P38 Air Con Compressor Clutch - Why won't it work?

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kooky_guy

kooky_guy

Well-Known Member
Posts
2,410
Location
Sandhurst, Berkshire
Hi dudes,

Been playing with my air con again today. I've removed the shim behind the clutch to reduce the clearance to about 0.4mm, but it still won't engage.

Checked the gas by pressing the valve on the pipe - definitely loads in there.

I'm getting a bit baffled now.

Checked the voltage going into the clutch - 12V with it disconnected.

Resistance of the clutch itself is 3.5 ohms if I'm reading my multimeter correctly.

What I assume is the thermal cutout is showing no resistance to earth which I assume is correct or I can't see how it would work at all.

How does the clutch itself work? Is it supposed to be able to slide on the splined shaft or does it just turn with the magnets? I was wondering about lubing the splines with copperslip or something but that seems slightly iffy.

One thing I did notice is that the clutch seems to be very slightly warped - the clearance isn't quite the same all the way round. Don't see why that would make any difference though.

I'm stumped now.

The air con used to work occasionally, but seems to have given up the ghost completely now. I thought removing the shim would do the trick but it hasn't.

Any thoughts/suggestions anyone?

Guy
 
Petrol.

Just tried a direct feed to the clutch and it worked a treat.

I just can't understand why it won't work normally. There seems to be 12.4V there at the positive to the clutch disconnected. There seems to be continuity.

I'm sure I'm just being thick.

I seem to remember a similar problem with the fog light on my old series. Voltage was there until I connected it up then it just went to 0 and didn't work. Fixed it eventually, but I can't remember how now.

Grrr. It's so frustrating!

Guy
 
kooky_guy said:
Hi dudes,

Been playing with my air con again today. I've removed the shim behind the clutch to reduce the clearance to about 0.4mm, but it still won't engage.

Checked the gas by pressing the valve on the pipe - definitely loads in there.

I'm getting a bit baffled now.

Checked the voltage going into the clutch - 12V with it disconnected.

Resistance of the clutch itself is 3.5 ohms if I'm reading my multimeter correctly.

What I assume is the thermal cutout is showing no resistance to earth which I assume is correct or I can't see how it would work at all.

How does the clutch itself work? Is it supposed to be able to slide on the splined shaft or does it just turn with the magnets? I was wondering about lubing the splines with copperslip or something but that seems slightly iffy.

One thing I did notice is that the clutch seems to be very slightly warped - the clearance isn't quite the same all the way round. Don't see why that would make any difference though.

I'm stumped now.

The air con used to work occasionally, but seems to have given up the ghost completely now. I thought removing the shim would do the trick but it hasn't.

Any thoughts/suggestions anyone?

Guy
Click to expand...

If you put 12 volts to the feed lead on the compressor the electromagnet should pull the clutch in. If you have 12 volts to feed and nothing happens check the other lead on the electromagnet is earthed properly. The pulley should spin freely until the magnet pulls the clutch in to engagement. On the clutch there should be flexible spring arms to let it move in or out. It being slightly untrue should make no difference. Clutch will not engage if there is low pressure in system, pressing valve and getting a pressure leak is not the best way of doing it. You will unless the system is empty always get some sort of pressure. That does not mean unfortunatly that there is enough.
 
wammers said:
If you put 12 volts to the feed lead on the compressor the electromagnet should pull the clutch in. If you have 12 volts to feed and nothing happens check the other lead on the electromagnet is earthed properly. The pulley should spin freely until the magnet pulls the clutch in to engagement. On the clutch there should be flexible spring arms to let it move in or out. It being slightly untrue should make no difference. Clutch will not engage if there is low pressure in system, pressing valve and getting a pressure leak is not the best way of doing it. You will unless the system is empty always get some sort of pressure. That does not mean unfortunatly that there is enough.
Click to expand...

I could see how it worked when I hotwired it!

Hmm, fair enough about the gas, but it wasn't just a small release, it was a HUGE high pressure burst.

