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Dash lights only work when lights off

 
Old 11-09-2010, 05:20 PM
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Oli928
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Default Dash lights only work when lights off

Okay, the title to this post sounds like an oxymoron...let me explain. The instrument/dash lights to my 1989 928 turn "off" once you rotate the headlamp/parking light switch. In other words, they work when you put the key in and start the car and work the whole time you are driving; they also work once you exit the car until they time-out or you lock the door, but once you turn on the parking lights or headlights, the lights to the instrument panel go out.

Items to note:
1) The computer display still works...always
2) The thumb-wheel under the dash has no effect when rotated (whether the instrument lights are on or off).

I searched the forums for this exact problem and couldn't find.

Should I pull the headlamp switch and see if a connection is backwards there?
Any clue where to start?

Thanks!
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Old 11-09-2010, 05:31 PM
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I have the same problem, although intermittently, so I'm curious to see possible solutions!
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Old 11-09-2010, 06:09 PM
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Not sure what the failure mode is but I know that many people end up needing to replace the rheostat dimmer switch (thumb-wheel). Do a search for the display dimmer switch (not sure about what the exact terminology there) and see if the symptoms match.
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Old 11-09-2010, 06:52 PM
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I wonder if in the interim, i can simply remove and bypass the dimmer. That way, the lights would always be on and at full illumination until I get the replacement dimmer from Roger. If the problem reoccurs... it ain't the dimmer...
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Old 11-09-2010, 06:54 PM
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That's the way it is supposed to work. Bright during the day (without lights on) so you can see the instruments, less bright at night (with lights on) as not to ruin your night vision. The reostat should work though. You might take it off and clean the contacts, check the resistance with a multi meter, otherwise buy a new one.

On a '89, there's also a light sensor on the top of the dash (above PDRN21) which regulates the brightness. Shine a flashlight on it and the lights go brighter. That's to compensate for sunshine on the dash.
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Old 11-09-2010, 06:56 PM
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until I get the replacement dimmer from Roger
Have you confirmed that your symptoms match those of a failed dimmer?
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Old 11-09-2010, 06:59 PM
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Those darn 1989's


'89 S4 GP White/Black
'76 912E Silver/Black
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Old 11-09-2010, 07:09 PM
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Originally Posted by Manfred View Post
Have you confirmed that your symptoms match those of a failed dimmer?
no, but I have confirmed that I don't want to dismantle the pod, and that's the easier way to get started... Do you have another troubleshooting suggestion, because if you do, I would be grateful to hear it !

:-)
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Old 11-09-2010, 07:23 PM
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Forgive this ignorant question...I have not twisted my body into a figure 8 to look at the dimmer; is this piece removable without tearing apart the pod? When the rheostat fails, is this the symptom?
To clarify, at night (headlights "on") the dash lights are zero. I had a 944 before, but not a 928 / I didn't know the dash light is normally on. I'll check the light sensor as well.
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Old 11-09-2010, 08:08 PM
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Originally Posted by Oli928 View Post
Forgive this ignorant question...I have not twisted my body into a figure 8 to look at the dimmer; is this piece removable without tearing apart the pod? When the rheostat fails, is this the symptom?
To clarify, at night (headlights "on") the dash lights are zero. I had a 944 before, but not a 928 / I didn't know the dash light is normally on. I'll check the light sensor as well.
I'm not positive, but I think a couple screws hold it in place and it can be remove without total pod demolition.

EDIT: I'll know tonight, when I bypass the rheostat.
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Old 11-09-2010, 08:18 PM
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Originally Posted by Oli928 View Post
Forgive this ignorant question...I have not twisted my body into a figure 8 to look at the dimmer; is this piece removable without tearing apart the pod? When the rheostat fails, is this the symptom?
To clarify, at night (headlights "on") the dash lights are zero. I had a 944 before, but not a 928 / I didn't know the dash light is normally on. I'll check the light sensor as well.
go here...

https://rennlist.com/forums/928-foru...cs-part-i.html

and scroll down...

Dwayne does probably the most detailed and awesome step by steps here...
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Old 11-10-2010, 10:56 AM
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Thanks for finding the link to that post. Once I have time, I'll give it a *shot (*bypass dimmer/trouble shoot dimmer) and post the result. That link also answered one other issue (the scraping sound in the steering area). I always wondered what that was; I figured it was just some piece of plastic that deformed with age and/or heat.
...from reading other posts, the dimmer also looks as though why the back-lights to the a/c controls are out...besides a possible wire unplugged near the clock.
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Old 11-10-2010, 11:54 AM
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My 89 has the same intermittent problem, with headlights on, the dash lights go out. When the dash lights are out, none of the lights in the center console work either. On the times when the dash lights do work with headlights on, the console lights up (HVAC head being the most notable). I am pretty sure my problem is in the dimmer, because every once in a while I can get them to come back on by working it back and forth quickly. I would try bypassing the dimmer and see if that fixes it.
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Old 11-10-2010, 12:30 PM
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Originally Posted by tlsmith1999 View Post
My 89 has the same intermittent problem, with headlights on, the dash lights go out. When the dash lights are out, none of the lights in the center console work either. On the times when the dash lights do work with headlights on, the console lights up (HVAC head being the most notable). I am pretty sure my problem is in the dimmer, because every once in a while I can get them to come back on by working it back and forth quickly. I would try bypassing the dimmer and see if that fixes it.
dimmer bypass didn't work. Anyone have any other suggestions? I'd love to hear them.

Thanks,
R
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Old 11-10-2010, 12:56 PM
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The only other items I saw during my 45-minute search to see if this topic already had a thread was:

1) Apparently there is a cluster of wires/connections (behind/below?) the clock...someone said to pull the ashtray unit out and check all connections behind there and clock area. Also, maybe something got disconnected during radio swap/re-installation work?

2) Another post said to pull every fuse related to lights and not just check them visually, but check them with a meter for continuity.

3) ...the obvious, check to see if wire at dimmer is shorted to ground or has too much resistance (dreaded green wire syndrome). My wire to the fuel pump had the green monster...had to run new one a few months ago.

4) Possibly the light sensor above the gear indicator lights is causing a fault (I may have misread this...might be related to some other issue). The person said to shine a flashlight unto the sensor to see if the dash dims when they are on.

5) Related to stereo work / someone may have tried to connect an aftermarket radio's light to the headlight switch fuse area and shorted/disconnected something by accident.

Again, I only searched for about 45 minutes; there may be other threads that answer this better.
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