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MG MGB Technical - Third Brake Light flashes

I have a problem that I'm not sure how to solve.

I added a 12-LED third brake light to my 1980 MGB. It is mounted to my rollbar.

The light is wired to the double bullet connector for the driver's side brake light (positive) and the double bullet for the ground for the driver's side rear marker light.

The third brake light works fine when the brake lights are on, but it begins to flash when the flasher is also on at the same time.

Any ideas? Perhaps a weak ground?
sliedavs

I am assuming that the third brake light only flashes when you put the left turn signal on, is that correct? Do you know if the side light flashes with the turn signal? Finally, have you watched your turn signal lights from behind and are they working correctly, particularly when the lights are on at the same time? All of these questions are aimed at making sure that the rest of your brake/tail light circuits are working correctly and if the answers to the questions can help us isolate your problem. Meanwhile, the problem certainly sounds like a defective ground. You can check this out by disconnecting the negative wire for the third brake light from the ground lead that you have it going to and jumpering it to a seperate ground (someplace that is directly attached to the chassis of the car). If it works there, then you have found your problem and also know that you have a ground point to fix. See my article on ground point preparation at: http://www.omgtr.ca/technical/General%20Technical/grounding.htm
Good luck - Dave
David DuBois

David,

The third brake light flashes when both the left and right flashers are on. It only lights when the brakes are engaged - whether the taillights are on or off.

The side lights and the brake lights do not flash when the flashers are on. All of the lights function normally.

I still get this problem even if I run the ground directly to the negative post of the battery.

Based upon one of your earlier posts, I began using Kopr-Shield on all terminations. The ground strap is also new.

Any additional insight is appreciated.
sliedavs

One more possible clue:

The third brake light flashes "out of phase" with the flasher. Therefore the third brake light goes off when the flasher bulb goes on. When the flasher bulb goes off, the third brake light goes back on.
sliedavs

"The third brake light flashes "out of phase" with the flasher" - This reinforces my thought that the side lights should be flashing with the turn signals. Somebody that is more familiar with the later MGB than I am may be able sto shed some insight on this, but I know that many late modle cars were set up so that the side lights flashed witht he turn signals, but were always out of phase with them, like your description of your third brake light. It is always possible that some previous owner tried to set this up in the your MGB and failed, but left the wiring in place. What you need to do at this point is to carefully probe the wires going to your tail light/brake light/turn signal assembly with a meter and see what is happening. I suspect that you will find that the wires you are hooked to are not what you think that they are or that somebody previously mis-wired things in that area. I would also not be suprised if you find that someone has substituted the standard flasher in your turn signal circuit with a heavy duty (or hazard) flasher and returning it to a standard flasher got you signals that either flash very rapidly or not at all.
Good luck - Dave

PS - All would be restoreres sholud have their wire cutters conficated prior to being turned loose on any car. Every MG that I have ever purchased had the wiring hashed up to some degree. The worst being on our TD, where someone had cut out the front half of the wiring harness and rewired it to some totally unknown standard. When we first got the car, whenever we stepped on the brakes, the front side lights came on!
David DuBois

Check your grounds at both tail lights. It is possible that a loose ground at one corner is allowing voltage from the turn signal to feed back through a bad ground to the brakes. Since the LED requires less current and is to a better grond it will noticable light up where the others will not.

doug
Doug Gordon

If david is correct and there is some bonkers flasher reason in the side light circuit, try running an extended lead from your headlight circuit at the front (proper headlights, not side lights). I am assuming you wouldn't have the headlights flashing with indicators so if the brake light stays on correctly then there is some turn-flasher highlighted problem.

~PHIL
Phil

This thread was discussed between 20/07/2004 and 21/07/2004

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