Diagnosing A/C Problems - PeachParts Mercedes-Benz Forum

jkoebel · #1 01-23-2008, 10:10 AM

I've been having some intermittent A/C problems lately, my compressor won't kick in. I can fiddle with the knob turning it to "MAX", the blower to "3", manual climate knobs to full cold, and it'll just blow outside-temperature humid air in.

It's so bad it makes the car completely un-drivable at night because it fogs the windows up instantly.

When it does engage (rarely), it cools just fine as if there were no problems.

Where should I go for diagnostics? I've cleaned and reseated the electrical contacts at the compressor itself, with no change noted. How can I get behind the face of the CCU and the panel with the buttons to clean/re-seat/check stuff there?

**manual CCU sorry**

ez31011 · #2 01-23-2008, 11:24 AM

I don't have answers to all your questions, however on my 85 300d, i was having a similar problem and it was just low refrigerant level. And that's a pretty cheap fix.

tobybul · #3 01-23-2008, 11:33 AM

As said above, it could be low on refrigerant, electrical connection at compressor, or CCU not working right.

If refrigerant is fully charged, start with checking that the electrical connector at the compressor is tight and good. Next, try making sure the CCU terminal connectors are clean. If that does not do it, try the resoldering trick of the CCU board.

JimmyL · #4 01-23-2008, 11:36 AM

Most likely cause as stated above is low freon. You could jumper across the pressure switch to see if that causes it to come on every time..... {in lieu of pulling out the AC gauges}

tangofox007 · #5 01-23-2008, 11:38 AM

Quote:

Originally Posted by jkoebel

When it does engage (rarely), it cools just fine as if there were no problems.

That would suggest that low refrigerant isn't the problem.

E150GT · #6 01-23-2008, 11:38 AM

did you check the klima relay?

JimmyL · #7 01-23-2008, 11:41 AM

Quote:

Originally Posted by E150GT

did you check the klima relay?

That would be hard to find on an '83 240......

jkoebel · #8 01-23-2008, 11:43 AM

9 times out of 10, it works perfectly when its daylight out.
9 times out of 10, it won't work at all when it's night.

Manual CCU, there is no board as far as I know of, right? It's just a blower knob, a wheel for the compressor, and two valves attached to knobs.

jkoebel · #9 01-23-2008, 11:44 AM

Quote:

Originally Posted by JimmyL

Most likely cause as stated above is low freon. You could jumper across the pressure switch to see if that causes it to come on every time..... {in lieu of pulling out the AC gauges}

Pressure switch....what/where is that? Is that the temperature switch thing that's on the dryer, controlling the aux fan?

I don't have a set of A/C gauges, just one that came with a little can of R-134a that hooks in to the low-pressure side. I haven't added anything from the can, though.

JimmyL · #10 01-23-2008, 11:49 AM

Quote:

Originally Posted by jkoebel

Pressure switch....what/where is that? Is that the temperature switch thing that's on the dryer, controlling the aux fan?

I don't have a set of A/C gauges, just one that came with a little can of R-134a that hooks in to the low-pressure side. I haven't added anything from the can, though.

There should be 2 switches on the drier. One is longer and has wires and spade connectors going to the wiring to aux fan. Another switch screws in to the rec/drier with 2 tabs sticking up. That is pressure switch and can be jumped.
You might be teetering right on the edge of correct freon pressure, which could cause it to work intermittently. Since it is winter it will be easier to achieve a cool temp from a marginal system.
As stated, might not be the problem, but it is an easy check...

rickhou · #11 01-23-2008, 11:58 AM

I had a similar problem. Just bad connections at pressure switch.I took pliers and squeezed the connectors . No problems since.

jkoebel · #12 01-23-2008, 12:07 PM

Does one of them have vacuum lines coming from it? "Electropnuematic switch" was what its called in EPCnet I think

There was something that was hooked up between the engine block and the EGR which had a vacuum line and some electrical lines, right by the A/C dryer. I disconnected the vacuum line to that switch when disabling EGR.

AMH · #13 01-23-2008, 12:34 PM

Pressure switch....what/where is that? Is that the temperature switch thing that's on the dryer, controlling the aux fan?

Ok, things are getting scary. Another user trying to fix their ac with the Auto Zone fix all can. It's your car and your eyesight.
The pressure switch is located on the receiver/drier. It will have two wires with spade connectors and should be the larger of the two switches on the drier. Disconnect these wires and jump them together. The compressor should engage.

I don't have a set of A/C gauges, just one that came with a little can of R-134a that hooks in to the low-pressure side. I haven't added anything from the can, though.

Throw that can of **** away and get yourself a good set of A/C gagues. Now for the next question. Does your car still have R-12? If so read the first sentence.
I agree with the posting regarding operation between day and night. It could be low on freon. The only way to tell is to check suction and return pressures with the compressor engaged.
Tell us what freon is still in the system and more answers will follow. It's a Type I system, one of the best produced.

Alan