1985 3.2L Carrera - Interior Light Circuit

[cfleury14]

Hello all - I have read all the threads on this that I could find, but still having issues.

I am repeatedly (intermittently, not immediately) blowing the 5A fuse in the interior light circuit in my 1985 3.2L carrera. I know that the following loads are on this circuit:

Frunk light
Glovebox light
Dome light x2
Clock

I have removed and inspected the dome light and glovebox light fixtures and everything looks good, tested with multimeter and nothing shorts within the switches themselves. I have felt inside where they mount and dont feel anywhere the hot wire could be shorting to the body. I have also disconnected the frunk light permanently.

However the darn thing continues to blow the 5A fuse intermittently. The lights work for 20 minutes or so, then the fuse blows. Using a DMM, I have confirmed that when they are working, the circuit is only drawing a few hundred milliamps - so it's not like it's just continually shorting. Tells me that something is intermittently shorting to ground.

Having looked at everything else, it seems like it's time to disconnect the clock and then see if the fuse blows. (The clock has never worked as far back as I can remember, which seems suspicious). In taking out the clock, I came upon this red cylindrical spring-loaded electrical connector that I have never seen - anybody know how to get it off without breaking it ? (picture below)

Any other thoughts as to what may be shorting? (I know the door switches have been a hot topic of debate in previous threads, but I believe that a bad door switch would result only in the light staying on all the time, NOT a short to ground, since it is on the ground side of the load (light bulb).

Thanks as always

Chris

[bmcfarland]

The red cylindrical spring-loaded electrical connector in your photo is the light socket. Gently pry between the metal ring and black holder to remove. However, instrument lighting is on a separate circuit (black-blue wiring and unfused unfortunately), so it’s probably not the source of your problem.

I have some of the 3.2 Carrera wiring diagrams. Send a PM and I can e-mail them to you.

[mrdstr]

the door switch, if loose, can arc against the body and blow the fuse. I would make sure those are tight.

[GH85Carrera]

On my 85, there is a little white connector at the bottom of my photo. I have always assumed it was for the mechanic to pull apart and kill the dome lights, and trunk light and of course the clock. On my car, that little connector was going bad, and had intermittent issues. It drove me nuts until I talked to a old time Porsche repair master mechanic, and he said that connector is important and to check it. I ended up having to replace it as mine was just not stable.

It is handy to shut off the trunk light and dome light when working on the car.

[cfleury14]

Thank you all for the replies. Tomorrow I will check for that little white electrical connector just below the 5 amp fuse to see if it is present, and if so if it is bad. I upgraded my fuse box to the blade fuse box 10 yrs ago so it is laid out a little differently but all of the circuits are the same. I have studied the wiring diagrams in the Bentley manual but so far dont see anything else jumping out at me. Bmcfarland - thank you for your offer - I have DM'd you to see if there are any other wiring diagrams that may be of assistance.

I did have one more thought. I completely disconnected the frunk light over a year ago, when I began chasing this intermittent electrical gremlin. I was in there replacing my windshield washer check valves last weekend and I happened to look at that wiring, and I noticed that I can not find the hot wire that used to go to the frunk light. It could very well be that wire that is bouncing around in there and intermittently shorting to ground, since it is now disconnected and unaccounted for. Does anybody have any pictures of where that wire runs, how it gets to the frunk light, what color it is, etc? (Of course color may have changed but will be a good place to start)

Thank you all - hopefully getting closer ...

Chris

[spunky.goat]

I had the same issue and it turned out to be the glove box light. My problem was that the fixture was not getting fully seated into the opening which would allow it to move. The movement was enough such that one of the contacts was able to contact the metal dash.

[bmcfarland]

My car is also an 85 and the trunk light is wired as shown in the photo. An excerpt from the applicable wiring diagram is also included and the complete pdf version of the Porsche-issued wiring diagrams for MY85 has been e-mailed to you.

I hope this helps. Good luck!

[bmcfarland]

Sorry, the previous version of the photo had an error which is corrected in this one.

[cfleury14]

Well I am still hunting down this problem. At this point I have both dome light fixtures out of the car, as well as the glove box light fixture and the clock. The fuse continues to blow. I have studied the circuit diagrams and have come to the conclusion that the door switches can't possibly be shorting if the light fixtures are not in place (because they just ground the fixtures), but I am going to take out the door switches today and make sure they dont have a hot wire or something obvious touching them.

It seems to me that the most likely problem at this point is the missing hot wire that goes to the frunk light - cant find that thing for the life of me.

I have ordered a 12v buzzer that I am going to use as a test buzzer - I will just take the fuse out and wire it across the fuse, and whenever the circuit is shorting to ground the buzzer will buzz and that way I can work around the car and manipulate various things to see what makes it stop buzzing. (basically analog of a test light, but dont have to be able to see into the frunk to know if its on or not). Does anybody have any other ideas?

[bmcfarland]

When you remove the door switches, be careful to not allow the ground wire to fall into the body panel. Others on this forum have mentioned it is very difficult to fish out.

I will attempt to shoot and post photos at the firewall of the primary black wire running between the frunk light and the glovebox switch.

[bmcfarland]

The wiring diagrams show the primary circuit comes from the clock, to the glovebox light, and finally to the frunk light. However, the photo of my MY85 seems to indicate my frunk light is being fed first, then the glovebox. In spite of that apparent discrepancy, I hope this helps when to continue your efforts to locate the frunk and glovebox light circuit in your car.

