fonetico

Thank you Wally. You're undoubtedly the Oracle!
I'll try you're imaginative technique this weekend.

Manfred

OK. Almost a year since I started this thread. I have finally found time to dive into this again. Greg (RIP) had been working on this for the past few months to no avail. Obviously, the car is back in my hands now and I'm digging in. I picked up at Ed's suggestion to measure resistance from the potentiometer connection. He said it should be zero when the short is present, higher when the short removed. Well I measured the resistance at 0.6ohms (which I'm guessing is close enough to zero) before I started. Then I removed the pod (disconnected all the switches) and the instrument cluster and measured again and the resistance was 14.4ohms. I'm guessing this means my short is somewhere in the pod area. I'm going back out to connect each switch one-by-one to see if suddenly the resistance drops again.

Ed Scherer

It seems rather unlikely that a switch would cause this (at least internally), although perhaps there could be an external short across its terminals.

It's more likely that by moving stuff around, you've already "fixed" the problem (unpinched a BK/BL wire-to-chassis-ground short).

As Wally said in post #27 and I mentioned as well:
FWIW, Wally's suggestion in post #30 for using a lamp in place of the fuse is a really good one, for when you're ready for your next test.

Good luck. Keep us informed.

Lizard928

More likely a slightly bare wire against the metal part of the pod

Manfred

Hi Ed,
Yeah, I just plugged each switch back in, one-by-one. The resistance dropped from 14.4 to 13.7 after plugging the fog light switch back in and then dropped again to 12.4 after plugging in the hazard light switch. Other than than, it stayed constant. I wasn't thinking that switches themselves were causing the short but I thought that it might narrow down which circuit I had to trace down.

I have been looking for a BK/Bl wire being pinched and can't find one anywhere.

I bit the bullet after reconnecting everything (pod cover still off but pod and all switches reconnected) and reconnected the battery and turned the ignition on. The resistance jumped to 65ohms. I turned on the headlights and the resistance stayed at 65ohms.

I think the next step is to reconnect the potentiometer, put in a fuse (more simple right now than making a Wally 12v light indicator) and see what happens. I have a sinking feeling I'm going to be down another fuse.

Manfred

If all else fails I may be forced to paint all the metal surfaces or cover them in electrical tape!

Manfred

So, I don't know what/where the problem was, but....

I fixed it, I fixed it, I fixed it, I fixed it!



Anyway, put the #9 fuse back in, plugged in the rheostat, turned on the lights and, voila, no blown fuse, HVAC, clock and cigar lighter light come on and they dim and brighten with the rheostat!. Sweet!

Now to put it back together without shorting it out again.
BIG FINGERS CROSSED

Thanks for everyone's help.


Dan

fonetico

It's bizarre how many of the issues I've had to deal with in the last twelve months have cropped up again on this site in recent days.

Since originally contributing to this thread and trying the various suggestions I took the car to two different auto electricians, eventually getting the car back with no lights whatsoever. By that I literally mean anything that involved illumination of any kind no longer worked!

In desperation I took the car to Paul Anderson (the 928 expert in Gloucerstershire in the UK) who took all of twenty minutes to diagnose the fault.

Shortly before the problem appeared, I'd had a leather panel in the driver's seat replaced. Apparently, when the seat was reinstalled, a connector for the seat adjustment electrics had been damaged.

According to Paul the central console lighting and the seat electrics share some sort of common circuit.

Once again, I apologise for my unscientific explanation.

Manfred

Nice going on getting your problem fixed. Yeah, I spent a number of days reading Alan's guide to reading the Porsche electrical manuals and studying the circuits. In the end, it helped but it basically led me to the conclusion that I would have to take off the pod and poke around if I was going to get anywhere.

fonetico

Well Dan, it's great to hear you're illuminated again but it must be frustrating not to know exactly how and why.

Since I first posted on this problem I've discovered there are quite a few as in the dark as you and I were. If you ever find out precisely where the gremlin was, please let us all know. I gather there are a number of others who've given up any hope of ever shedding light on the subject.

Manfred

Well, in putting the console back together, I left the ohm meter hooked up so I could make sure I would know it if the short returned. Everything went back together smoothly until I plugged the defroster switch back in. Everything else was already back together; literally, it was the last step.

As I plugged the defroster switch in, the ohms dropped to zero. Unplug it, and the resistance returned. I guess my short must be somewhere in the harness for the defroster--someone else can chime in but I don't think the switch itself can be the source of the short--it's just that the switch completes the circuit and allows the short to happen. So, I guess off comes the pod again so I can look critically at the defroster wires.

Also, stupid mistake made on my part. I had the battery disconnected during my work. Put everything back together and then went to reconnect the battery. I did not realize that a few of the switches (headlights, hazard lights) were in their "on" positions and I got a few sparks when reconnecting the battery. I'm not sure if this could've caused a problem but now the digital computer display has uneven lighting (the right half of the right display is dim). It could be that while moving things around a bulb went out but it was working fine when I tested everything with the pod cover off. DAMN! I'll see if I can take a picture that shows it.

Alan

It wasn't the battery incident, its just a bulb or connection.

All the main pod switches are illuminated - you know this right? with the switch out - test the switch. Test the bulb - it is located between pin K & 31. Pin 58 is the normal illumination input - (K is constant high illumination from the rear defroster relay when high def mode (push) is on) there is a diode & resistor in the switch for low variable illumination - you should have connectivity with some small resistance between 58 (+) and K (-) and Between 58 (+) and 31 (-) - IF the bulb is still good.

...Or just pull the cover off the def switch and remove the bulb (use some plastic tubing to grip it) - blow out the socket...

Alan

Ed Scherer

One other thing... if/when you pull the bulb out (using plastic tubing as Alan mentioned if necessary; they're hard to get a grip on!), shine a flashlight in there and take a close look at the contacts in the socket. IIRC, they're rather flimsy. If somebody really jammed a bulb in there without making sure it seated correctly, it could bend the contacts enough to make them touch. Likewise, the bulb leads themselves could be bent in such a way that they are shorting out.

If/when you pull the bulb out, take note of the position of the leads. The process of extracting the bulb might straighten them out again, possibly covering the tracks of a potential short, but it's still worth a close examination.

Manfred

Thanks Alan. I'm not sure if you understood my last post. I was worried the battery incident may have caused the digital display to falter--the one on the right ( either the tripometer, or stop watch depending on which is selected via the computer) is dim on the right hand side now.

As far as the defroster switch goes are you saying the switch itself could be the cause of my short? I can definitely test the bulb but I'm not sure what you're thinking.

Dan

Manfred

OK. That's Ed. So I guess you and Alan both think the switch itself could be the cause of the short. That would be nice--I don't want to have to take the pod off again. I swear every time I fix one thing another thing breaks.

One thing I did notice which was strange. The female receptacle (plug) for the switch has one less metal contact than than the switch has male prongs. Is this normal? I didn't see a loose wire dangling back there where the female part fell out of the plug. IIRC, it's contact #88b.