Giantman

My A/C compressor clutch won't engage! I tested with multimeter and only about 9v are making it down the wire to the compressor but I don't know where the voltage drop is occuring? Could a faulty relay be doing this?

Thanks,

Simon.

scottb

Do you have a low/high pressure cut-off switch in the system? If so, and if you're low on refrigerant, that could be causing your problem.

Giantman

Thanks for the reply Scottb. I don't know if I have the pressure cut-off switch (how would I tell?) but I have just recently replaced the receiver/drier and had the system pressure tested, evacuated and re-gassed so I don't think the pressure is low. In fact when I connected an external 12v batter directly to the compressor to power the clutch the system worked well and blew cold. There is just not enough voltage getting to the darn compressor.

Simon.

theiceman

Giant man I think you are right and it is a voltage drop issue, low pressure safety cut off would send no signal to the compresor, not a reduced one, but scott is correct that a low cut off would not allow the compressor to start, I am sure you have verified this but the multi meter should read 0 with ac off and then =12v with the ac turned on. I have a Bently and can check the schem if you have an SC, try to tell me what year. I don't think that turns on through a relay but you never know. I would suspect a loose or damaged connection, I thgink a relay may be swapable with another , but I iwll check the schem if I can.

ice

theiceman

Just had a quick check of the diagram. there is green/ white wire that comes off the centre pole of the ac fan switch ( B) . this wire goes to the AC temp switch ( this is usually mounted right on the evaporator on most cars and is a safety switch to stop the evaporator from freezing , it is normally closed and opens if the evaporator gets too cold. the wire that exits that is green and goes to a connector where it splits and powers the coil side the cooling fan relay ( connector 86 ) and then to ground, the side your interested in remains a green wire and goes to clutch.

If I were you I would check connector 86 of the cooling fan relay and see if you get 12v. if you do you can eliminate the ac temp switch. If you don't I would go back and see if you are getting the 12v from the back of your ac fan switch connection B .
Don't be fooled by the cooling fan coming on, the set voltage on a realy is significantly less than 12v and the 9 could be enough to set the relay for your cooling fan , but not enough to pull in the clutch.

Oh and one more thing, when you checked for the 12v at the clutch was it disconnected? The clutch itself could be loading it down. try disconnecting at different points until you get your 12v back , that will be your answer.

Good luck and let us know how you make out ..

Giantman

Thanks for the detailed reply theiceman. Following your suggestions I checked the switches (I had already checked the relay). Turns out the A/C cooling switch is on the fritz. Jiggling the wire (green from compressor and green/white to a/c fan switch) got it going. Detached and reattached both wires and was able find a point where it seems to work (just). The switch needs replacing but hopefully vibration won't cause it to fail again until I change it out.

Again thanks for pointing me in the right direction.

Simon.

PS Voltage reading at compressor (disconnected) is now 12.6v whereas before it was 9.6v.