P38 Air Con Clutch Not Engaging

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Radar1946

Radar1946

Active Member
Posts
349
Location
Lichfield, Staffordshire
I have now fitted a new a/c receiver and new tri switch and single switch, as I was not sure which was faulty,
and I did not want to have to remove the bumper again, so I bought all new. I asked the air con fitter to come back out and refill the gas and check everything over. He filled it with gas and tested it and found the clutch on the compressor was not working. He said it was either the compressor or the hevac faulty.
Is there anyway I can test them both before buying new ones.
 
Radar1946 said:
I have now fitted a new a/c receiver and new tri switch and single switch, as I was not sure which was faulty,
and I did not want to have to remove the bumper again, so I bought all new. I asked the air con fitter to come back out and refill the gas and check everything over. He filled it with gas and tested it and found the clutch on the compressor was not working. He said it was either the compressor or the hevac faulty.
Is there anyway I can test them both before buying new ones.
Click to expand...
Year of car a P38? if a P38, depending on year, there may be a relay between the HEVAC and the compressor clutch.
 
DanClarke said:
Can't you swap with another relay for A.N. Other service to check its definitely the relay? Some of them are common in the fusebox and can be swapped over for testing I think.
Click to expand...
Perfectly OK to swap relays of the same colour. It'as also possible to take the lid off the relay & check the contacts.
If the relay is OK, it may be a problem with the sockets in the fuse box.
 
Your right, I never noticed that one. I changed it but still will not work. I checked the voltage on 86 and it is only 4volts, so I tried all the other relays terminal 86 and they are all 12volt.
I am thinking a bad wire under the fuse box, what do you reckon ?
 
Radar1946 said:
Your right, I never noticed that one. I changed it but still will not work. I checked the voltage on 86 and it is only 4volts, so I tried all the other relays terminal 86 and they are all 12volt.
I am thinking a bad wire under the fuse box, what do you reckon ?
Click to expand...
Fuse box problems are quite common, but I suspect your HEVAC has been replaced at some time for the earlier version which did not use a relay. The relay coil does not present a sufficient load for the HEVAC output to operate., are you sure the relay energises fully? Take the cover off to check. If it does energise fully then the problem is either from fuse 27 which supplies 12 volts, or from pin 5 to the dual pressure switch and then onto the clutch.
 
Last edited:
Took the cover off the relay and it is not energising.
When I try it on the bench with 12v it energises.
I put the relay back in and closed the contacts with my finger, then cool air started to come through the vents.
When I close the contacts I can hear a click down by the compressor.
The problem has got to be the low voltage going to the relay contacts.
What do you think.
 
Radar1946 said:
Took the cover off the relay and it is not energising.
When I try it on the bench with 12v it energises.
I put the relay back in and closed the contacts with my finger, then cool air started to come through the vents.
When I close the contacts I can hear a click down by the compressor.
The problem has got to be the low voltage going to the relay contacts.
What do you think.
Click to expand...
Feed from pin 7 Hevac to pull side of relay.
 
Radar1946 said:
Took the cover off the relay and it is not energising.
When I try it on the bench with 12v it energises.
I put the relay back in and closed the contacts with my finger, then cool air started to come through the vents.
When I close the contacts I can hear a click down by the compressor.
The problem has got to be the low voltage going to the relay contacts.
What do you think.
Click to expand...
Nothing to do with the voltage going to the contacts, you have proved that by manually closing the contacts causing the compressor clutch to engage, it's the voltage to energise the coil that is missing. Look at RAVE circuits section Heating and Ventilation with aircon.
I think you have an early HEVAC that did not use the relay. I seem to remember a mod to get round the problem.
It's worth pulling the HEVAC out, checking the connectors and the part numbers.
 
It must use a relay IF when they manually close the contacts it all works and goes cold.
I think @wammers has been suggesting to look here:
upload_2019-9-25_9-1-30.png
 
Datatek said:
Nothing to do with the voltage going to the contacts, you have proved that by manually closing the contacts causing the compressor clutch to engage, it's the voltage to energise the coil that is missing. Look at RAVE circuits section Heating and Ventilation with aircon.
I think you have an early HEVAC that did not use the relay. I seem to remember a mod to get round the problem.
It's worth pulling the HEVAC out, checking the connectors and the part numbers.
Click to expand...

Which ever Hevac is used, it still outputs 12 volts from pin 7 to C0792 connector later models or C244 early models. Which is the same connector with changed definition. Early one pulls the compressor later one pulls the relay. Both give same effect. However do as Keith says and check Hevac part number yours should have latest number. Latest will work on all cars but some early ones will not work on later cars.
 
Last edited:
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