Tail Lights Not Working

Brake lights, headlights and turn signals are all working, but for some reason the running lights in the rear are not. I checked all the fuses and relays, all seemed ok. Next I checked all the connections to the rear light assembly and they seem ok as well. I even put new bulbs in just to be sure and still no dice.

Any takers on this one?

Thanks,

JP

Check the grounds.
I would estimate that 85% of lighting issues are ground related.
Actually remove the ground, inspect for corrosion and ensure metal to metal contact.
Clean, reinstall and recover.

If it isn't the issue at least you can check it off the list.

RS Technic wrote:Check the grounds.
I would estimate that 85% of lighting issues are ground related.
Actually remove the ground, inspect for corrosion and ensure metal to metal contact.
Clean, reinstall and recover.

If it isn't the issue at least you can check it off the list.

Agreed. 99% of the time I find that just cleaning the sockets, grounds, contacts, and frequently the bulb itself, will bring them back to life on all the old sports cars I've played with. And even a budget multimeter can be a huge help when sorting out problems that have other causes.

So finally got around to checking and double checking the grounds, bulbs, etc. and it all checked out. Took it to a local shop that also had no luck. Everything had power and checked out. They suggested that it could be the actual switch for the headlights. The headlights however work. Is it possible that the switch is still bad in terms of the connection to the rear running lights?

Yes, I had a similar problem. It is easy to pull the headlight switch. If it is bad there will be some brown burned area on the outer surface.

Well, I replaced the headlight switch and still no running lights.  I must have some serious electrical gremlins going on. Next step is seeking out a 944 specialist here in town. Just not looking forward to spending more money after dropping $800 at another local tuner trying to figure it out. Woof...

jpkinerk wrote:So finally got around to checking and double checking the grounds, bulbs, etc. and it all checked out. Took it to a local shop that also had no luck. Everything had power and checked out. They suggested that it could be the actual switch for the headlights. The headlights however work. Is it possible that the switch is still bad in terms of the connection to the rear running lights?

You say everything had power? The bulb socket showed power?

On the passengers (right) side taillight barrel connector, attach your positive lead of a voltage meter to Pin 4 Gray/Red and the negative lead of the meter to Pin 6 Brown. If you see +12V, everything forward of the car is fine. Reconnect the barrel connector and check for +12V at Pin 4 on the other side of the connector (back-probe). If good, remove the black plastic taillight cover from the taillight from inside the hatch compartment and check for power by touching the meter probe to the back of the bulb socket with the gray wire attached and the negative lead to the assembly.

Also, check fuse numbers 40, 41 , 30 and 31; all are 7.5A fuses. If your front running lights are on and the rears are off....that would be a neat trick.

To check the headlight switch, touch your positive meter lead to check pins 58L wire colors Gray/Black and 58R Gray/Red. If 12V+ out of the pins, its not the switch. Use Pin 31 of the switch as ground (wire color Brown) for your negative meter lead.

If your license plate lights work but the running lights don't, its not the ground. If the under-hood engine compartment light works but the running right side running light doesn't, then the fault must be in the right side tail light connector. Check Pin 4 Gray/Red for +12V to Pin 6 Brown.

I've had issues where my lights don't work on my car and the fuse is good, however replacing the fuse brings the lights back to life.

Don't know why that works, but it does...

Awesome info. Thank you. I've been out of town for work, but I will run down this punch list with a multi-meter and try to get this sorted out. I understand the principles of automotive wiring but it's by no means my forte, so I really appreciate you taking the time to write that up. 

Cheers,

JP

I've had the same experience as Doc. Hours of diagnostics and head scratching, changed a fuse that was fine, but old, and presto!
Also, I've found that over the years many chassis have had bulbs that would work, but not the correct specs installed at one time or another. The correct bulbs help as well.

Well, I finally figured out what was causing the short, and by "I" I mean a friend who happens to be an electrical engineer (big thanks to Chuck and Alan). So to recap, all the grounds and fuses checked out, so we dove into checking the harness with a milli-ohm meter. After coming close to pulling out the whole harness front to rear, we discovered that it was the power and ground wires touching each other from an unused side turn signal lamp. 

Yup, I felt dumb. I was all my fault from not capping them off when I did the original tear down and discovered I wouldn't be using it. Anyway, I have running lights now! 

First track day is coming up May 10th at Putnam. On to breaking more things! Thanks for all the suggestions.