The brake lights on my '91 5.3L V12 coupe recently stopped working. Checked and cleaned all fuses, relays, bulb sockets, bulbs, etc. The 3rd (hi-mount) brake light works fine, as do all my other lights and signals.

Got to be missing something...advice?

Thanks.

 

Since the upper level brake light works most of the brake light circuit is OK.

Power from the brake light switch splits off to the upper level brake light and the brake light failure unit. Power exits the failure unit and goes to the regular brake lamps.

I suspect the brake lamp failure unit if on the blink, or perhaps just dirty/loose connections. It's in the console, aft of the gearshift, metal case.

Cheers
DD

 

Thanks for the information, made good progress today - for a while.

I managed to find and remove the brake light failure unit, its on a bracket just in front of the center console and under the lower end of the ski slope. Sure enough, one lug had broken off - the one that connects to the thicker power (12v) wire. The plastic jack the wires terminate in had also cracked, and one side looked like it had melted a bit.

Opened the unit up and resoldered the broken lug wire connections. There was also a gap between two wires that appeared to have broken apart, so I soldered them together as well. This is not a simple relay, there were a pair of coils, a resister, a diode and a transistor (I think) inside.

Connected it back up, with the ignition on and brake pedal depressed measured 12v between each of the two lugs that feeds a brake light wire and the center ground lug. Hooked up the brake light connections, and brake lights worked! Seemed to come on a bit slowly, but I figured my battery was weakened from all the testing I had been doing.

Started the car, the brake lights came on much faster and brighter - until they stopped working again. Also thought I smelled a faint burning metal smell. Measured again and was still getting correct voltage from the unit, but only when the brake light wires were disconnected. No voltage when they were connected. Bulbs are fine.

So...either I rewired the unit up wrong, or it has a bad component, or both.

The part is known by various names, Lamp Defect unit, Stop Lamp Failure Transmitter, etc. but appears to be part C42291. Manufactured by both Lucas and Smiths Industries.

Does anyone have a schematic of the unit's internals so I can rebuild? Otherwise, I'll have to buy a replacement. Found a few NOS and used ones at various prices in various countries.

 

I think you have knackered the unit! It relies on a certain current passing through it, and if a bulb is out that current is lowered and this somehow triggers a switching system in the unit that illuminates the warning light. Chuck it and get a new unit from somewhere, I suggest!

Greg

 

Thanks, ordered a used unit I found locally, and if that doesn't work, will purchase a NOS unit I found in another part of the world (where it costs twice as much to ship as the part itself!).

Meanwhile... wired up the brake lights temporarily via a Y cable connecting the power wire to the brake light wires.