Dan87951

Ok I made a big booboo I left the low pressure switch disconnected thats why I wasn't getting 12 volts at pin 9 forgot I had it disconnected. Pin 9 does have 12 volts. I'm loosing power at the compressor wire. I'm taking a break from this for now as im making to many mistakes and will catch back up with you guys. thanks for all the help.

dprantl

I made another post before this, but it disappeared for some reason?

Ok make it simple. Leave everything connected, disconnect the A/C clutch wire and measure voltage to ground with ignition on and A/C on. If you don't get 12V there but do get it at Pin 9, then at least one of your problems is the wiring going from Pin 9 through the harness to the A/C clutch wire.

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

Dan87951

Disconnected AC compressor from power wire and 12v is there. The moment I connect the compressor it shorts out and I get 0 volts so I did a continuity test from the AC compressor power wire to a ground and I have connection.

dprantl

Disconnect and jumper both freeze switch and low pressure switch. Do you then get 12V at the clutch wire with ignition on and A/C on and clutch wire connected?

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

Mrmerlin

what happens if you add a jumper to the compressor wire from the hot post with the engine running?? is the wire making sparks?? its possible the power wire to the compressor is shorted to ground, or the compressor clutch has an internal short

SQLGuy

These symptoms sound like the relay in the control head. The relay fails because its contacts get overheated and burn. This raises their resistance. With no load, they can still carry a bit of current - so, with the clutch disconnected, you would still read 12V. As soon as you apply a load, though (the clutch coil) the current increases considerably, and the resistance of the contacts becomes very noticeable - dropping the 12V instead of acting as a proper conductor.

You can see here, the burned/overheated contacts (right side) of my original AC relay.

Dan87951

I get 12 volts at the clutch wire as long as the compressor is not connected. The moment I connected the compressor it goes to 0 volts or some really small voltage .023v for example.

Dan87951

I did this without the car running but I could hear the clutch engage (click into spot) when I connected it to the jump post. No sparks.

dprantl

Ok, so bypassing both switches still shows 0V on clutch wire with it connected. Do as Stan says, disconnect the clutch wire and with the A/C on and engine running, connect the wire directly to +12V (i.e. the jump post). Do you see the clutch engage and spin (the center part of the compressor)?

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

Dan87951

I think my next bet is probably to remove the HVAC control unit and take a peak. Is it hard?

Dan87951

I did this without the car running and it engages I can hear it click and there are no sparks.

dprantl

Yup, sounds like you are losing your +12V on the little relay in the A/C control head. It's easy to remove it, just carefully pull out the plastic surround (it just snaps into place), then remove all the philips screws by the sides (6 I think), then just pull out the warning bar and control unit far enough to unplug it.

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

SQLGuy

Not too bad. The tricky part is getting the thin bezel out from around the radio.

Here's a good write-up (steps 11 through 17): http://members.rennlist.com/pirtle/svc_cons.html

Dan87951

Thanks. I got the Control Unit out and took the cover off but I don't see any thing that looks like a relay inside unless its hidden under more plastic I can't see.

edit I found a thread on it.
http://homepage.mac.com/ew928/928rel...toAlbum62.html

SQLGuy

This is what it looked like with the cover on on my '88 (little dark blue box - other MY's have different color and different shape relays):

And, second picture, with the larger, lighter blue, relay I replaced the original with.