting. Where are these relays located? Under the hood or under the dash? Also, does anybody out there have a part number that I could search for?

Thanks!

http://catalog.peachparts.com/ShopByVehicle.epc?q=1982-Mercedes--Benz---40d-Climate--Control&yearid=1982%40%401982&makeid=63%40%40MERCEDES+BENZ%40%40X&modelid=6142%3AMBC%7C1489%3AED%7C1 0000129%40%40240D&catid=242213%40%40Climate+Control&subcatid=242253@@A%2FC+Evap+Temp+Sensor&mode=PA

It's under the dash. The probe is inserted into the evaporator case.

I'm sorry. I don't know much about the A/C system, so I have lots of questions.

- How would I go about jumping it to see if that is actually the problem.
- How difficult is it to replace?

Thanks!

Same problem on our '82 300TD. I replaced the modular plug to the compressor, but it didn't solve the problem.

Sometimes it takes 10-15 min of driving before the compressor kicks on. I was considering replacing the CCU in the dash, but will be following this thread closely!

I found that I was having this problem too. I thought it was a bad relay but after some troubleshooting I found that my relay (Hella brand, not OE) was not making firm contact in its seat and would back itself out and not work 100%. I zip tied it down and now it works fine.

Disconnect the wires at the regulator, then jumper the wires together.

It's not difficult.

I have a question, too. Does the compressor ever disengage while the a/c is operating? For example, the a/c is operating normally for 10-15 minutes and then just starts blowing warm air? Or does the compressor only fail to engage after the a/c has been turned off?

A negative response to the first question would make the regulator less of a suspect.

That switch (temp seleector) is unrelated to the operation of the compressor. The compressor should engage in all modes except EC and off, regardless of the temp selected.

This former Fair Oaks resident extends his sympathy and understanding!!!

It does have a pressure switch that disengages the compressor if freon pressures are too low. I added a Hi/Low switch in case the pressures get too high as well.

The temp regulator at the evaporator need to checked, as does the relay as suggested previously.

mjquillici, I didn't see in your post if the car blew warm air, or nothing worked. If the compresser clutch is not engaging, make sure the electrical plug is pushed all the way in, at the compresser. I, and others here have had the plug come loose or off. Easy, quick, and free check. Good luck.

---------------------
1984 MB 300SD Turbo

I am sure that is not really what you wanted to say.... once you think about it...

it is also possible that the clutch on the compressor is weak or its gap is set too wide. You need to unplug it and throw a test light or meter on the wiring to see if it is getting voltage. If so, then you know it is the clutch which is at fault.

I had my AC fixed at German Motors in Lakewood last year. When completed it had this symptom and they claimed it was a common problem on older compressors and the gap on the clutch is too wide. They said they could adjust it but it was not easy and they could not guarantee they could get the adjustment accurate enough to solve the problem. So I said, let it go. I found that if I revved the motor at idle, it usually engaged. I can hear it engage from the driver's seat.

As I understand the operation of the ACC, the temp is regulated by varying the flow of coolant through the heater core, not by cycling the compressor off and on. It is my belief that the compressor should run continuously except in the EC and OFF modes, where it does not run at all. In other words, if the normal, bi-level or defrost modes are selected, the compressor should be engaged regardless of the temp selection.