W124 air con wiring diagram?

[leylandp38]

I have a late '92 300TE-24 with climate control. I had a problem with steadily reducing airflow into the car which I have now found to be the aircon evaporator freezing solid, blocking all air flow after about an hour. I think this may be caused by a faulty relay as there is a constant 12v feed to the compressor irrespective of the EC setting in the car or the temperature, so the compressor runs constantly.

The wiring in the car (and the car overall) is very clean and shows no signs of any bodgy PO tampering. I did have a minor fault with the twin electric fans not actuating however this was a salty and corroded electical contact which I have fixed. I'm guessing something similar may be behind the compressor actuation problem.

With the compressor unplugged from the loom the evaporator does not ice up, and that's OK for winter but I'd like to trace the wiring fault. Does anyone either:
1. Know where the relay for the compressor would live, or
2. Have a wiring diagram for the aircon / climate control. Haynes, useless as ever, doesn't include it.

many thanks in advance,

 

[Bolide]

As I understand it the compressor should be cycled in & out by the high & low pressure switches on the system. They're under a 4-pin plug on a chrome pipe on the front rhs of the engine bay down by the rad over the chassis leg

You say there's a constant 12 volt feed to the compressor. But is the compressor kicking in and out? If the compressor's running all the time I wouldn't expect the system to last long. I'd get an aircon specialist who knows these cars well (anyone out there??) to take a look at it asap

Nick Froome
www.w124.co.uk

 

[leylandp38]

There is a plug with 3 wires on the compressor (red, yellow and green), which I have shown below. I have for the present unplugged it to stop the icing symptoms. The green wire to the right (closest to engine) is the 12V feed and I can verify that this causes the compressor to run and start to ice up within a short space of time. It seems to run constantly (my fuel consumption is noticeably better with it disconnected).
(click to enlarge)


I have also attached a pic of my high/low switches below - they look slightly different from your description (2 x 2 pin switches) but maybe you are looking at a later model or I am looking at the wrong thing. They make no difference, even when totally unplugged I get a 12V feed to the compressor via the green wire, which is why I think something isn't right


Oh, the system was flushed and recharged recently in order to help diagnose this problem, but i thought it would be fixed and as it takes an hour to show up the aircon guy didn't spot it. I didn't mention it as I thought it just needed a service. Of course now it just ices up better :-|

 

[television]

I am thinking that the fault is in the control unit, do you have air con or climate.

Malcolm

 

[leylandp38]

It has climate. So yes, I was wondering if it may have a fault, however the heating (with the aircon compressor disconnected) works perfectly, responding immediately to any change in the rotary controls. Also i expect the ECU would have a very low current through it, not enough to switch a compressor clutch, hence my suspicion that there must be a relay somewhere.

I am going to try to find time to trace the wiring back further, I was hoping someone could help me better understand what controls what in the system. I have verified that the high pressure switch does actuate when the pressure reaches a certain point, so that works OK, but it does not appear to cut the voltage to the compressor.

Does anyone know what else controls the compressor other than the input from the high pressure switch?

 

[television]

It has climate. So yes, I was wondering if it may have a fault, however the heating (with the aircon compressor disconnected) works perfectly, responding immediately to any change in the rotary controls. Also i expect the ECU would have a very low current through it, not enough to switch a compressor clutch, hence my suspicion that there must be a relay somewhere.

I am going to try to find time to trace the wiring back further, I was hoping someone could help me better understand what controls what in the system. I have verified that the high pressure switch does actuate when the pressure reaches a certain point, so that works OK, but it does not appear to cut the voltage to the compressor.

Does anyone know what else controls the compressor other than the input from the high pressure switch?

put up the first 6 Vin numbers, and I will look to see if there is anything recorded

Malcolm

 

[leylandp38]

My VIN is WDB1240912F210335

thanks in advance...

 

[Bob.R]

Could be a faulty evaporator temp sensor. This is a common fault on W210's.Though not much info on 124's
You should be able to get live data from the climate control panel. With a/c running hit left then right auto buttons, set both temps to 22C. Press Rest button for 5 seconds.
Display should then show 1 in left side and in car temp deg C in right side.
Press left auto button: 2 " " Outside temp in deg C
Keep pressing left auto until you get 5 in left display - this will show the live reading from the evaporator temp sensor, If the evap is frosted it should read below zero. If it reads higher you have found the problem! Change the sensor..
To return display to normal press rest again.
Full list of functions and wiring diagram - PM me if you need them.
Bob. R (tech@CarAircon.co.uk)

 

[leylandp38]

Hi, I think you are referring to a slightly later edition than mine. It is climate control but does not have any display. It has manual rotary knobs side to side with the three buttons in the centre.

I don't think it is the temp sensor in any case as pressing the "EC" button (for Economy Mode" should definitely disable the 12v feed to the compressor, but it doesn't. If that didn't work then I am also assured that unplugging the high pressure switch at the front of the car near the receiver/dryer should also disable the compressor. But it does not.

So... I have a constant 12v feed to the compressor clutch no matter what the controls do. Removing the fuse does stop the 12v but it also stops the entire climate control system.

I still think I have a rogue relay somewhere - surely the 12v feed to the clutch does not go direct from an ECU but via a relay?

In any case if you have a wiring diagram for a late 1991/mid 1992 24 valve W124 that would be great.

thanks in advance...

 

[leylandp38]

Coincidence and more information

Due to a failure of my fuel pump relay I discovered that it is also the climate control ECU

According to Mercedes, on the high spec 24v W124's (ie 300-24's and 320s) the same ECU controls aircon (climate control actually), fuel pump, electronic auto box and possibly some other engine management functions. See the thread on http://forums.mercedesclub.org.uk/showthread.php?t=19457&page=2 for details of how I found this out with some help from another member.

So I'm now trying to find the module photographed below to fix my problem, but number MAS 012 545 48 32 (the one shown is for a non-aircon car). If anyone has one going spare please contact me.