Brake Lights Dilemma

flathead34

flathead34

Senior Member
On our '34, when we have the brake lights on and then when the headlights are turned on the brake light goes off. This only happens on one brake light, the other performs normally. In studying this problem, we observed that only one filament was being used by both. We changed out numerous double-filament bulbs and they all performed the same, the brake light and the head light used that same filament on that side. What do we have to do to correct that and not have that brake go off when the headlights are on?
 
Are you sure it's the brake light and not the tail light that's going out?
If it's the tail light it could be a grounding problem. Only one head light filament
should be on at a time. One filament is low beam and the other is high beam.
 
It sounds like a grounding problem. Run temporary grounds to each taillight housing and see if that solves the problem
 
Really confusing when you say headlights when you are talking about the taillight. Yes headlights and taillights come on together, but the headlights have nothing to do with this problem.

As said above, this is a classic symptom of no ground at the bad light's socket. Brake light is using the taillight filament and wiring to find ground at the other side's good socket. Only the taillight filament lights up because it is smaller. And then this strange path to ground gets blocked when taillights have power on.

To fix this, look at how the socket is attached. Needs clean, tight, metal-to-metal contact between the socket and truck body. Sometimes it's a coat of paint breaking the contact.
 
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Sounds like the problem is the connections or grounds to the one light that goes off. What light bulb are you using? the brake light is just one filament of the dual filament light bulb with the tail light being the second.
 
Thank you guys for your responses We will look at the grounding. There is some confusion the way I described the problem when we turned the headlights on. We step on the brake pedal and the brake lights come on. With brake lights on we turn on the headlights. One brake light turn off. Do you think this happens because improper grounding on that one tail light? The other brake light works properly with the headlights on.
 
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