jpitman2

I am having the common issue with min temp driving the AC ok, but as soon as the temp slide is move a little it goes to full heat. I dont think its a vacuum problem - after shutdown I can hear the flaps moving each time I turn the ign on for at least a dozen times. There is no vacuum leak in the HVAC controls, altough my mity-vac shows a slow leak in the cruise unit when its isolated.
I have checked the outside temp sensor in the freezer and it seems fine - high ohms when cold, resistance decreasing with temp. The connector is clean and snapped fully home. Next, where do these 2 semsor leads terminate inside? Somewhere on the central panel plugs?
The book shows what looks like a single line coming from the sensor, going to the inside sensor then into the controller - is the other lead not shown 'cos its ground? is there a connector near the controller where I can find the inside sensor leads so I can check them? Is there a connector I can test the inside sensor blower at ? Can I get any closer to the inside sensor by removing the glove box and/or the parcel tray?
BTW, if the in and out-side sensors are in series, how does that work? How can the controller know whether its seeing hot/out cold/in or cold/out hot/in scenario when both can produce the same total resistance?

Update: Found the test connector at the setting motor, and I seem to have open circuit on pins 4-12 per Wallys test - now to find the real culprit. Also, curiously where manual says pins 3-11 should give battery voltage, best I can get is 10.5V, on a car with a fresh run today, and on a trickle charger. I suspect this implies a poor connection somewhere on the way to the source for the HVAC. In fact it may be related to a couple of other things - when the turn indicators are going, the voltmeter changes as they flash, and turning on the lights lifts the temp gauge reading a couple of mm....
Is there no way the inside sensor comes out of the dash forwards? Does anybody have a pic of an inside sensor out of the dash please?

thanks
jp 83 Euro S AT 51k

Bill Ball

The only way that I know of to get at the interior temp sensor is to remove the center console.

Tails

JP,
I have a digital photo and I sent you a pm on it.
Tails 1990 928S4 Auto

Mrmerlin

Bills right, the console must come out to fix/look at the interior temp sensor.
Before you go there though are you absolutly sure that the connector barrels in the front/outside temp sensor are connected?
If they are not the problem you describe will happen, I removed my outside temp sensor through the headlight area after removing the headlight and i found the connectors even though they looked connected have a shoddy barrel type connector and this can either get loose or just not make contact. Do a search for, fix you hvac system, there are some pictures there i will bet that this is your problem, Also check if all of the interior connections are made to the temp control head. the console is a pain to remove and since this is so the wires dont usually get disturbed.

jpitman2

The other thing I forgot to mention was that I had the whole interior leather refurbed a year ago..complete resurface of all the leather. I wonder if the leather guy tried to pull the sensor out and damaged something?
If I could find where the outside sensor terminated inside the car, I could test that it was (or not) open circuit there, before having to get at the inside sensor...
thanks
jp

Mrmerlin

Well with this new info then yes is quite possible that the temp sensor /fan on the dash has been damaged its a bit tricky to remove as the securing fastener is on the inside of the console, and there are wire harness connectors that might not have been reconnected, you can test to see if the fans working in the dash by holding a cigarette next to the round grate with HVAC system on see where the smoke goes, if its getting sucked into the grate atleast you know the fans working

jpitman2

Ok, with some help from tails, I have located the connectors for both sensors, and proved interior one is OK! Blowing down the vent changes the reading, big relief. No to trace back from the outside sensor and findout whats going on there. The sensor itself reads ok off its connector in the wheel well, so now I will have to trace the wires back. Anybody know if the sensor leads merge into an engine bay loom, or are discrete for any distance? Will try a network cable tracer and see where that leads me...
jp 83 Euro S AT 51k

Mrmerlin

the most likley break in the circut is as i described above. Do a search for......... fix your hvac system
in the 928 forum by Mrmerlin

Emickelsen

Damn, thats good stuff!! I have the same problem, but haven't looked into it yet. At least now I can find this thread easier in my user CP.

jpitman2

OK, Mrmerlin had the correct location, not quite the right cause. Checking sensor resistance with pins into the sensor side of the connector was good, but on the loom side, bad. I got the connector out past the headlight without dismantling anything. Getting the connectors out was another thing though. B came out well, and was solidly soldered. With A, it came half out, even with wd40 on it and warmed a bit, then the wire pulled of the solder. A bit more brute force finished the extraction. Stand alone the wire connectivity to the console connector was good. Finally found the inside of the A connector was corroded, and needed a good clean up inside with a 2mm drill, so both sides got some of this. Re-soldered the A wire, fitted back in the plug, back onto sensor, and we have a good reading up at the connector inside now. Break for lunch, and re-assemble the interior, we should be good to go. Next time I will check the easy options first - clean the socket as well as the pin side....
Next - hot start issue.
thanks,
jp 83 Euro S AT 51k