rifitz

The S3 is on the road after 18 mos of engine-out, interior, wiring & fixing lots of bad PO and shop work...etc.etc.etc... runs like a champ. I'll post the comeback story a little later...Came back from the tint shop tonight with both headlights and A/C on at the same time. First night run since the rebuild. Strange thing is that if I turn on the parking lights, the A/C quits...voltage gauge hangs around 12 with lights on...(know that can be inaccurate). Alternator is one of the few things I didn't replace in the process.

Car has a new 14 pin connector, engine harness looked good, HVAC controller is rebuilt. Any theories before I start digging through the wiring diagrams again? Overall voltage goes too low, not enough to energize the a/c clutch??

Any help is appreciated--Thanks

MainePorsche

Well, well. This is almost exciting. Don't mean to appear odd, but I have not yet heard of this issue on Rennlist other than me having had it. It took almost 3 years for me to get to the bottom of it. It was one of my first threads on Rennlist. This Gremlin was elusive, and I capitalize his moniker. When I started with it I was new here and knew practically nothing about the 928.

First off, with the Headlight switch OFF how does your AC work ? Be specific.
Second, do you have any other electrical oddities or issues ? Specifics

Here are my threads related to the Vanquished. Note how the first thread has a similar title to this thread.
https://rennlist.com/forums/928-foru...c-problem.html
https://rennlist.com/forums/928-foru...-relay-xi.html
https://rennlist.com/forums/928-foru...ne-update.html
https://rennlist.com/forums/928-foru...-question.html

Post back. It will be done

rifitz

Thanks for the prompt reply...I actually googled the forums after the post and stumbled into the middle of your thread on this symptom...indeed it's a long one... Car has no electrical oddities after I fixed all the bad PO work. PO did have a separate new wire run all the way from the a/c head to the compressor when i got it (idiots!) cleaned up that mess and replaced the head, and thought I was good. A/C works great until lights come on--then quits. I'll replace the supressor relay this weekend as a starting point as I read through your saga!!

How's the Star City? Small world, I lived in Easton, Fort Fairfield and PI before going to So. Portland--then a 19 year road trip through the US, currently in the Chicago burbs..

Appreciate your help!!

MainePorsche

Hah ! Still quite nice up here. Good for the kids - that's why we came here some 5 years ago. We even have sushi now. We have it with the potatoes !

Anyway, what you're going to have is a corrupted ground. MP IV (Masse Punkt IV) is the ground point under the steering wheel back on the bulkhead. The HVAC controller is grounded there and so is the Head Light Switch. The cig lighter, dash/center console illumination, and I believe the clock also ground there. I'm not using my laptop so I can't confirm some things right now.

The first things I would do are to evaluate MP IV to see if it is properly 'earthed/zeroed' when the lights are on, when the AC is on, and when both are on. Then I would measure voltage at the compressor wire (the little black one off the wire harness just to the right of midline near the dipstick). Get value with lights off and on. I bet you are at 0.0v with lights on and not some below threshold value to activate compressor clutch. Post back.

Add: Don't change the Supressor out. This is not really a relay and doesn't often fail. Do the above first.

rifitz

Glad to hear how far PI has progressed!!-- I need to get back to visit-- no place quite like it. I'll chase down that ground point. I've replaced the pod, but don't remember looking at that ground. I've restored a few Porsche models along the way, but the 928 community here is the most capable and willing to share I've ever seen. Info like you've supplied saves so much time.. Love this car, always have. A newer 911 4s and I were near each other on the road Saturday. A carload of teenagers were killing themselves trying to get a look at the 928 and figure out what it was... funny to see.... I'll report back later--thanks

MainePorsche

Back on my laptop now.
Here's the Power Path for the AC system from an old post. Note on your MY the fuse may be different (I haven't checked).

https://rennlist.com/forums/9890673-post6.html

Points to measure voltage where switched would be the anti-freeze switch and the low pressure switch. Your issue will still be a corrupted ground. You just have to narrow down to where.

rifitz

Got it-- I've been through the A/C circuit before (freeze switch/low pressure to compressor, etc) I was verifying everything when I replaced the 14 pin connector due to the the previously mentioned PO hack job on the front harness.. Thanks for the info-- hope to dig into it this weekend and report back..

MainePorsche

Just curious, do you remember how the PO tapped in the the HVAC controller output ? Do you remember what you did to clean it up ?

rifitz

Sorry- I don't recall as that was early in the process. Based on the other "work" that was done to the car, I'm doubtful that the improvised solution even worked. The car wasn't driven after I bought it to even test the A/C. It would run, but the bent valves ended up in a backfire that blew the airbox off. I must admit they did an almost-respectable job patching the airbox top from a prior big boom with sheet metal, rivets and silicone.

The r-12 charge was almost non-existent as well so I didn't bother to jumper the low pressure switch to see what happened. Just went about cleaning up the mess--

MainePorsche

I was just wondering if the PO through in some relay along the line possibly stealing/jeopardizing a ground or power source from somewhere.
You say you have good AC now with the Light OFF right ? I know you know an inadequate charge will keep the compressor from activating. Sorry if I'm getting to elementary on you.
Will wait to see what you find especially at the compressor clutch feed with the Lights ON.

MainePorsche

If you're out there, I'm just curious on your findings.

rifitz

Hi there-- still here, but haven't had time to dig into it yet-- work has been insane... another symptom I noticed was that flashing the headlights (H4's) killed the radio momentarily-- so if the systems your describe all tie back to that ground point-- it looks more and more like you are correct that MP IV is the culprit-- maybe I'll climb under the dash over the long weekend.

I'll let you know---Thanks

MainePorsche

I'm very curious on this one.
Best Wishes and Happy Hunting.

rifitz

Well- after a long pause, this thread is back. The issue is turning on parking or headlights kills the A/C compressor. I redid the pod, and verified that ground point MP IV (on the steering column support) is all good. I did discover that one of the ground points above the CEP was loose--so I thought I had found the issue-- Unfortunately not. I'll check the voltage drop at the compressor next. Need to look at the wiring diagrams again, its been so long..

MainePorsche

Ok, now try this.

Disconnect the Black/Blue wires that attach to the dimmer rheostat for the console/pod lights. The Black/Blue system has 2 branches off the rheostat - one to pod switches/dash illumination - the other to center console illumination (including HVAC controller bulb).

Now start car and turn on AC -> now turn headlight switch on and report if compressor engages or not.