Headlights on = no brake lights!

[akkbhart]

All:
Looking for some guidance. I was tracking down some voltage issues, and cleaning up all the grounds throughout the chassis and engine area. As I went to put everything back to order and check voltage with my multimeter, with engine running, I noticed that I didn't have my driver side brake light or third brake light. I did have the passenger side, however.

After replacing the driver side bulb, I can activate the brake lights with the engine running but, NOT with the headlights on. They don't activate at all, anywhere. I'm puzzled. I did check and clean up all the fuses, and everything seems good there.

Coincidentally, (or not), I have been working to sort out an overcharging condition, and believe that voltage has spiked on occasion. My multimeter has indicated high voltage on sporadic moments, and I see the battery has leaked pretty well over the past week-- all signs of a bad voltage regulator and/alternator. It will be replaced this week.

But what about the brakes? Related to voltages, or something I did in cleaning the grounds and moving around the wires and cables? Where do I start, and what should I test? Your collective advice in tracing this down is greatly appreciated!

Model = 1988 Carrera, 148,000 miles, Daily Driver.

All the best...

[Porboynz]

All to do with the earth return for that rear light cluster. If the earth return path is missing or high resistance then when the head lights are on the rear light finds an earth return path via the brake light filament, through the wiring and via the earth in the working earthed rear light cluster on the passenger side. When you try to activate the brake lights there is no return earth path. Confused? Its simple to run a temporary connection from the faulty rear light cluster housing to any convenient earth point, maybe use a battery jumper lead or something similar. Then you need to make a permanent earth repair.

[akkbhart]

Thanks for the quick reply.
I'll definitely look at the ground in that cluster. Electrics are a weakness for me, and it just didnt occur to me of thinking about the ground path and flow that you described. I hate rusty and unclean things anyway -- so this will give me the addded incentive of getting in there and cleaing things up!

If if cant clean up the connection and existing ground path -- can I run a ground wire directly back to the grounding point behind the fuel filter? Or is there an easier connection?

Thanks again.

[T77911S]

if it worked before you did the work, go back and check what you did.
me, i would avoid running another wire jsut because its not original.

good job porboy.

[E Sully]

In post #9 in this thread I posted a wiring diagram for the Carrera lighting. Yea, another electrical ? Ian found his problem was degraded wires.
In the diagram the headlights and brake lights are independent circuits. You don't mention whether the rear running lights work with the headlights on, since the ground is common for the brake and running lights. One thing you can do is step on the brake pedal and check the black/yellow wire to chassis with a meter and see if there is voltage. If you have voltage, then check for voltage to the brown ground wire. If the first check shows voltage and the second doesn't then you need to check the brown wire connection to the chassis.

[akkbhart]

Yes, for sure, I am going to re-check my work from last night. I simply cleaned up all the fuses with contact cleaner, and unscrewed, cleaned, and reinstalled all the ground connections -- especially two in the engine area. There I had to jam my hand behind the filter to get the connection clean. Its possible I didnt re-connect things quite right. I'll trace back my work this evening.

ESully also makes a good point that I didnt think of: I do have running lights/rear tail lights.(on both sides) Its just that I dont have the brake lights -- with the headlights on. If the headlights and brake lights are on different ciricuits -- Now, I am really confused!

Does the overcharging alternator play into this, at all?

[akkbhart]

UPDATE:
Brakes lights are working again!

This evening, I removed and replaced all the bulbs, checked the voltage on the third brake light, and sprayed electrical contact cleaner inside each light socket as best I could. I also cleaned and sprayed the 6 pin connectors at each side. Finally, I went underneath and scraped away the road gunk from the wires and around the back of the fixture. (not sure what that accomplished, but it made me feel better. Also, rechecked the all grounds that I could reach easily, and gently checked the wires/connections in the engine area.

Originally, I tried to remove the entire fixtures not inspect the connections and grounds in the back, but i stripped one bolt, and spun another that never backed out. I'll leave this job for an entire weekend when i can devote the proper time for complete fixture removal, electrical cleaning, ground checks, etc.

For now, everything works...

Thanks to all for the advice and suggestions!

[Porboynz]

Maybe your earth was via the mounting screws and when you loosened them the earth vanished on the affected housing. Now they are cleaned the earth path is back, but maybe not for long. Make sure the factory earth is good, where it mounts seems to vary with model.

[T77911S]

Originally Posted by Porboynz

All to do with the earth return for that rear light cluster. If the earth return path is missing or high resistance then when the head lights are on the rear light finds an earth return path via the brake light filament, through the wiring and via the earth in the working earthed rear light cluster on the passenger side. When you try to activate the brake lights there is no return earth path. Confused? Its simple to run a temporary connection from the faulty rear light cluster housing to any convenient earth point, maybe use a battery jumper lead or something similar. Then you need to make a permanent earth repair.

you could pull all the bulbs except the one that is working. if the working bulb goes out, it is getting ground thru another bulb.