[cfleury14]

Wow - interesting. Mine looks slightly different, I think. Wondering if yours is the same and I just can't see it from the picture, or if it is indeed different and causing the problem.

I have the same wire that runs along the firewall from the clock to the first wire connector (green arrow). BUT in mine this is a bundle of two wires - a black wire (orange arrow) and a brown wire (yellow arrow). The black wire goes through a connector to another black wire, which then gives off the supply to the frunk light (blue arrow) and then continues into the harness penetrating the firewall to the glovebox.

Back to the wire along the firewall from the clock - there is also a brown wire in this bundle (yellow arrow from before). The brown wire goes to an inline spade connector (nothing connected, black arrow), then through a connector that joins it to two wires: (1) a brown wire (double black arrow), which runs through the firewall separately. and (2) a brown/white wire, which runs into the same harness penetrating the firewall to the glovebox (double blue arrow).

Photo below, with the single and double arrows identifying the various points of the wire:

But to me this all makes sense, and I dont see any obvious source of a short from the wiring in the picture. Please let me know if you think different.

[bmcfarland]

Agree, given the nest of wiring, photos are difficult to follow, and I did not label the ground wiring. However, your primary circuit appears to be like mine - black from clock to glovebox light with a black-white branch to the frunk light.

Somewhat following the component isolation process you have been using, can you unplug the connector, thus disconnecting the frunk and glovebox lights, then see if the fuse still blows?

[cfleury14]

Just tried this - unplugged that connector so that frunk and glovebox are out. Fuse blew within 5 seconds.

My test buzzer and some more fuses are arriving tomorrow - hopefully will be able to make more progress then

Presumably the clock wiring comes off before the frunk and glovebox (despite what the circuit diagram shows). Do you know what part of the circuit the right and left dome lights come off? I would guess the left comes off the clock wiring before the connector I unplugged, and the right comes off past the glovebox light, but I am not sure

[bmcfarland]

Electrical shorts are the worst. Sometimes it’s best to step away for a period if marriages and religions are at risk.

The red-lined wiring diagrams indicate the clock (82) gets fed first, then the left and right interior lights (16).

A branch connection on the clock feeds the frunk light (18) and finally (as we discovered) the glove box light (20).

More experienced folks need to weigh in, but your continuity check of the harness, with everything disconnected, seems like the next logical step.

[bmcfarland]

The focus of attention has been on the primary circuit, which seems to make sense when looking for a ground fault that is blowing the fuse, but does anyone know how a malfunction in the alternator diode (911 617 108 00) and clock relay (83) that are tied to the door switch ground circuit (blue-lines) influence current flow?

[bmcfarland]

Sorry, cfleury14, I’m not trying to hijack your tread, but you have clearly checked the obvious. With my curiosity now getting the best of me, I’m wondering about the less obvious.

The wiring diagram in my previous post covered “Illumination (USA,Japan)”. It showed the alternator diode and clock relay on the interior light ground circuit (your clock reportedly doesn’t work so could you have a clock relay problem?).

The wiring diagram in this posting is “ROW” and is different. It shows the window relay on the interior light ground circuit. I know from experience that if the door contacts are not functioning properly (in my case because they were oxidized), the windows will operate with the ignition key off and parasitic battery draw occurs. That seems to make sense if the interior light ground circuit includes the window relay???

So now I am not sure how the ground side of the circuit you are troubleshooting is actually wired in our USA spec cars, or if your over-current problem could be related to the aforementioned components on the interior light ground circuit???

[cfleury14]

Very interesting thoughts. I am at work today so won’t get a chance to work on chasing it down until tomorrow. Will keep you updated. Thinking about checking continuity along the harness as a next step, which is a bit more work but seems like the next logical thing.

You are right about electrical shorts - they are frustrating and sometimes I just need to step back for a bit before getting back into it. The good news is that everything else on the car works perfectly, I just put my momo prototipo steering wheel in and love it, it’s my daily driver and I am lucky to have it running so well and reliably. The dome lights are a small nonessential part. Occasionally zooming out helps with the frustration of these electrical gremlins

I will do some more investigating tomorrow, and will keep everybody updated!

Chris

[wazzz]

Originally Posted by bmcfarland

... It showed the alternator diode and clock relay on the interior light ground circuit (your clock reportedly doesn’t work so could you have a clock relay problem?)...

The clock relay actually has nothing to do with the clock, except for the name. The clock works by itself and needs no relay.
This relay is also called warning relay, warning chime, seat belt control unit, etc. in the Bentley manual.

As for the diode (just an ordinary diode, not an alternator diode), it seems to only appear starting from 86 models. As far as I understand it, it is used to separate (isolate) ground provided by door contact switches (when a door is open) from ground provided by seat belt contact.

Here is another schematic from Bentley manual.

[bmcfarland]

Thanks wazzz. Further study of the wiring diagram yielded the same conclusions. Although I’m still confused about USA,Japan (where the diagrams show the “clock relay” and diode on the interior lighting ground circuit) vs ROW (where the diagram does not show those components, instead shows only the power window relay is connected). In either case, do you think it is an electrical possibility for a malfunction in that ground branch to cause the over current situation cfleury14 is attempting to solve? I don’t want to send him down a rabbit hole.