My AC always blew nice and cold until last Thursday, when it started to get warmer and warmer in the car as I was on the Garden State Parkway heading down to the Jersey shore. When I checked, it seemed to be that the compressor clutch wasn't actuating. I disconnected the wire going to the compressor, and I measure +12v at the end of the lead coming from the engine, so the relay must be OK. But, when I reconnect it to the lead going to the compressor clutch, the voltage drops to150mv at the connector,which doesn't seem right. It's a Kuehl unit installed by the P.O.when he converted to R134 years ago. The unit doesn't have a separate ground wire,so I guess the ground is through the body of the compressor. The fans also come on as soon as the AC button is pressed-I don't know if that's normal or not. With at least a few more months of hot weather ahead, any advice or suggestions will be appreciated!

 

You're sure there's no independent ground line from the clutch to the mount? Mine broke recently and it was a PITA to find. Without it the AC was non-operational. On mine there's a small (IIRC 8MM) bolt that secures the clutch ground to the mount that you can access through the passenger side front wheel well. (Not easily, there's suspension bits in the way, but a loooong driver will do the job). Not a long wire and not easy to locate at first. Mine worked fine after the ground was re-connected.

 

I'm willing to bet the HVAC head unit relay is unable to send the proper amount of amps to kick the compressor on. It will still show 12v but when a load is put on it, it fails. Had that happen on a few cars and the first time it was a bit interesting to find the actual fault.

 

Yep - volts ain't amps.

 

I had exactly the same voltage symptoms and my anti-freeze switch was the cause. It is easy to disconnect both leads going to the switch and bridge them together to rule that out.

Dan
'91 928GT S/C 475hp/460lb.ft

 

If it isn't grounded, amps aren't going to help either until we get to the arc weld point at least.......
Try the easy stuff first.....
And you can rig a test relay to confirm if there's a potential head unit issue......
Without having to dig in there right off the mark.
Or, to confirm the clutch is operational, feed direct 12 volts from the battery positive. If that works, it's likely the relay (Head Unit). If not..it's the grounding or the clutch itself is done for.

 

I've got mine bypassed right now too...seemed to be cutting the clutch too much and too soon.

 

http://www.nichols.nu/tip020.htm

 

Had same symptoms and it was anti-freeze switch that was culprit.

But head relay failure will show same symptoms (current not sustained under load). You have had good advice to first check clutch with known good 12V source - take it from jump post on RHS of engine bay.

If compressor and clutch checks out OK differentiate between freeze switch or hvac relay failure by pulling off spade terminals from freeze switch (small rectangular metal box under plastic cover in front of windshield) and linking the wires.

 

The easy check is to use the under hood power post to provide power directly to the clutch........does it operate?

The compressor is bolted to the engine which in turn has a large ground cable.

 

If it engages work your way back to the low pressure switch and then to the freeze switch (under the rain guard).

More than likely the relay.

Michael

 

That's what confused me too.....I had thought the mounting braket would provide more than adequate ground to the block and chassis, but apparently not......

 

I've done a lot on this car, but never paid any attention to the AC. Looks like a good project for this weekend if I can get an early start!

 

Im confused as well then rob as I changed the compressor on the S4 with a brand new unit......there was no ground wire

 

The wire is grounded to the compressor body by a screw, the screw also holds one end of the hot wire to keep it from being pulled out of the clutch.