Michael Benno

In my 88 S4 with sunroof, I am having an issue with the two overhead rocker light switches. When turning on the overhead lights with the ignition on and doors closed, the light is very dim. However when I have the lights switched to the "activate when doors are open" mode, the lights illuminate full brightness.

This only happens for the two overhead lights, the door lights are full brightness in both modes.

I do have LED bulbs installed in all locations. These bulbs do not require polarity, they work in both directions.




I am wondering if this DIM mode an actual feature, or is there an interaction issue with the LED bulbs? Or is there something more devious going on with the electrics, or possibly the relay?

I am not sure how to test this with my multimeter. When I tested using volts across the switch (no bulb) I got about 12v on the door open setting and -12v on the ignition on setting. Seems like the circuit is providing the correct voltage. Not what I expected. What other test should I run?

I have researched a bit and found serval people were using the same LED without issues.

Thanks in advance.
Mike

docmirror

It's just corrosion along the path somewhere. a bit of higher resistance in one leg than the other. Chasing it down could be a challenge.

martinss

I agree with doc, but the first place I'd look for a path difference is the switch in each lamp. When you throw the switch a contact moves between two parts of the lamp - one connected to a permanent ground and one to a switched ground. I found some corrosion, but also some erosion of the contacts from sparking, likely due to poor contact and/or corrosion, in some of my lights. They would work in one setting, but not in the other, until I cleaned, filed and bent all the contacts to make sure they touched firmly.



The bulb holder actually rotates a bit as you move the switch. The design is poor IMHO. One of the reasons others have moved to newer VW lamps.

Alan

From what you describe the relay and pin switches work fine - they control the door switched mode. If that mode works fine then the +ve supply works fine too. Your problem seems to be in the ground line for the roof which is used for the local switched mode (brown wire). Try adding a backup connection to a solid ground to prove this is the cause - then trace the roof ground...

Alan

Michael T

Since the ground completes the circuit, this is very likely a ground issue. The door ground being good (the door switch), but not otherwise.

Michael Benno

Thanks for the suggestions on testing grounds.

I did a little more sleuthing yesterday. Pulled the roof lights. The front light is connected to the rear light and then they both go to the ground point trunk in front of the battery (MP VI). I tested for resistance between from the ground leads to the main battery ground point (MP V). The tests showed no resistance. So that tell me there is not an issue with grounds. Correct? FYI, I did take the opportunity to clean/Deoxit both ground points.

I gave the lights a good cleaning and Deoxit treatment on both lights and re-assebmled. Now the ignition setting on both lights does not work at all. Grrr.

One thing I noticed was there were some non-stock wires connected to the rear lid light. I want to pull the trim panel to do some additional inspection. I think I may have pulled a wire loose somewhere upstream. How do I remove this trim panel? I removed the sun visors but can't figure out how to remove it and the WSM is useless.

Alan

Someone has messed with the connectors (at least), makes you wonder why - see what else is connected?

Your description means there is an issue with the brown wire grounds - so keep looking. You are looking for resistances of a few ohms - some meters won't detect this very well, you are better just doing the test as proposed. It is easy to do once the lights are out.

You talk about "ignition mode" - but there isn't one...?

Lights are either off permanently, on permanently or on when a door is open - all controlled by the switch unique to each lamp - the ignition state doesn't enter into it . The extra resistance could be in the switch or the wiring - but if both lamps behave the same way then it pretty much has to be the wiring...?

Another thought - remove the bulb from one roof fixture and test the other one as before - what happens in the different modes?

Alan

j.kenzie@sbcglobal.net

Hi Michael,
That blue spade connector with a brown and a black wire should be a constant ground. It does look like it has been extended, so any of those splices could be a problem.
(Sorry this is only on my 911. The 928 may have non-metallic mounting surfaces. BTW my switch grounds perfectly well through the body of the switch touching the sheet metal. I don't even have a constant ground.)
Good luck,
Dave

j.kenzie@sbcglobal.net

It also looks like the white wire connects to both the constant ground terminal, and the switched ground terminal. That can't work properly. The connector toward the front in your picture connects to the body of the switch and should have a constant ground connection.
Good luck,
Dave

Michael Benno

I'll do the direct ground connection test, I couldn't find my alligator clips and didn't have enough hands. I'll have to take a look at it again later this week.

@Alan, in my car the non-door open position would only illuminate with the ignition in the on position. Are you saying that this position should be illuminated regardless of whether the key is on? This could be a clue!

Alan

Yes - the permanently on position should not be dependant on anything else. The power in this circuit does not come from an ignition switched source. The switching for the lamps for all modes is on the ground side - so it is very odd that there would be any interaction with any ignition modes.

Alan

Michael Benno

Thanks for all your tips. I figured out the problem was as simple as the light switch over the rear seats was plugged incorrectly. 12v was going to connected to the spade connector imbedded in the plastic, rather than the correct spade on the opposite side.

It took me a little of connectivity testing with the switch and a multimeter to figure out how the switch worked and thats when I realized it was plugged in incorrectly.

Once I had it plugged in correctly, the switch now works correctly in all the settings (same brightness) and I wasn't having the weird, and incorrect, behavior of only working with the ignition on.

Thanks for all your support, I was able to clean up all the wiring and replace the connectors with stock style female spade plugs and heat shrink. Much more stock looking now.


Mike