911 SC - Tail light electrical gremlin - can anyone help?

[TurboTim14]

Hi All,

Having a strange issue with my 911SC tail light that I cannot figure out.

With the headlights off and brake pedal not depressed, all turn signals function fine.

However when I turn the headlights/tail lights on, and use the turn signal for the drivers side, the driver side tail light goes into a sequence of alternating all the bulbs blinking (see video). The plate light on this side blinks as well.

The other problem I have with this driver side tail light is when I have headlights off and depress brake pedal the same time as using the left turn signal, the left brake light blinks and the signal bulb does not.


https://youtu.be/YHxaZDabfBc


The passenger side tail light functions fine.

Initially I thought it was a bad ground, especially when I pulled the passenger tail light and saw how corroded the ground was to the frame. I touched the wires and they broke off from the the ground just by touching them. I wire wheeled the mount and bolt, and wired a new ring eye connection for the wires. Bolted it in to place, and no change.

-Some additional background on the car. I have only owned it a few months and Im still learning the car. For 2 of those months the car had been in service and then paint. I remember shortly after getting it and checking it out, both plate lights worked. But the passenger side tail light side marker light did not work. I put in a new bulb on pass marker and nothing. When I took it for service they said they would look at it. It now functions fine so they must have repaired it.

-Also, now the passenger plate light does not work at all. I switched the parts from DS plate light and nothing, like the wires are dead?

-Immediately after service it went to paint for pass. rear quarter panel, where I assume they had to remove the pass. tail light. I just got the car back and noticed these turn signal/brake light issues when someone I know said something to me about it after following me home. Not sure if this problem has anything to do with the service shop or body shop, but it is something to mention. Also not sure how long this problem has been like this. The only thing Ive noticed differently since taking possession of the car is the PS marker light works, and the PS plate light does not now.

One other thing, the car does not have an engine compartment light, and I cannot find the wires for it. Not sure if that could be related?

I am still thinking this is a grounding problem somewhere... but is there another ground to check besides the one behind the pass. tail light that I already repaired?

Long winded post , but the more detail the better! Thank you

[Targa Me]

That IS a weird problem.
I would start with cleaning all your ground wires and make sure they are tight.
My 3.2 had a weird blinker issue and it turned out to be a bad ground.

[bemmett]

I had a strange issue a few weeks ago (swapping back & forth between stock bulbs and LEDs on exterior indicators) where the drivers side indicators worked fine, but the entire passenger side blinked 2x as fast as it should. Grounds were fine, relay was fine, but the culprit ended up being corroded connectors to the bulb itself in the front passenger turn signal housing. Cleaned that up, and everything started behaving properly.

So, maybe check the fronts as well, as it seems like it can impact the behavior of the whole system (in my experience, anyway).

[GH85Carrera]

Originally Posted by Targa Me

That IS a weird problem.
I would start with cleaning all your ground wires and make sure they are tight.
My 3.2 had a weird blinker issue and it turned out to be a bad ground.

Grounds are often the issue.

But the bulb connectors themselves can be a problem. Scrub and clean every possible ground. Then go to each bulb, and remove the bulb, and check the spring tension on the bottom of the bulb. There is a spring like part on the bottom of the connector that presses on the bulb bottom. Use a pick, and pull the spring to press harder on the bottom of the bulb.

Check the side marker bulbs and all the bulbs that are on the signals.

[flat6pilot]

First of all, where are you located? Maybe someone nearby could assist.

It's important to note is 'was' just working a short time ago.

Are you sure all the bulbs are correct for their application? You know the pax side works, try swapping everything over to the driver side and see what happens.

Otherwise, I'd try to find what circuit(s) might be causing this. This could help isolate where the problem might be coming from.

I'd first start testing circuits on their own (including the front driver side marker/turn signal bulbs):
Brakes on there own (With other bulbs removed)
Parking light bulbs only
etc.

See what happens.
Like people often mention, it usually does have something to do with a ground. Or the wrong bulb.

[sigchuck]

I wish I had a better memory on this point, but had very similar issue with driver side tail light in my 83 that was a result of engine compartment light harness shorting. If you can’t find the harness, you might want to dig a little deeper if cleaning grounds doesn’t work.

[targa80]

It is more than likely a running light or flasher ground problem in the left rear light bucket. One of the features of our cars is the night time parking light. If the directional is left in the left or right position with everything else off the front and rear parking light will illuminate on the side the turn signal switch was selected. This was a design by Porsche to allow the owner to illuminate the side of the car at night on a narrow street for safety reasons. Test to see if this function works for the left and right side of car. Do all of the running lights work normally with and without the the flashers.What happens if you turn on the four way flashers on with and without the running lights?

[TurboTim14]

All, thank you for the helpful replies. I will try the things that have been suggested and get back to you soon.

@sigchuck, this could be possible as I mentioned my engine compartment light is missing and I cannot find the harness for it. I will have to keep digging.

[TurboTim14]

Few more things I have tried thanks to the replies.


-I replaced all bulbs on the DS tail light to match what was in the PS tail light. 2 bulbs were different compared to what was on the PS. After replacing them to match, the problem is still there.

