Captain_Slow

So I drove my 928 at night for the first time since probably Frenzy last September. Between then and now I've had the console out, I've fixed up wiring that had been cut and spliced, etc. I've replaced the old Sony head unit with a new Alpine. And I gave the CE panel a Deoxit spray bath, with all the relays and fuses removed, then put them all back.

I can't remember the actual last time I drove it at night. The HVAC unit is also not illuminated, but the AC switch lights up when pressed in and the compressor comes on (the climate control unit probably needs a bulb). Because the ashtray light is on I have ruled out fuse 44 (and I just cleaned up the CE panel a few month ago).

Looking for some suggestions on what to try next.

Note: It hasn't happened recently, but there have been a few instances when I had to wiggle the key to get lights to come on, and included other functions - headlights, tail lights, brake lights, marker lights, window regulators, etc. This appears to be only the instrument lights. Still, could this be caused by a bad ignition switch?

Thanks,
Jon

MainePorsche

Interesting, the hook up for the ash tray light. I remembered something about this years ago on a posting.
The ash tray bulb gets its feed through a BK/BL wire DIRECTLY from the CE panel at F24. It does NOT go through the potentiometer as all other BK/BL wires for pod/console illumination. This would result (in the functioning state) all the illumination would dim when on except the ash tray and the headlight pod switch.

Check your dimmer rheostat. There should be 2 BK/BL bundles attached there for output to the bulbs (one for pod/one for center console). The one BK wire spade that should be there is power in off fuse 44 off F24 at the panel.

Captain_Slow

Thanks Craig,
I found the old thread referencing fuse 44. The fuse chart also shows instrument illumination and ashtray getting power from fuse 44. So down that rabbit hole I went.

The observed behavior would make sense if after coming off fuse 44 power goes to the potentiometer/rheostat then on to the bulbs for illumination. It would also explain why the HVAC control backlight is out.

I'm not sure what you mean by F24 at the panel. Is it another fuse that provides power directly to the ashtray light? If so, the fuse chart I downloaded from a vendor site is incorrect.

I'm pretty certain I did have dash lights post all the wiring clean up I did while the console was out for HVAC actuator replacements.

So now I'm thinking either power isn't getting to the rheostat or there is problem with the rheostat. Time to go check fuse 44 first.

Jon

MainePorsche

Jon,
It is 44 that regulates output to these bulbs. On the WD for MY 88 you can see fuse 44's output within the CE 'box' feeding F24 at the socket there. From there, there is one Black/Blue line directly to the ashtray, and one Black line that feeds the potentiometer.

MainePorsche

G33 is the page coordinate for the feed to the ashtray.

Captain_Slow

I forgot to mention my light switch (headlights, etc.) is not illuminated. I'm a novice with wire diagrams, but would 44 have anything to do with this switch is not illuminated?

MainePorsche

The headlight switch bulb stays illuminated all the time. Safety thing I guess if driving during the day with headlights off, then entering a dark tunnel so one can see the switch. If your's is off, likely the bulb.

MainePorsche

You can see on the WD above the headlight switch bulb (circle with the x in it) is powered by a BK (black) line from a completely different source.

Captain_Slow

Craig - great info on the headlight switch always illuminated (and also likely to be blown bulb...similar to the D indicator in the cluster being illuminated for long durations). I checked fuse 44. It is pristine. I think my rheostat was a bit finicky - rolling it would result in some dimming but it could also cause the illumination to go out until I rolled it enough to bring them back. Very erratic. Could the cleaned CE panel result in the full current (that it hadn't seen in many years) taking out a rheostat that was on its last legs?

I've studied the diagram above. It makes sense. But I'm still not sure what F24 is - though I do understand that it sends power to the ashtray bulb directly and another wire to the rheostat. Can something be wrong with "F24" such that it doesn't send power to either or both the ashtray bulb and rheostat? Or, does the fact that the ashtray bulb is working suggest the rheostat has failed?

And.. Does the rheostat also provide illumination dimmer control to the climate control unit and the transmission selection indicator at the shifter?

Captain_Slow

Craig - I found this related thread (includes a post from you) about removing, cleaning, testing the rheostat (by jumping the wires and looking for lights). This is my next task.

https://rennlist.com/forums/928-foru...ht-dimmer.html

MainePorsche

Here's a pic of the CE panel. The sockets are seen at the bottom and are labeled by letters. Each sockets has 10 male spades/pins in it. The five on the left are labeled 11 - 15 from top to bottom, and the five on the right would 21 - 25 from top to bottom. Hence F24 would would be second pin up from the bottom on the R.

If you're good at the ashtray, it means that current has come through F24 at the panel socket. If your lights don't work on the pod/center console the defect is either in the Black line to the rheostat or the connection of the Black/Blue lines coming off the rheostat.

Yes the HVAC unit should dim.
If you have illumination at the shifter for an auto, it had been added on later. The PRND at the shifter was not illuminated at the factory. I did mine off the clock or cig lighter Black/Blue.

MainePorsche

Just a thought on why the cig lighter remains at max illumination when headlights are and interior lights (possibly) dimmed - a little safety thing like having the headlight switch always illuminated. Possibly the engineers didn't want us to poke around with the hot element trying to replace it in the receptacle in the dark.

Captain_Slow

Thanks Craig. I had a feeling F24 was related to the CE sockets. I noted the lettering when cleaning the CE. OK...looks like I have to investigate the wiring.

I really appreciate all the detail you've provided. Hope this helps others down the road.

Captain_Slow

It was the dimmer potentiometer. I followed a post suggesting I remove the connections from the dimmer and bridge with a 5 amp fuse. Instrument lights restored. Very dirty. No hurry to replace it since it adjusts between dim and dimmer.

Ed Scherer

That's one horrible looking rheostat! Has your car been under (sea!) water or something?

It's worth an attempt to resurrect it, though. Quoting my own post from a while back:

That one is bad enough that it would probably be good to do a two-pass cleaning/conditioning: pass one with loads of contact cleaner to flush away as much crud as possible, and pass two with some 100% DeoxIT.