Can't solve brake light problem.

[thismonkeyhere]

Odd problem. Brake and tail lights are a combined unit, and have a dual element bulb, one for the tail, another for the brakes.

What should happen of course is that when lights are on, braking will result in the tail lights getting much brighter, or if lights are off then just coming on as brake lights.

What currently happens is that if lights are off, works as normal. If lights are on, they just turn off completely so neither element is lit.

Foxing me completely, never experienced anything like it before.

Anyone able to shed any light?

[deggles]

Assume you've tried replacing the bulbs? If both elements are in series and one fails that could explain it. Otherwise it's a wiring fault, need to know what car and see a circuit diagram really.

[thismonkeyhere]

deggles said:

Assume you've tried replacing the bulbs? If both elements are in series and one fails that could explain it. Otherwise it's a wiring fault, need to know what car and see a circuit diagram really.

On inspection the bulbs seem fine, with intact elements. If it's the bulbs then maybe not a visible problem.

[52classic]

This problem is to do with the earthing of the bulb housing. Poor earth can't cope with the power of the tail lamp AND the big wattage of the brake bulb so they go off. Fix is strip and clean all the connections especially from the lamp unit into the wiring harness.

[E-bmw]

Could be that the bulb has been forced in wrong previously & done some damage.

The bayonet lugs on combined bulbs are staggered so that they can be fitted only 1 way, if forced in the wrong way the bulb sits much further in than it should & can stress/bend the contacts.

If you can, get the bulb & holder in a position where you can see well enough to check the contacts & see if there is any damage to the socket that the bayonet lugs sit in on the bulb holder, it could be an easy fix with a small screwdriver/long nosed pliers.

[gazza285]

52classic said:

This problem is to do with the earthing of the bulb housing. Poor earth can't cope with the power of the tail lamp AND the big wattage of the brake bulb so they go off. Fix is strip and clean all the connections especially from the lamp unit into the wiring harness.

This.

[Megaflow]

gazza285 said:

52classic said:

This problem is to do with the earthing of the bulb housing. Poor earth can't cope with the power of the tail lamp AND the big wattage of the brake bulb so they go off. Fix is strip and clean all the connections especially from the lamp unit into the wiring harness.

This.

Yep. Rear light working normally when one light is on, but then going weird when two are in use, is almost always an earth problem.