R129 air con problem

[Midnight Callers 300SL]

Hello, I have a very nice 1990 R129 300SL. I just had it regassed yesterday, all went well, holds gas etc. Only problem is the compressor only works if you put a 12v feed onto the blue and green stripped wire on the compressor itself. The only other time the compressor will run is if you have left the car off for some time and you start it up, for about two seconds the clutch will engage and then trip out.

I think I'm being a bit hopeful, but does anyone have a wiring diagram for a 300SL, or maybe able to point me in the right direction?

Is there a relay for the air con? If so where is the blessed thing?

The receiver/dryer switch is closed indicating pressure (as far as I'm aware) in the system.

I have checked the compressor fuse, that's fine.

 

[LostKiwi]

Malcolm is probably your man for this - hopefully he'll be along shortly. If not I'll ask him tonight. Is yours the digital display version or the two thumbwheel / two dials version?

 

[Midnight Callers 300SL]

Malcolm i probably your man for this - hopefully he'll be along shortly. If not I'll ask him tonight. Is yours the digital display version or the two thumbwheel / two dials version?

Mine is the twin thumb wheel with just air con and dehumidifier. I'm so close yet so far, it all charged up well and if I hold the 12v feed on, using a laser temperature tester the internal vent temp goes right down to 1.6C. which is impressive and rather chilly!

 

[LostKiwi]

Which lights are on on the switches?

 

[television]

I am short of time today, I will look at the circuit for this shortly

 

[Midnight Callers 300SL]

Which lights are on on the switches?

I've got very very tiny red lights, one on each button and they both illuminate when pressed.

 

[Midnight Callers 300SL]

I am short of time today, I will look at the circuit for this shortly

Thank you, your help will be most appreciated.

 

[LostKiwi]

I've got very very tiny red lights, one on each button and they both illuminate when pressed.

But which ones are lit when you're testing?

 

[Midnight Callers 300SL]

But which ones are lit when you're testing?

The one that I'm operating in particular is the snowflake air con button, I have also tried the dehumidifier but not so much.

 

[Ned]

I have attached a wiring diagram for the air con, I can't remember if it's for the digital display or for yours but the air con is controlled by the relays in the MAS unit which also controll the idle speed and fuel pumps.
I hope this helps.

Ned

 

[Midnight Callers 300SL]

I have attached a wiring diagram for the air con, I can't remember if it's for the digital display or for yours but the air con is controlled by the relays in the MAS unit which also controll the idle speed and fuel pumps.
I hope this helps.

Ned

Thanks for that Ned very helpful, I think that the diagram is for the climate control type but I'm going to guess that mine is still driven from the MAS unit which is worrying me. I'm now wondering why it's not keeping the clutch engaged for more that two seconds at a time from a cold start? Hummm...

I wonder if the RPM sensor on the compressor is telling the MAS that it's spinning? I'm going to guess that if that was faulty it might not throw the clutch in?

 

[LostKiwi]

Have you tried various combinations of switch positions?

 

[Midnight Callers 300SL]

Have you tried various combinations of switch positions?

Yes, I've tried them all. My gut feeling is the brain isn't getting all the right signals, or it's not interpreting them correctly.

 

[Ned]

The manual air con is still controlled by the MAS unit.
Have you tried opening up the 3 pin wiring connector on the air con compressor as one of the rpm sensor wires was broken on mine inside the plug. They do get brittle from heat over time.

 

[Midnight Callers 300SL]

The manual air con is still controlled by the MAS unit.
Have you tried opening up the 3 pin wiring connector on the air con compressor as one of the rpm sensor wires was broken on mine inside the plug. They do get brittle from heat over time.

I think that I will try that next, it all looks bit dry and powdery down there. When I check for ohms between the compressor terminals for the speed sensor I get a reading of 599 what ever that means?


Just found that I have good continuity from fuse box power out to MAS pin 21, then MAS power pin 22 to compressor, and both wires from MAS pins 5 & 6 to compressor speed sensor.

 

[Shadychris]

I think that I will try that next, it all looks bit dry and powdery down there. When I check for ohms between the compressor terminals for the speed sensor I get a reading of 599 what ever that means?


Just found that I have good continuity from fuse box power out to MAS pin 21, then MAS power pin 22 to compressor, and both wires from MAS pins 5 & 6 to compressor speed sensor.

I realise this is very late - but I am having exactly the same issue with my 95 SL 320. According to service history the AC pump had a refurb and AC system serviced about 15 months ago. I bought the car 12 months ago and with UK weather had never really used the AC until a couple of days ago and noticed it seemed to be blowing warm. I have exactly the same symptoms as you - every time the car is started and AC turned on the AC clutch pulls in for a couple of seconds and the aux fans spin up, but after a couple of seconds it just disengages. Supplying a 12 V feed manually to pin 3 (Blu\Yel) will keep the clutch engaged and the aux fans spinning and I get icy cold air.
Did you ever find the cause of the problem with yours?

 

[Blobcat]

I realise this is very late - but I am having exactly the same issue with my 95 SL 320. According to service history the AC pump had a refurb and AC system serviced about 15 months ago. I bought the car 12 months ago and with UK weather had never really used the AC until a couple of days ago and noticed it seemed to be blowing warm. I have exactly the same symptoms as you - every time the car is started and AC turned on the AC clutch pulls in for a couple of seconds and the aux fans spin up, but after a couple of seconds it just disengages. Supplying a 12 V feed manually to pin 3 (Blu\Yel) will keep the clutch engaged and the aux fans spinning and I get icy cold air.
Did you ever find the cause of the problem with yours?

If you don’t use the AC the seals dry pit and the gas leaks out. It’s recommended to use it all the time

 

[Shadychris]

If you don’t use the AC the seals dry pit and the gas leaks out. It’s recommended to use it all the time

Thanks, I have read that it is good to turn on the AC every now and again, but that's not really the cause of my problem here....

 

[mattkh]

The manual air con is still controlled by the MAS unit.

Q. What is the MAS unit.???

 

[alexanderfoti]

Your problem is likely with the RPM sensor on the compressor, or the wiring to/from it (devoid of any insulation?) The ecu sees no compressor rotation and disconnects the compressor for safety.