-The problem is still there if i use the hazard flashers with headlights on. Without headlights on and flashers only , no problem.

-I removed the passenger side tail light lens to inspect the bulbs on that side when I wanted to be sure they matched. And one thing I noticed on this side is, when headlights are on and DS turn signal is blinking, the DS AND PS reverse light will very dimly blink too. When I would remove the PS reverse light bulb, the DS blinker bulb would get very dim and the DS revese light would not be lit or blinking anymore.

-I opened up the back side of each tail light, and everything looks good. No loose connections from what I can tell. Also, all bulbs seem to be making good contact with the metal spring tab.

-Another thing I noticed was the PS tail light is very bright, almost like the brake is being depressed. This was the side I didnt think I had a problem with, but maybe something is going on with this light too.

Nothing really solved so far, but a few more things I have noticed.

How many grounds are there in the tail light system? As far as I know, there is only 1? The one behind the PS tail light that I have already repaired.

[TurboTim14]

Also, Where does the harness go for the engine compartment light? I believe it runs down the DS decklid hinge and then where? I cant seem to find it, but I am not sure where exactly to look. I am looking on the DS engine compartment wall.

[sigchuck]

Interested to know where the harness connects too as I'd love to remove mine!

One thing I did recall is that I had actually blown the fuse. I spent some time chasing this issue because I assumed there was no way that all of the other lights (brakes, markers, etc.) wouldn't be on the same fuse as the tail light. I was wrong, but happy that the fix was isolating the engine compartment wires and replacing the fuse.

You might start there if you haven't checked already. It should help rule out the engine light being the source of the issue (or at least the harness shorting against itself). Sorry if you had ruled this out and I missed it.

[timmy2]

Lid light harness connects to a single pigtail plug coming out of the back of the 6 pin connector that feeds the tail lights. Grey wire, drivers side.

[targa80]

Did you try the night parking lights with no key turn the directional switch for left or right. Check the front and rear parking lights should be on for the side the directional switch is in. It will help eliminate a portion of the light circuits.

[targa80]

Night parking light with left directional on and no key in ignition.
Try this test with both left and right parking lights to verify they work the same.

Front parking lights



Rear driver parking lights



Rear driver parking light with 4 way flasher on (center bulb flashing)

[TurboTim14]

Originally Posted by timmy2

Lid light harness connects to a single pigtail plug coming out of the back of the 6 pin connector that feeds the tail lights. Grey wire, drivers side.

Thanks, I will have a look.

Originally Posted by sigchuck

Interested to know where the harness connects too as I'd love to remove mine!

One thing I did recall is that I had actually blown the fuse. I spent some time chasing this issue because I assumed there was no way that all of the other lights (brakes, markers, etc.) wouldn't be on the same fuse as the tail light. I was wrong, but happy that the fix was isolating the engine compartment wires and replacing the fuse.

You might start there if you haven't checked already. It should help rule out the engine light being the source of the issue (or at least the harness shorting against itself). Sorry if you had ruled this out and I missed it.

I inspected all the fuses, the ones under the frunk and the ones in the engine compartment on the DS wall and all looks good.

Originally Posted by targa80

Did you try the night parking lights with no key turn the directional switch for left or right. Check the front and rear parking lights should be on for the side the directional switch is in. It will help eliminate a portion of the light circuits.

I did, with the key out of the ignition and just hitting the directional switch left or right, the front signal, front side marker, rear side marker, and rear tail light are all lit. The same lights come on flipping the switch to either side. I actually was not aware of this feature.

My issue seems to only arise when the headlight switch is pulled in the first or second position. Or if I use the hazards while the headlights are on.

[TurboTim14]

Originally Posted by targa80

Night parking light with left directional on and no key in ignition.
Try this test with both left and right parking lights to verify they work the same.

Front parking lights



Rear driver parking lights



Rear driver parking light with 4 way flasher on (center bulb flashing)

Thanks for photos. When key is out and directional is pushed up/down for left/right, mine looks same as your photos. But when the directional is in the DS position and I hit the hazards, my problem is there with the alternating light sequence in my video in 1st post on DS.

[targa80]

The flasher unit has many failure modes that can occur. Below are two write ups I did and it might add some insight to your issue. The mod I did was easy and I used a
$16.00 3 prong relay from the local flap store. There is no guarantee that it will fix your issue but it might be worth a try. Recently I had an issue of no parking lights, directional or hazards. It was the 4 post flasher relay.

This is what the 4 prong relay looks like on the inside.



The link provides a write up on the tach directional lights that explains a possible cause of the fault.

https://forums.pelicanparts.com/porsche-911-technical-forum/819548-flasher-hazard-mods-my-80sc.html

The below thread provides the how to mod that I did to my flasher circuit to fix the dim turn indicator lights on the tach.

911 Turn signal indicator issue

[spoke]

You have a ground issue with the left bucket. Notice that both brake filaments light up as well as the reverse light too. The only common connection of the running light, brake light, reverse light, sidemarker, and turnsignal is the ground. With your ohmmeter, remove one of the light bulbs and check the resistance from the socket base to engine case or ground lug inside the engine compartment. You should read zero or near zero ohms.

[The Dude]

So what happened? Problem fixed? Got something similar.