Brake Lights Always on....

[jpcj-7]

Ok, I have read all the old "No Brake Light" post and they didn't seem to answer my questions. I have turn signals and marker lights, however it seems that my brake lights are consant on. Any ideas what would be causing the stop lights to "stick on"?? Thanks!

 

[Blood_of_Tyrants]

Poorly adjusted brake pedal switch.

 

[jpcj-7]

Poorly adjusted brake pedal switch.

How is that adjusted?

 

[503m715]

I had a brake light switch that leaked around the two electrical studs. When brake light switch harness was plugged in, it would fill with fluid and short the two terminals together. (brake lights on all the time) I replaced the switch with a new one from A B Linn and problem went away. Check your switch with an ohm meter. If there is a short accross both studs without pedal pressure, change the switch. They are cheap. I think I paid 12 or 13 bucks for a new one.

 

[neb4x4]

I don't think that it is adjustable. My brake lights don't work unless the pedal goes to the dirt, and only when its running. I looked for an adjustment, but there is only a pressure switch that is in the master cyl. It has 2 wires going into it.

I just bought one for like $28 or something from Saturn. But have yet to install it.

I bet if you go under the rig to the master cyl and unhook one of the wires at the switch the lights will go out...... If not, than I bet it's in your master light switch. Or short to voltage, but does it go away when the "MAIN IGN/ACC SWITCH IS TURNED OFF???"

 

[Green_gator]

I had that happen when pulling a trailer not long ago. I found that one of the wires running into a rear light was loose and shorting against the frame.

 

[jpcj-7]

Thanks everybody! When I get home tonight I'll check the brake light switch closer.

 

[dabtl]

My first thought is to unplug the brake light switch. If the brake lights go out, probably a bad switch. If they stay on you have some kind of short.

 

[Seth_O]

Long Live the Search Function! I have this exact issue and was able to troubleshoot it after reading this post. Thanks (belated) to those who contributed here - and bump this post for anyone else having the same problem.

 

[Digger09]

Had that happen to me and it was the brake light switch,

 

[rlwm211]

Other than a short there are two possibilities that can cause the brake lights to stay on.

1) A defective switch that keeps them on.
The test for this is to unplug the switch and if the lights go out you know the problem is not a short in the harness.

Before condemning the switch you need to do step 2

2) It is possible that the brake fluid pressure is not releasing in the master or is releasing very slowly. Since the switch is in the air pack and not on the master you need crack the brake line bleeder at the top of the airpack and if the lights go out the problem is upstream and the master is not allowing fluid to return to the resevoir. This also can cause dragging brakes. If the lights go out when you do this, the switch is ok.

If you determine that the problem is pressure in the master not releasing, check the freeplay of the brake pedal. You should have about 1/16-1/8th inch of freeplay where the adjustment rod enters the master. If there is no play, adjust until there is as I describe.

If the rod is adjusted properly, you may have a plugged relief port in the master. To test for this, open the master cylinder by loosening the breather with 3/8 wrench and then take the cap off with a 3/4 wrench.

Press the brake pedal with your hand and release watching in the resevoir for a return flow of brake fluid. An upwelling should be visible when you release.

Explanation: There are two ports in the master to feed the piston assembly and let the fluid back. The big port is where the master refills the area that the piston uses to pressurize the brake system. The small, even tiny, port is ahead of the main piston seal and allows fluid into the resevoir when the piston is at its rear most position.

One last item is that the airpack itself can be defective and keeping air pressure on the main piston and thereby apply enough pressure to keep the brake lights on, and also the service brakes most times as well.

The first test checks for pressure in the lines and if everything else in the brake system has passed the tests I have described here then you are looking at dealing with the air pack.

I hope this is helpful

 

[PureMayhem]

I don't think that it is adjustable. My brake lights don't work unless the pedal goes to the dirt, and only when its running. I looked for an adjustment, but there is only a pressure switch that is in the master cyl. It has 2 wires going into it.

I just bought one for like $28 or something from Saturn. But have yet to install it.

I bet if you go under the rig to the master cyl and unhook one of the wires at the switch the lights will go out...... If not, than I bet it's in your master light switch. Or short to voltage, but does it go away when the "MAIN IGN/ACC SWITCH IS TURNED OFF???"

I just picked up my M923A2 from San Marcos today. On the way home my SAG wagon called and said that my brake lights were on all the time. When I got home and turned all switches off the brake lights stay on. Is this a short to voltage?

 

[juanprado]

Strange because with the light switch off, you should not have power to the brake light circuit. Even if the light switch is on, with the battery switch off the circuit should still have no voltage.

I was going to suggest the brake light switch in the inside frame rail around the fuel tank is faulty if always on but you have some other wiring gremlins. Need to pull ground off or you will run down batteries till you isolate the problem.

Keep us posted.

 

[dslucas]

Not sure of the wiring on a 923. You may have better luck posting this as it's own topic in the 5 ton section

 

[TsgtB]

I'm doing my research, and wanted to ask... instead of making a new post.
I think I may have a sticking break switch also.
the hazzards/turn signals work, but the signal bulb is not any brighter than the tail lights are in service drive.
I'll get someone up in the cab to try the break pedal so i can see whats going on.