240D fickle A/C compressor
 

Still sorting out the small bugs in my new-to-me 240D. I love the manual climate control (verses the push button debacle in my 300D) but it has a peculiar quirk. Most times when I move the blue temp wheel to the cool area and turn on the fan, the fan comes on in fine order but the compressor does nothing and blows just ambient air.....however a while later driving down the road I may try again and feel the compressor kick in and the cooling begins. I thought at first that perhaps the compressor would not engage because of the low pressure switch but surely it wouldn't come and go as it's doing. The only thing I could find by searching related to this problem was that the ignition key sometimes affects the climate control. Any help on this intermittent problem?

Ok, two points:
I have seen bad ignition switches give some really wierd climate control results. One car I had , the fan would sometimes not work. Then I would turn the key back, just a touch and the fan would run fine. I replaced the switch and all was well with the car.
On my '83 300D, it has the R4 compressor. The connections on it are not always great. Just a pair of 1/4 in female connectors in a common plastic housing. If that is what you have, try cleaning the connectors and maybe pinching the metal part a bit to get a solid electrical connection.

OK....not the ignition switch and I giggled the wires going to the compressor to see if the connection was bad. Compressor still didn't kick in. I thought I would next try to jump the connecting wires to the low-pressure switch for a half-second to see if it will turn on....but that seems unnecessary because I already know that the compressor kicks in sporadically. If the system was low on Freon, the compressors would NEVER come on instead of hit and miss...correct?
Thanks

The compressor will "cycle" more when freon is low. When the froen level drops below a certain pressure point, I do not know off hand what that is, the compressor wil stop kicking in.

Use a volt meter and check for 12v at the compressor connections. Do this with the engine off. Put the ignition switch in position 3, and turn on the A/C. If you are getting 12 volts, and the clutch is not engaging, could be a bad clutch.

Good thought....and if there is NOT 12 volts at the compressor with the ignition switch in position 3 and the A/C set to come on, then this would mean the problem is BEFORE the compressor? Thanks!

I may be wrong on this, but it is my understanding that the clutch is not supposed to cycle on this system. If this is true, I would suspect a restriction, such as the expansion valve or accumulator. Or, as you mentioned, a low charge of refrigerant.

You're saying on a 240D the compressor is not supposed to cycle? Runs all the time?

From what I understand, they generally do not cycle, but they will engage and disengage based on the opening and closing of the pressure switch.

Yes, if you are not getting 12 volts at the compressor, then the problem lies somewhere up the line. Also check your grounding cable. Poor grounding will cause all sorts of issues.

Check the A/C relay. You can switch it with the fan relay.

Could you please expand on the suggestion? Where is the A/C relay and what about the switch?
Thanks

The A/C relay is on top of the driver side fender weld...remove the plactic cover, about 4"X5" ...You will see two silver looking relays, the one in front is for the A/C and the other one is for the fan. both are the same....switch them around and see if that helps.

The compressors are supposed to cycle to keep the evaporator from freezing up. Don't give up on those connections at the compressor just yet. One of mine came loose and I replaced it, worked fine. A few months later the compressor decided not to turn on one day. I got under there and jiggled the connections, still no engagement. Luckily I didn't give up and I went ahead and replaced the other connection with a fresh one connected to fresh copper wire. Fixed it and haven't had any problems with it since.

Oh yeah and replacing that $0.02 fuse would eliminate another possibility, as I've also had that happen with my new system.

Great suggestions. Thanks for all the help. I won't know until I get back to work on the car this weekend, but hope springs eternal. Hope I can repay with my own advise one day on another subject.

I'm learning so clarify......
 

The compressor "cycles" to maintain a pressure, correct? So if the compressor is maintaining a fairly steady pressure at idle, shouldn't the clutch rarely cycle?

And I thought the expansion valve has a temp sensor to prevent icing when the temp is too low.

It is my understanding that is kind of a balance between the two.