Oldtee

My AC won't come on. The switch light won't light and the compressor won't start. It looks to me that to work a bunch of switches must be in the correct positon and I took a test light to several places on the main circuit board on that line, but did not get power on the line that connects them all. The clock works. The AC fuse is OK. Since I had the instrument panel apart (which I don't want to do again but ...).

Where is the AC control switch?
Where is the thermoswitch etr (evap switch)?

I haven't jumpered 12V to the line yet for fear that because the LH Control Unit is in the circuit I could do serious damage. Can I? or better Should I put 12 V to that circuit?

I swear that before I buckled up the right side of the instrument panel and center consol I checked and got a light.

Some help would be appreciated.

SteveG

Jumping the pressure switch on the dryer won't damage anything if you don't leave it on for more than a few seconds. If it is low on R12, you will have to do that to top it up. But first you should want to know if it leaked out and where. Or with the ignition on, A/C switch on, you could check for 12V at the switch and work your way back from there.

Oldtee

Thanks, I'll give that a try tomorrow. Note: The compressor works on/off in Defrost Mode.

I have 134 and know I have a small leak but it takes months to leave. I didn't know that low feron would give this indication. Nice to know, if true.

dr bob

A very common 'failure' after the dash has been apart is the connector on the back of the AC switch panel. I'm speculating that your 85 is similar to my '89. In the '89, the AC button sits in a narrow mini-panel below the main climate control head in the console. Behind the button in the panel (forward of the back of the button in the car, where front is front of car...) there is a small electrical connector that plugs onto the pins on the switch. It's a friction-fit only with no retainer mechanism, so it pulls off rather easily. Since this sit right above the radio, generally it's radio work that gets the blame.

That little plug is accessible through the driver's (left side on US cars) side panel on the console. A couple screws hold that panel on, and the rear screw access generally requires that you put the seat at its rear-most and lowest position. Once that carpeted panel has been removed, you can reach in above the radio and reseat the connector. If the fit is too tight you can remove the radio for additional access, but remember to hold the connector together as you slide the radio back in so the connector isn't pulled loose again by radio wiring.

If that isn't your problem, the next step is a basic check of fuses, then the current flow through that switch to the control head. With the fuse good and the connector on the back of the switch properly engaged, the light in the button should come on when the switch is depressed, regardless of whether the AC clutch is engaging or not.

The current from the switch goes to the climate control head through an edge connector on the forward left side of the controller as it is installed in the car. Forward as towards the front of the car. With that same side panel removed, you can verify that the edge connector is seated properly on the controller.

There is a relay inside the controller that's notorious for being undersized for the task of the AC clutch. There are several good write-ups about diagnosing relay failures.

The most common 'failures' on 928 AC systems involve leaks. Since you state that the light doesn't come on at the AC switch and the dash was apart, I'll stop here and let you do your testing. If you get to the point where the light comes on but still no clutch engage, there are several more good writeups about checking the system pressure, temporarily jumpering the safety switch, etc, as diagnostic methods.

WallyP

The power for the system goes thru Fuse #17 to Relay X, the Fresh Air Blower Relay. Before getting too deep in complex troubleshooting, check the fuse. If that doesn't help, swap Relay X with Relay XII (horn) or Relay XXII (Backup Lamps on auto transmission cars).

Oldtee

Thanks Dr. Bob. You were spot on in your diagnosis and solution. It took longer to replace the carpets and put the tools away than it did to do the repair. All good now. This is a great site.

Wally, are you gonna be at SITM this year?

WallyP

Planning to be there...