Kevin in Atlanta

Have a tail light warning on a my new 88 I got running recently. It comes on with and stays on after start-up. Could this be a bad bulb control unit issue?

OTR18WHEELER

Probably as simple as mis-matched bulbs, nickel based bulbs are preferred.
Also check the license plate bulbs.

Alan

What does it say exactly? don't just guess go check it?

Alan

Kevin in Atlanta

Tail with a light symbol after it. I have verified all the lamps work properly,

Alan

Not the brake lights? doesn't go out when you hit the brake pedal?

Alan

Kevin in Atlanta

Tail. Brake light goes off after a tap to the brake.

I find it strange that the tail light warning is on when they have not been turned on.

SeanR

Most likely a bulb warning control issue.

mdkelly1

The Bulb Control Unit is telling you that it senses a fault, probably a difference in resistance within the tail light circuit.

I've learned the hard way that it is best to trust the BCU initially until other possible causes can be eliminated.

The fault might be the result of:

1) A bad/corroded connection in one of the bulb sockets, harness plug sockets & pins, or an issue with one of the harness wires.

2) Mismatched bulbs (sometimes even different brands with same wattage) as already suggested

3) Or a problem with the BCU itself

You can experience a "Tail Light Error" or fault without having the lights on, at least that was my experience.

It would be helpful to know exactly what the tail lights and brake lights look like from the rear when the error message is displayed and the brake pedal is held depressed at the same time, even though you are only getting a "Tail Light" error dsiplayed.

Is one of the lights slightly dimmer than the others? Does the brightness change over time as you watch it with the brake light depressed?

Also, are the side marker lights working correctly?

A clue in your description is that the "Tail Light" fault comes on when you start the car and does not go off when you drive the car normally with the lights off (but using the brake pedal, etc.). The BCU monitors both the tail light and brake light circuits.

It will be interesting to see what you find.

Alan

Actually you cannot have a valid fault until the lights are turned on. Due to the way it works - it cannot check for a fault until the filaments are energized.

The brake lights have the warning default on until you press the brake for that reason... this is to encourage you to put your foot on the brake to test the brake lights at startup. Porsche considers the brake lights to be the most important rear lighting, and wants you to test they are working - before you might need them to work in anger - but it can't test them until you use them...

So something is up with the bulb controller.

Alan

mdkelly1

Hi Alan,

Thanks for the clarification as always.

I should have been more precise in describing the experience I had with receiving a "Tail Light" error message while driving with the lights switched "off" and depressing the brake pedal as described in this thread:
https://rennlist.com/forums/928-foru...long-post.html

Alan

Yep understood. This would be an invalid error due to circuit connections compromised and cross feed between circuits.

Alan

Kevin in Atlanta

Replaced the BCU, repaired the the harnesses as needed, replaced all the bulbs with new ones that are the correct per other threads and the markings on the back of the taillight housings, verified the lights work and still get the Tail ))) warning at start up.

I need to spend some time going over the wiring diagrams.

Alan

Likely still the bulb controller - from the way it works any error reported on the markers before they are turned on means the bulb controller is faulty.

One extra test: remove the controller - NB your lights just won't work like this - do you still get an error at startup? if not then it's certainly a bulb controller fault.

Bulb controllers vary a lot across model years - so make sure you get exactly the right type.

Alan