Blowing (R) parking light fuse

[CarlosDanger]

Hi there,

I recently picked up a '80 SC. The right side parking light isn't working. I checked and the fuse was blown. Put in a new fuse, turned on the lights and it blew again. I replaced the bulbs and fuse, turn the lights on and it blows the fuse.

I'm very novice when it comes to electronics. But this is the information I have so far.

- No voltage at the sockets.
- Voltage at the fuse.
- Fuse connections look clean.

It seems like something is getting grounded somewhere?

Any help is much appreciated.

PS. I'm having a terrible time finding the yellow 5 bullet type fuses. Where do you guys pick them up?










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[DRACO A5OG]

I would remove the light assembly and look behind the back panel.

You may have a wire that is shorting out in the assembly. The connectors are mounted on a PCB like plastic board that becomes brittle over time and constant replacement of the bulbs.

[CarlosDanger]

Originally Posted by DRACO A5OG

I would remove the light assembly and look behind the back panel.

You may have a wire that is shorting out in the assembly. The connectors are mounted on a PCB like plastic board that becomes brittle over time and constant replacement of the bulbs.

Thank you. I will check tomorrow.


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[930T86]

Fuses sold by Porsche dealers, part number: 900 173 006 90 (10 packed in each bag)


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[Marc Bixen]

Also check for a short or pinched wire at the engine compartment light on the engine lid. It's hooked up to the p-light circuit.

[CarlosDanger]

Well not much luck. The tail light assembly is pretty much rusted shut...



And my car had a whale tail delete, so no engine compartment light.



The wiring out side of the assembly looks alright.



Seems like next steps might be to get into the assembly which I may have to replace after.

I don't see any obvious wires dangling around where the engine compartment light would be.


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[timmy2]

Unplug the six pin connector to the rear lamp inside the engine compartment by the rear engine mount, and see if when you replace the fuse and turn on the lights, that the fuse holds. This will ensure the problem is isolated to the rear lamp fixture.
If the fuse holds, you have found the culprit and will need to remove the lamp assembly and fix it.

[CarlosDanger]

Originally Posted by timmy2

Unplug the six pin connector to the rear lamp inside the engine compartment by the rear engine mount, and see if when you replace the fuse and turn on the lights, that the fuse holds. This will ensure the problem is isolated to the rear lamp fixture.
If the fuse holds, you have found the culprit and will need to remove the lamp assembly and fix it.

Will do, thank you. I pulled that connector and re wrapped the wires but did not test the fuse. First thing after work tomorrow.




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[Cajundaddy]

Chaffed wire or connector is shorting to ground. Pay particular attention to where wires run through a bulkhead or firewall. Rodents will often chew and expose wiring while gaining access.

[CarlosDanger]

Originally Posted by timmy2

Unplug the six pin connector to the rear lamp inside the engine compartment by the rear engine mount, and see if when you replace the fuse and turn on the lights, that the fuse holds. This will ensure the problem is isolated to the rear lamp fixture.
If the fuse holds, you have found the culprit and will need to remove the lamp assembly and fix it.

Unplugged the rear lamp and still blew the fuse. What is next? Hunting down where the engine compartment light was?


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[timmy2]

Well, at least you know it isn't the rear lamp.
On the drivers side there is the same plug. The engine compartment light is usually spliced into the harness in that location and runs from there behind/under the fuse panel and up to the hinge area to the lamp location.

But, before going there to check things out, I would suggest you get yourself an ohmmeter and test/confirm the bottom of the fuse holder for continuity to ground. One of the two wires (for front and rear running/parking lamps) is grounded somehow/somewhere.

You need to isolate which one by removing each from the fuse panel, separating them and testing each to ground. Once isolated as to which wire is the culprit you will know if it is bad wiring to trace out in the front or the back of the car. You can pull the carpet back and by following the wires see if the bad one goes forward or back.
Once you know which way to go there are more things to check.

[CarlosDanger]

Sounds good. Will do tomorrow night. I have a volt meter, not sure how to use it yet but I'll figure it it.

Thank you very much for your help.


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[timmy2]

You need to be able to measure resistance to ground in ohms. Is your voltmeter a multimeter?
If not, a cheap harbor freight or other discount store one will work for now.
Google and read up on how to test resistance.

[Michael T]

With regard to the fuses, you can find Bussmann brand fuses on Amazon. I found copper ones.

[CarlosDanger]

Okay, so I think I'm testing resistance correctly (multimeter set to ohms at 20k, negative touching the negative battery terminal and positive touching the wire in question).

The first wire, grey with thin red stripe, zeros out.



The black with red stripe jumps all over the place. Seems like that is the bad one then?

EDIT: After further testing the second wire also zeros out...



I can't tell which wire goes to the front and which goes to the back. Seems like I would have to pull the fuse box to get a good look.



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[CarlosDanger]

Not sure if it is related but I found the wire for the trunk compartment light poorly taped up and just dangling. I untaped it and plug it into where it belongs - and the light came on. I'm assuming this can't be part of the problem because the light is on and the blown fuse it out.

Also is there a better way to check if a fuse is going to blow than using and actual fuse? Like maybe a really thin piece of wire? I'm going through a lot of fuses trying to figure this out.

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[timmy2]

Zero to ground is bad. It should be really high. The red/black one jumping around is good as the meter cannot find a value as it is measuring the front lamp in the circuit.
So, as per my wiring diagrams, grey/red goes to the right rear side marker light and the right rear tail light.

This is not a good sign if you left the right rear 6 pin plug disconnected while performing the continuity test. If you plugged it back in, unplug it and repeat the ohm test.

Try unplugging the left rear 6 pin plug as well and take the same measurement. There may be some funky cross grounding going on through the stop lamp, but I doubt it.

If the meter still goes to zero, (likely) there is a fault to ground in the grey/red wire that runs all the way from the front to the back of the car in the main front to rear harness.
I would bet it is likely a wire pinched behind the fuse panel as that wire is only exposed by itself in that area, and then wrapped or sleeved all the way from the front to the 6 pin connector at the back of the car.
You would need to remove the bolts holding the fuse panel to check. ** After disconnecting the battery**

[timmy2]

Stop using fuses to test. That is what a meter is for, diagnosis.

[timmy2]

As your fuse diagnostic test has confirmed it is not the engine light as that is wired to a different circuit.
The fault is in the grey/red wire somewhere.

[timmy2]

In this photo of an earlier year fuse panel and harness you can see the two wires coming off of fuse 3 and going into the front and rear harnesses.
That is the extent of how much wire is exposed before it disappears into the main through body harness and is normally well protected. (About a foot max)