[fildyb]

Hi,

I have 1992 300D. Everything was working nice when suddenly AC cut out at stop light. Came on a minute later. The next day, started the car, AC worked, started driving and it died after a few minutes. Now it works for a minute then dies, sometimes compressor comes on for a few seconds.
I performed the "cut off" diagnostic. It's ok.
What I have noticed is there is no ground signal from pushbutton unit (CCU) on the pigtail to high pressure switch. I tested 3 different pushbutton units, and the symptoms are the same.
I replaced my evaporator sensor with one from pick-n-pull, no difference.
Any thoughts? What's next? The charge is good, everything is nice and cold when pigtails are jumped.
Aux fan is good with its relays. Fuses look good.

Thanks,
FD

 

[Floobydust]

I always thought that the CCU produced +12 to call for A/C which is routed via the high/low pressure switch to the KLIMA relay pin 10 to activate the compressor.

Given what you have already done, I would:

Read the CCU diagnostic codes on Pin 7 of the Diag connector.​

Ohm out the connection between the high/low pressure switch and pin 2 of the CCU.​

Suspect the pick-n-pull high/low pressure switch.​

Suspect the KLIMA relay.​

Suspect tach signal from the electronic diesel system.​

Good luck,

- FD

 

[tuttebenne]

What condition is your belt tensioner in?

 

[fildyb]

What condition is your belt tensioner in?

Replaced last year. Belt doesn't slip.
I would like to add that when AC cuts off, the aux fan goes on HIGH. That makes me wonder if it's the cheap coolant temp sensor that is faulty, since it feeds info to CCU to turn on the fans on high.

 

[sbaert]

Aux fan going on high speed when A/C was running is a classic sign of a high pressure condition. Could be bad/clogged receiver/dryer unit, clogged expansion valve, etc, etc, etc.

 

[Floobydust]

^Yes. The pressure switch in line with the CCU signal to the KLIMA relay is cuts out for both over pressure and under pressure. This would explain why the system runs when this switch is bypassed.

BTW, I did find some documentation that the CCU does provide +12 to pin 10 of the KLIMA relay to call for cooling.

 

[fildyb]

It's not high pressure situation per se, high aux fan is high temperature situation, as the input for it goes from coolant temperature sensor to CCU, then bypasses preresistor and goes straight to fans. Of course high pressure switch will be closed by then and first fans would run on low setting. CCU also turns off AC. I checked the sensor, it read way too low (1.5ohm instead of 500ohm). Replaced it and everything is fine. The lack of ground on high pressure switch was indicative of lack of "ON" signal from CCU to compressor, so the problem was at CCU or at one of its inputs. I replaced evaporator sensor but initially forgot about the coolant one. Thanks

 

[Floobydust]

Glad all is working now. Interestingly, the bad (essentially shorted) temp sensor should have shown up as error code 12 on Pin 7 of the Diagnostic connector.