I would have thought that if it was the HEVAC thinking there wasn't enough gas then it wouldn't be supplying 12v to it in the first place, which it seems to be doing.

I'm going to try bridging the earth and see if that makes any difference....

Guy
 
Ok, I'm fed up with it now.

I seem to have 12v going to it whether the air con is on or off. So presumably it's switched via the earth connection, but shorting the earth out didn't make any difference either.

I'm baffled now - time to give up and have a nice cup of tea in the sun instead.

Stupid car. :(

Guy
 
If you jumper the two wires to the thermal switch (Round thingy on top of the compressor to the rear) and everything works o.k., that means the thermal switch is faulty. Just loop it out, most compressors don't have that switch, they rely on a pressure switch (Which the P38 has) to cut the compressor out when the pressure is too high...remember that heat is related to pressure in the aircon system, so you are still protected.
 
Irishrover said:
If you jumper the two wires to the thermal switch (Round thingy on top of the compressor to the rear) and everything works o.k., that means the thermal switch is faulty. Just loop it out, most compressors don't have that switch, they rely on a pressure switch (Which the P38 has) to cut the compressor out when the pressure is too high...remember that heat is related to pressure in the aircon system, so you are still protected.
Click to expand...

Thanks Irish.

I can't work out how to disconnect the big weird looking connector at the front though. Duh.

I don't want to chop wires on the offchance.

Guy
 
!2volts with the clutch disconnected does not mean you have 12volts when the clutch tries to draw current, there could be a high resistance connection or relay contact which drops the voltage when under load.
 
It's a sqeeze job on the outer socket.
Why not just leave the wires intact, cut a short length of the insulation to bare the conductors and then connect a short piece of wire to "Jump" them. You can always tape the areas up after you have proved whether the switch is faulty..and the cable cores remain intact.
 
Datatek said:
!2volts with the clutch disconnected does not mean you have 12volts when the clutch tries to draw current, there could be a high resistance connection or relay contact which drops the voltage when under load.
Click to expand...

Hmm. You're right there - it's about 0.3 V with it connected although I'm a bit confused as to why I get any voltage there at all with the air con switched off.

Isn't there a relay in the fuse box that can cause problems like this? Is there an easy way to check it?

Guy
 
Need to look at the wiring diagrams on RAVE. You could try hot wiring 12 volts only. If it engages without switching on the aircon then it must be positive switched and you probably have a relay or connector problem. If it can be controlled by switching the aircon of and on then it is negative switched and you just have a bad connection somewhere. Fusebox is a likely candidate.
 
Datatek said:
Need to look at the wiring diagrams on RAVE. You could try hot wiring 12 volts only. If it engages without switching on the aircon then it must be positive switched and you probably have a relay or connector problem. If it can be controlled by switching the aircon of and on then it is negative switched and you just have a bad connection somewhere. Fusebox is a likely candidate.
Click to expand...

Good idea.

Will have to wait for another day though as I've got to take the dog out now.

Just had a quick look in the fusebox and noticed that one of the fan relays is a bit brown (burnt) looking. I don't suppose that is related to the air con is it?

I'll have to get another one - I vaguely remember that the fan relays are a bit prone to overheating.

Has anyone ever installed the link harness kit that was produced by LR to help resolve air con clutch problems? Was it a recall job and if so would they still do it? Is it still available? I've got the instructions for it here and it looks ever so complicated.

Guy
 
kooky_guy said:
Good idea.

Will have to wait for another day though as I've got to take the dog out now.

Just had a quick look in the fusebox and noticed that one of the fan relays is a bit brown (burnt) looking. I don't suppose that is related to the air con is it?

I'll have to get another one - I vaguely remember that the fan relays are a bit prone to overheating.

Has anyone ever installed the link harness kit that was produced by LR to help resolve air con clutch problems? Was it a recall job and if so would they still do it? Is it still available? I've got the instructions for it here and it looks ever so complicated.

Guy
Click to expand...

You can look at tech bulletins in RAVE for that info. Think it was to cure a voltage drop problem.
 
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