MainePorsche

Do this.
I believe the answer to your problem is not far off.

rifitz

You called it-- I had already bypassed the dimmer rheostat and jumpered the wires together. With it disconnected, lights on- A/C continues to work. Reconnect it and the compressor stops..

MainePorsche

I'm glad for you, as now you will know what it is. Was a hassle when it functioned like that.
So here's the issue.
You have a faulty diode on Track/Trace 4 of your HVAC controller. This Trace attaches to the Brown ground at the harness. If I remember correctly there are 3 diodes along this Trace. The problem one is the first one off the harness connection. What is happening is your bulb filament, which ties in to the Trace just after this diode, is acting as your 'ground'. Turn the lights on and the HVAC relay is losing its ground, hence no function. If you want to read my threads on how I deduced it, you will see I determined the fault to be somewhere along Trace 4 (this was after a tremendous amount of work). I still didn't understand what precisely was causing it though, for Trace 4 had no visible defects. Alan, our venerable Yoda of 928 electronics, presented to me the bulb filament grounding scenario.

Unfortunately, though you might be happy to know the cause, there is now the fix. You can verify the diode's death and replace it, or you can get a rebuilt HVAC controller. I was considering doing the diode replacement but did not for two reasons. First, I am not very good at micro soldering work. Second, I would have had to dissect the plastic infrastructure of the unit to get good access to the diode in order to do my questionable micro soldering repair. I would only get a Greg Brown rebuilt unit.

Well sorry for the bad news, but now the mystery is over. Happy to help. Made it much quicker than my ordeal with the Gremlin.

Craig

rifitz

Craig-- No real bad news here-- saves me many, many of hours chasing it. Electrical isn't exactly my strongest suit.. Thanks so much

Again-- folks like Craig in this group willing to help out makes this one of best resources out there.

MainePorsche

Thanks Rifitz.

I was just in the garage and a smile came to my face when I took notice of the 'shrine' again. The 'shrine' is a box on a shelf labeled 'old HVAC Controller' - this is the one with the Gremlin. I've kept it where it is for it was really responsible for me learning the 928, and bettering an already good car. I knew basics of electrics and mechanics, and even less about the 928 when I started with this Gremlin on Rennlist. As a result of tracking down this Gremlin I learned to read our WD's, learned well the AC circuitry and did general servicing of the entire system when there, learned the significance of the 14 pin connector and all its components - serviced everything attached to it, removed the entire power harness from starter/alternator to the front points - fabricated new quality cable and connections - opened looms and repaired all aged wires - did an ace job in re-protecting the harness, removed the PO alarm system in its entirety and all the wire sacrificing 'scotch' connections that were used - found several issues with other wiring in the pod and dash that were repaired, etc...
I leaned a lot along the way which helped me in the general maintenance/preservation of the car, and the Gremlin was the one who pushed me out of the door in the fuselage at altitude - I was the one who then pulled the cord. I do owe this Gremlin accord, and that is why he has his 'shrine'.

Alan

I'd actually say that the diode replacement is a rather easy task even if it is your first such repair.

There isn't much to damage electrically, you can have a couple of goes at it pretty easily, you can easily test it after you have completed the fix & its not micro by any means, not even mini - more of a maxi these days.

All you need is tools: Good soldering iron, rosin flux solder, desoldering braid or solder sucker

And a new diode: maybe 2(+) Amp, 30(+) Volts

The mechanical dissassembly/reassembly aspects are harder than the electrical - though really not that bad IMO.

Given the cost of a new head unit I'd certainly try this first (unless there are other issues with it)

Alan

MainePorsche

I had all the tools and was prepared to to this, but my practice runs on something else were rather 'sloppy' and I wanted to have faith in the union. If you're good at board soldering I would do that. I could suture a lacerated lip back together and you would even know that I was there, but I am all thumbs with board soldering.