Possible causes of AC compressor not engaging
 

The AC compressor is not engaging on my 85TD. I've ignored it for a couple of years since I haven't needed AC, but I'm heading across the midwest next week so I'm considering fixing it. I know a low charge of freon will stop engagement. Are there other possible causes I can check before checking the charge. It worked when I first got the car. If my memory serves me right, I switched the pushbutton unit and it was after that that the AC stopped working. Can a bad pushbutton unit stop compressor engagement. Is there a way of testing if a low charge is causing the problem by jumping a sensor somewhere? Fuse seems fine in the fusebox and blower is working.
I haven't spent much time working on AC systems so I am fairly ignorant on the topic.

Low Pressure cut out.....
 

on your right / passenger side fender....front, below the Water Tank....there is a receiver / drier with a low pressure cutout switch......if I remember correctly power to the Compressor Clutch passes thru this switch....little round thingy......jump the 2 terminals.....and the AC clutch should pick up.....if the Aux Fan runs....it's the other one......

SB

yes jump the pressure switch but do it with the engine off and the key on. you will hear the coil on the compressor click. no sense turning the compressor when it is empty, thats what the switch prevents.

Here is what I did to troubleshoot my 300SD.I am sorry that I do not have my wiring diagrams with me so I cannot give you specific wires/sockets. You will have to chase themk on your own or use a wiring diagram.

When I jumped the switch at ther receiver/dryer nothing happened.

I unplugged the connector from the compressor and tested for continuity back to the switch then I went from the switch back to the fusebox. In the fusebox is a black relay marked "KLIMA" I pulled the relay and checked each socket for continuty back to the reciever/dryer switch. I found the line that goes from the KLIMA back to the compressor and marked it. I then turned the key to "on" and using the voltmeter found the switched 12V line that goes into the KLIMA. The KLIMA takes this 12V feed and connects it to the compressor when it recieved a signal on a third wire.

Anyway, I jumped the 12V feed and te compressor lines and the compressor clutch worked.

You can try doing this and once you get the compressor spinning, you can check the refrigerant charge with the right gauges.

I would not run the compressor this way for any length of time as it will not turn off if the evaporator freezes. It will still turn off if the switch at the receiver/dryer cuts it off.

In my case it turned out that the KLIMA was not getting the signal to start the compressor. I traced the signal wire back to the connector on the CCU and when I opend the CCU I found that that connector needed to be resoldered.

Do a search on "fixing CCU" or "resolder CCU" for more details.

Good luck!

The 1985 300D, CD, and TD have a "Klima" relay on the left inner fender, just ahead of the brake booster. This relay looks at a number of inputs and "decides" whether the compressor's clutch gets permission to engage or not. In addition to a missing input, the relay itself can fail. Without it, the compressor will not work.

The inputs are:
(1) a full throttle microswitch on the valve cover (NOT the kickdown switch)
(2) a low-pressure switch in the a/c line
(3) temperature control from the pushbutton unit
(4) an anti-icing switch in the evaporator.

The Klima relay and all of its inputs must function correctly or the compressor will not engage. Source: "Model Year 1985, Passenger Cars, USA Version, Introduction into service" by Mercedes-Benz (October 1984).

Many people simply wire around the Klima relay when it fails.

Historical note: "Klima" is a company name; it's like calling a starter solenoid a "Bendix."

Jeremy

On my 85 300D, when the tachometer stopped working, my A/C compressor wouldn't engage. The tach pulse is another input to the Klima. If your tach works, then you don't need to worry about this. If it doesn't look here:
http://www.peachparts.com/shopforum/diesel-discussion/138818-1985-300-d-tach-problem.html

Cheers,
Brian

call for ac with the button unit on the dash, jump the low pressure switch on the drier (round bit sticking up straight is the one). does the compressor come on ? if yes I think thats good to an extent, low freon, leak!, big? small ? how to test for it? and then how to recharge and what to recharge with ? thats all another thread. if it doesn't come on then you have to look at some other stuff, 85 specific the klima relay, the egr computer, the overvoltage protection relay. Or the push button unit in the dash. make sure the compressor isnt locked up. seems like on a lot of 85's, water gets in from somewhere and gets on this egr computer/computer connector and messes that all up. Its in the secret door on the passenger footwell, could be the connector or the computer or both, good indicator of egr computer functionality would be whether or not you have a working tachometer. lots of info on how to deal with a leaky ac system on the forum as well as info on klima realys & 85 funkyness. good luck

Ok so the 1985 system is more like the 1986 system on the 300SDL than it is the 1981 system on the 300SD If youhave the big Klima relay pull it off it's connector and jump sockets 5 and 7 that should kick on the compressor.

You can pull open the klima cover and look inside. On my SDL there was a burned out piece of printed circuit preventing the compressor wanted signal from going from the CCU to the kilma...

More data. Need more help.

I jumped the low pressure cutoff switch. This did not cause the compressor to engage even though the push button was in the AC position. I assume this means that lack of pressure is NOT the cause of inoperative AC

I then jumped sockets 5 and 7 on the Klima relay base. Compressor engaged but no cold air ensued.

Any further hints on diagnosis? Does this point to a bad push button unit failing to call for cold air?

Edit: More info.

Switched in a different push button unit (this is the one that was in there the last time it worked). AC still inoperative.
Jumped 5 and 7 on the Klima with the different pushbutton unit. Still no cold air.

Any good ideas out there?

I looked in the hidden door on the passenger footwell. I see the EGR computer and a large black plastic box. Is that plastic box the CCU? It looks like it might be a challenge to get it out of the door.

The only thing the FSM shows behind that door is the EGR computer. Let me go look in my own '85 . . . [looking] . . . the only things behind the secret door in my '85 are the EGR computer (black plastic) and the idle speed selector (a round knob-like thing with numbered positions).

Jeremy

the large black box is the egr computer. it will come out of the door. try this thread if you havent already read through it

http://www.peachparts.com/shopforum/diesel-discussion/172032-klima-basics-missing-forum.html?highlight=klima+jump

Does your tach work ?

Perhaps this is obvious, but "klima" is also German for "climate."

Yes, tach is working.
Thanks for the link, read the thread. I'll probably do some more diagnostics tomorrow.

Can anyone confirm that by joining the two wires going to the low pressure cut off switch on the receiver dryer (switch with spade connections pointing straight up) and NOT having the compressor engage means that the system probably has enough pressure and the problem lies elsewhere??

Also, can anyone confirm that the fact that when the compressor engages and runs, but I don't get cold air, the system is not getting the signal to produce cold air either because of a KLIMA problem or a problem in the push button unit?? Will jumping some pins on the KLIMA send a signal to produce cold air?

That suggests that you have a problem elsewhere. But it tells you nothing about the low pressure cut-off switch and nothing about the refrigerant level. If you want to confirm that the low pressure cut-off switch is in fact closed, check for continuity between the terminals.

If the compressor engaged and no cool air resulted, the system has insufficient refrigerant. Which is the most common reason for old A/C not to work.