Hi Folks.
The car in question is a ~1992 500 SEC belonging to my dad. Until recently the A/C system has been faultless but has now failed.
Apparently the symptoms are that the lights on the switch come on but the compressor does not start to turn. He says that the coolant is fine and there are no obvious faults.
Does anyone know how the compressor clutch operates? Presumably it's an electromagnet pulling in on a clutch surface or something similair? The main pulley turns fine with the belt but the clutch does not engage.
Is there any kind of internal diagnostic system which may be sensing a fault and not turning the compressor on - or is it the clutch system at fault.
If anyone knows how the clutch operates we might be able to do some testing and track down the fault.
Thanks in advance for any help.
Cheers
Rikll

 

if the compressor is locked or the bearings are bad in the clutch the speed sensor on the compressor will prevent it from engaging, or not engaging for more than a second or 2. This is to protect the drive belt from breaking. Check into that.

 

Thanks for that marknute.
We have tried turning the compressor shaft (well the body of the clutch section that is attached to the front of the shaft) by hand and it turns easily. This tends to indicate that the compressor is not locked up.
Can you tell us how the speed sensor in the compressor operates? Does it sense the rotation speed of the compressor once the clutch has engaged? And presumably it there is a problem with the rotation then this will disengage the clutch to prevent damage.
If this is the case then again presumably the clutch should engage first prior to the speed sensor coming into operation. If this is the case then the clutch must be at fault as there is no movement at all upon setting the switch. Just nothing happens. And the compressor can be turned by hand.
Do you know the mechanics of the clutch operation? Is it an electro magnet? Can it be tested in any way?
Sorry for being thick.
Cheers
Rikll

 

Check fuse 10 and 11 first, if they are found to be good you can check the relay for function. I do not know the inner workings of the relay but i can tell you that pin 5, (called 85 on the relay) is ground. put a thin wire in the socket of pin 85 and push the relay in, then touch the wire to a good ground under the hood, if the relay works, connecting the wire to ground should start the compressor. The relay is code J in the fusebox, I cannot tell from their diagrams which relay it is but it looks like it is in the middle.

And on second thought, your compressor may not have a speed sensor... here is the only diagram in the mercedes manual that shows anything about the relay:

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