Popoboy944

I finally have to waive the white flag and ask for more ideas on this one. hopefully someone out there has an idea. I'll start at the beginning so people know what i have done so far.
-intially my central indicator light was on every trip, with no other lights accompanying on the dash.
-found that bulbs were not installed in the dash for the stop lamp or taillight indicators.
-added bulbs, both stop lamp and taillight indicators were on in dash now on startup.
-head/tailights were not on at startup, which means taillamp circuit shouldnt be being tested, giving me a clue something might be wrong with the bulb control unit.
-went through stop lamps and taillamps anyway and made sure all bulbs were matching and had match pairs anyways, as this is also needed.
-still had stop lamp and taillight warnings on dash after bulb checks/replacements.
-pulled out the bulb control unit and inspected. found water had entered the cover and all components were completely rusted out.
-ordered good condition used ,control unit and installed. taillight warning now gone. stop lamp warning functions properly except it comes back on when door opens!!!???
-if i have all 5 interior dome lights set to come on with the door switch, every time i open either door the stop lamp indicator will come on.
-if i turn just one of the 5 switches to off, or on all the time, and open the door, the stop lamp warning DOES NOT come on.
-if i start the car, extinguish the stop lamp warning with brake press with the door open, then take out one of the stop lamp bulbs, then ,close the door and reopen, the stop lamp warning DOES NOT come on.
-i started thinking it was a voltage drop thing with all the lights coming on, so i recleaned all the interior light wire and bulb connections, as well as the 2 grounds above the fuse panel, which i believe one of them goes to ground for the bulb control unit. issue persists.
-i took apart the tail lamp wiring harness connector and cleaned that, as well as the contact points for the stop lamp bulbs. issue persists
-i found one of the contactors to the passenger side stop lamp bulb had a piece that cracked off, so that the bulb was only making contact through a small area of the contactor. i tried to improve the connection there by bending the contactor to make better contact with the bulb, but issue persists.

I would like to try a quick swap of tail lamp assemblies locally to rule out if the contactor is the issue. anyone in the detroit area up for a meetup and quick swap? no tools required, just fingers, and takes about 5 minutes.

Barring that, is there anything else im missing to try?

Mrmerlin

you have one or more of the 5 interior lamps mis wired,
though the door lamps dont matter post them anyway
disconnect the battery,
pull them out and post a picture of the back side of each one,
lets see if we can find it
You may need to replace a pin in the harness connector.
and usually its the rear side marker connectors that cause tail lamp issues

Popoboy944

Thank you for the help @Mrmerlin ! I did not notice anything amiss in the wiring when I took the pictures, but I am not the expert! Also just point of clarification, the stop lamp warning is the issue. Not the tail lamp warning.

Front dome


Back seat dome


Hatch


Drivers door


Passenger door

gazfish

Another suggestion, if you flash the full beams for example does that trigger the brake warning light. As you said it could be any voltage spike triggering it and not linked to the interior lights directly.

Mrmerlin

OK the interior lamps look like they are properly wired,
I would suggest to replace the 2 center lights with Jetta versions,
the Jetta lamp wont fit in the aft hatch.
Also put heat shrink any ground/ positive wire connectors so they cant ground .
Next check the connections in the 3 rd brake light, replace the bulb,
and the connections of the 2 pin connectors in the corners of the hatch floor for the side markers.
Use Deoxit 100 on everything you touch

Popoboy944

thanks for the idea on this. I previously found the thread with the technical bulletin of this light coming on with the headlights. I tried turning them on and it doesn't trip the warning. Only happens on door open, with all 5 lights set to the door switch. In that state it happens 100% of the time on door open.

Mrmerlin

the driver door harness may have a broken wire in the harness sheathing tube

Popoboy944

tonights update. thank you all all so far for the ideas, but no dice yet. i took apart, deoxited the 3rd brake light connections, and replaced the bulb. i also unhooked all of the permanent grounds to the 3 interior lights. no change at all.

@Mrmerlin im not sure why i would look at the side markers, theyre off, and i didnt think are involved in the stop light circuitry. what is the connection to the stop lamp circuit?

Also, not sure how i would check for a broken wire in the door, but in case it matters to your thought there, it does not matter which door opens, either door opening sets the light.

one test i did tonight that gives another clue was to open the hatch. the stop lamp warning did not come on. the 5 interior lights switch on, same as the doors, but the one difference i noticed is that the chime for an open door did not come on with the hatch open. seems like maybe i should check the circuitry going to the module contolling the chime as well.

next thing i can think to do is hook the door switched grounds to where the permanent grounds usually connect on the frame of the light to rule out the switch, as i dont have any of the vw lights.

any other ideas?

Mrmerlin

Connect the side markers they are part of the bulb warning circuit disconnect the hatch latch ground a 2 pin connector at the base of the receiver and leave it disconnected

Alan

Its a grounding issue. Clean all the ground points.

Whenever unrelated systems start interacting it is 99% of the time a grounding issue (the 1% is when they are actually related in some way that you don't understand yet).

Alan

dcbullet

My post isn't to give additional advice but encouragement. I had an issue where turning on my A/C would trip the stop lamp bulb warning light. Replacing the taillight harness ultimately fixed the issue.

Popoboy944

Update from yesterday's work. Thanks all for your support. I unhooked the permanent ground and swapped the door switched ground to the normal permanent ground location on all 5 interior lights. As a way of trying to test the switches in the lights themselves. No change to the issue. I also unhooked the chime module, no change with this either.

@Mrmerlin the sidemarkers have been connected, and functioning the entire time, i also checked and matched bulbs on these previously. I will try unhooking the hatch switch ground. This will lead to no lights on if i open the hatch, correct?

@Alan I have cleaned the two grounds above the ce panel, as well as almost every one labeled "MP" in the wsm in the past year. I have also been through the door pin switches recently as well. I will most likely do the door pins again. Are there any others in particular you think I should focus on?

@dcbullet you're encouragement is appreciated, and honestly lead me to circle back to the one thing in the whole system that seems less than perfect. In the passenger side stop lamp bulb receptacle, one of the contacts to the bulbs has the end cracked off. This sure seems like it could lead to a weak ground issue. I would really like to try a quick swap locally, it requires no tools and only about 5 minutes. Any s4 owners in the Detroit area up for a meet up and swap?????

PC-85-928S

sounds like a NAPA plug splice situation

(can't someone make a reasonably priced aftermarket S4 tailight harness?)



https://rennlist.com/forums/928-foru...l-sockets.html

gazfish

On my model year the interior lights and bulb check box use the MPV ground, along with a bunch of other stuff. It’s above the fuse panel so sounds like you did that but you could check the resistance to be sure.

Popoboy944

this was my understanding too. I did just clean MPV while working on this issue according to this picture for an 87. Checking resistance is a good idea though
https://rennlist.com/forums/928-foru...-on-an-s4.html