Troubleshooting Brake Lights

itskyb · Post by **itskyb** » Thu May 05, 2022 3:46 am

I recently purchased a '54 M38A1. Overall it is in pretty good shape. I had the brakes completely replaced with the master cylinder and all wheels. I have noticed that the brake lights do not work at all and I'm trying to figure out what to look at. I suspected the brake light switch in front of the master cylinder, but here is what I have tested so far:

1) New blubs (old ones fell apart as soon as I tried to twist and remove them)
2) I disconnected the two wire leads from the brake switch and jumpered them together, no lights.
3) While jumpered, I measured the voltage at the light socket. I get a weird reading of about 3V when the light switch is set to Stop, 0 V when the switch is set to off.
4) Other lights work just fine (black out, headlights, running lights, etc.).

A few questions:
1) Assuming the light switch is set to Stop and I have jumpered the two leads, should I not get 24V back at the light socket? 3V seems kind of weird.
2) Anything else that I should be looking at?

Thanks,
Evan

OKCM38CDN · Post by **OKCM38CDN** » Thu May 05, 2022 5:57 am

Is the light switch on in the "Stop" or "Service Drive" position??? If not lights will not work...

itskyb · Post by **itskyb** » Thu May 05, 2022 6:44 am

Yes, light switch is set to "Stop". I have tested in "Service Drive" as well.

Xamon · Post by **Xamon** » Thu May 05, 2022 10:13 am

if you measure the line into the stop switch to ground what is it saying?

Also with no bulb in put one lead on the power into the switch and the other to the lead out, with them off the switch. Does it read zero or are you getting current indicating a short?

itskyb · Post by **itskyb** » Fri May 06, 2022 7:56 am

I am getting voltage through the switch.

Xamon · Post by **Xamon** » Fri May 06, 2022 10:13 am

I am assuming that you mean you have 24V to the switch.
So you have a voltage loss somewhere on the run to the rear. A short in the wire or a bad ground would be my guesses. The ground seems unlikely since you have said the other light functions work correctly. I would resistance test the pins on the pedal switch to ground and the line to the back.

itskyb · Post by **itskyb** » Tue May 10, 2022 8:47 am

Thanks for the tips.

OKCM38CDN · Post by **OKCM38CDN** » Tue May 10, 2022 7:43 pm

FYI the brake switch is a make or break type switch. If eithor side connects to ground it is bad. Power comes in one side of switch and out the other to the lights. Only time is makes contact is when the brake pedal is pushed properly...

Same idea as the horn switch...