Brake warning light not working

[1968_K20_4x4]

Hey everyone, I have a few issues with the brake warning light and I need some help to troubleshoot the problem. I have two trucks and the light in both of them does not work, and I think the issue may be electrical. Both trucks have guages in the dash.

So my first truck is a 1968 K20. I re-wired the truck myself as per the wiring diagrams using a good wiring harness out of another truck. Before I rewired the truck, the brake warning light would come on with the key to the "on" position. However the brown wire was not connected to the prop. valve. Now my brake warning light does not come on when I turn the key to "on" and I have it connected to the prop. valve and everything else wired up like it shows in the wiring diagrams.

My second truck is a 1972 K20. The brake warning light does not come on with the key in "on" position. Also, this truck currently only has brakes on the front axle and no fluid in the master cylinder for the rear brakes (There's a leak somewhere, which is a separate issue I have yet to deal with) So the light should be on all the time, but it never lights up. This truck was rewired by the PO and it appears they did a nice clean job and knew what they were doing, but I can't say how they wired it up.

If someone can give me some guideance on how to troubleshoot this issue I would greatly appreciate it. I think I did everything right when I rewired the truck but would like some tips to check the function of the wiring so I can find the issue without having to pull everything out again. Thanks in advance!

[WorkinLonghorn]

Subscribed, I need to fix mine too.

I do know that the light should come on in the START position. In the "On" position it should only come on if there is a brake problem.

[1968_K20_4x4]

Thanks for the reply, you are right I should have said the light should come on in the "start" position, not "on"

[RustyPile]

First and foremost, with all electrical troubleshooting steps is to check bulbs and fuses..
The ignition switch supplies power to the brake warning light and the proportioning valve supplies the ground when there is a brake failure. When the ignition switch is in the START position, it supplies the ground, a sort of "test" for the circuit. Voltage will appear on the brown wire where it connects to the valve, although due to bulb resistance, the voltage will be something less than 12 volts.. Ground that wire and with the ignition switch on the ON position, the warning light should come on.. If the bulb is good, then you have a problem in the wiring.. Volt meters and test lights are the tools of the day..

[Dead Parrot]

Normally, the park brake is also included in the brake light circuit. If the park brake is set, the brake light will light with the key on, assuming the switch and wiring are still good. A convenient in cab way to test your brake light. Once you know the brake light works, then you can chase down any issues with the wire/switch for the hydraulic brakes.

[Andy4639]

I know of no 67-72 trucks that came with the emergency brake light. I have never seen it before except where one had been modified. I have done several of them myself.
If your light isn't coming on then as stated check bulbs & fuses first.

[1968_K20_4x4]

Hey guys,

Thanks for the replies!

So I did some checking with my test light on my 72 and had ground at the prop. valve. I also had ground at the brown wire in at the ignition switch and ground and the No. 2 terminal in the dash plug. When I pulled the brown wire off the prop. valve, no ground at the No.2 terminal. However when I put my key to "Start", I got ground! So that tells me that the wiring harness in the truck is all working properly. I pulled the bulb and socket and bench tested it and it works properly.

Only thing it can be now is the printed circuit. Next step I will pull my dash apart and check the printed circuit for issues.

Interesting about wiring up the parking brake to make the Brake light come on, in my research I did come across a few threads about this mod and definitely think it is a good idea. Going on my future projects list for sure!

Thanks again for the replies and advice!!

[VetteVet]

Here is a diagram of the printed circuit.






notice the no2 terminal goes directly to the bulb for ground, and the positive comes from the fuel gauge. The purple line is just the coloring they used.
Your trouble shooting is correct and if the fuel gauge works then you should have power to the brake warning light unless as you suspect the circuit board is bad at the light.
You could just have a poor connection at the light socket as well.

[1968_K20_4x4]

Originally Posted by VetteVet

Here is a diagram of the printed circuit.


Attachment 2005286



notice the no2 terminal goes directly to the bulb for ground, and the positive comes from the fuel gauge. The purple line is just the coloring they used.
Your trouble shooting is correct and if the fuel gauge works then you should have power to the brake warning light unless as you suspect the circuit board is bad at the light.
You could just have a poor connection at the light socket as well.

Thanks VetteVet! That diagram was exactly what I needed. I pulled my guage cluster and the printed circuit was the issue. I had a left over plug from my old wiring harness, so I was able to use that to plug in to the guage cluster and test it. I also have a new replacement gauge cluster that I was able to test as well, and the brake light worked! So I'll install that back in the truck and should be good to go. Now that I understand how the circuit works, should be able to resolve the issue in my other truck as well.

Thanks again for all the help guys!