Voltage Driving A/C Clutch

RandalJ · RandalJ's Garage

Have an intermittent problem with the clutch not engaging. It would not engage yesterday PM on the drive home, and when I checked at home, there were only 1.5 volts on the clutch wire (disconnected with ignition and A/C on). On the way to lunch yesterday, it engaged, but after lunch it blew warm air - I had to turn the fan switch off and on again - then the clutch engaged and it blew cold.

This morning, I measure again, get 11.5v, the clutch engages, and everything is fine. Took it for a 20 minute test drive, and it blew cold the whole time. Will start up again in a couple of hours to see if it fails again.

If it engages, it seems to stay engaged unless I turn it off, so it is not an intermittent problem once it's going. Just after I shut it down for a period of time.

Any ideas?

I did swap the cruise control relay for the A/C (same p/n) but I doubt it will do anything. I think the A/C clutch is not dependent on that relay, but maybe it is...

ruf-porsche · ruf-porsche's Garage

Actually there are two relays for the a/c system. One on the fuse panel in the front trunk and one in the smuggler box. The compressor clutch is also controlled by the temperature switch, it cycles the compressor on and off. Seems like somewhere you have resistance. If you battery is fully charged and alternator is working, the voltage should be 12+ at the compressor clutch lead.

Check out this thread for a wiring diagram for the a/c on sc.

http://www.pelicanparts.com/911/911_Parts/Electrical/911_electrical_82SC_AC.jpg

RandalJ · RandalJ's Garage

The second relay in the trunk is just for the evaporator blower, right? I'm having no problems with the blower. Could it also affect the operation of the clutch?

Do you know whether the temperature knob/switch still cycles if you have it on the max setting? Or should the clutch stay on all the time at that setting? If it cycles even at the max setting, what triggers it to cycle off? And what triggers it to cycle on again? Does it connect to that temp sensor near the evaporator?

I took several more trips today. On the first two legs, it ran fine. On the third leg, it would not engage. On the fourth it ran fine again.

Thanks

ruf-porsche · ruf-porsche's Garage

A capillary tube from the temp switch to the evaporator coil cause the temp switch to cycle the compressor on and off. Even if the switch is at the maximum, once the evaporator get cold enough, the temp switch will cycle the the compressor on and off. The compressor needs to cycle on and off to prevent the evaporator coil from freezing up.

The a/c fan switch also provides power to the clutch.

I would test the a/c system with the temp switch turn completely counter clockwise (min. a/c) and see what kind of voltage you are getting at the clutch lead. I will slowly turn the temp switch clockwise until you have maximum a/c and check the voltage to the clutch lead.

I would aslo test the a/c switch, first at the lowest setting and then at the highest setting (check the voltage at the clutch lead). You could also have a bad connection.

RandalJ · RandalJ's Garage

Thanks, ruf. A faulty temp switch or probe sounds plausible. I've not made any measurements yet, but I was able to turn it to min and get the clutch to disengage, and turn it back and make it re-engage. Could be a bad probe or cable. I think full throttle condition is also supposed to make it disengage. I'll experiment more during the week and check that the wiring for all these are OK.