Why would brake lights not work?

[Nickathome]

Guys;

Need help in this area. Although not all of my lights are working, I do have rear turn signals, and I do now realize I have rear running lights working while the headlights are turned on. Now whether this should be the case I am not sure. Anyway, I do not have any brake lights when I step on the brakes. I replaced the brake switch with a new one when I replaced my master cylinder so am wondering what else could be wrong to not allow the brake lights to illuminate when stepping on the brakes. Thanks.

[gwalker]

What type of t/s do you have on this truck? Power goes to the brakelight switch, then through to the t/s switch and to the rear lights. Ck if you have power at the brake switch an then if its going thru the switch when brakes are applied, then ck for it getting to the t/s switch. The operation of the t/s's and tail lights already confirms the rest of the circuit is ok.

[MSeriesRebuild]

I can add this also, several questions need answered before I can offer help however. Is your truck equipped with a military solid-state system that is correctly installed? If so gwalkers tips apply. If you have the old signal system with the large metal control box that was first used on M37's, the same thing applies to a point, but it all happens in a different way. I've also seen systems where turn signals were added to the original system using early civilian switches. Brake lights are not wired through the turn signal switch, but is a totally separate accessory when it's done this way. Regardless of which system is used, if the military master light switch is still utilized several issues can arise there. Many people have done many things to get lights not realizing how the functions of the military switch work. We've seen many cases where other circuits have been run when the circuit for the given function is already there & not used. This happens because folks simply aren't aware of how the military system functions. If you know exactly how yours is set up, a diagnosis & correction of the problem is likely a simple matter.

[N1VSM]

Ground, ground, ground, ground, ground, ground.

Try hot wiring a good, solid ground to a lamp bucket. I'd be willing to bet that your problem will be solved. The best way to check the ground s with a resistance meter - the resistance from the "-" side of the battery to the shell of the lamp socket should be zero. If it shows infinite resistance (or VERY big), you have no ground. If anything much more than zero means you have a poor ground. The worst kind to diagnose is an intermittent ground - exists when you measure it; doesn't when you need it (like stepping on your brakes).

The marker lights on these things (especially with T/S) have very strange behaviors (namely sometimes they work!) without any or a solid ground. It mostly comes that it can follow a path and ground itself through other circuits without blowing out lamps. Try using internal/external tooth washers like described in the article a few months back in MVM.

If you try this (on both sides) and it DOESN'T work, then we can help you trouble shoot your wiring system.

Oh, did I mention that it might be a bad ground?