Looking for some assistance in troubleshooting no brake lights on a 1975 F100 4wd.
Everything worked fine prior to removing the bed. Put bed back on and no brake lights.
Whats been done:

Ran new ground wire from Frame to bed.
Ohmed out brake switch at pedal - Opens and closes - Good
Ohmed ground connection inside of rear bucket housing (on bed) to frame - Continuity check good
Ohmed each connection to bulb sockets from rear harness connector to blades in socket - All looks good.
Checked voltage with harness connector plugged to main harness at rear light sockets - Have 12v from ground tab to bulb tab on both sockets while brake is depressed and no bulbs installed.(see pic below)
Plug bulbs in - no lights when brake depressed
Remove one bulb and check voltage again at socket - no voltage.

Summary - voltage is present when bulbs are removed - when one or both bulbs are installed, voltage drops out (Bulbs are good) both work when lights are on and blinkers work.



12V on this tab when brakes are pressed - both sockets

I hope this makes sense - Thanks for any input

 

Do your park lights work?

 

Yes - Turn signals, parking lights and head and tail lights all work. Just no brake lights.

 

that’s strange. Put the bulbs back in and check for voltage on both sides of the switch with brakes off and on. You could disconnect the downstream wire at the switch and use a test light to isolate the short to either the upstream or downstream side.

 

There is a constant 12v on one side of brake switch and 0v on the other. When pedal is depressed (switch closed) the 12v side drops to 0v . The othere terminal on switch still remains 0v as I would expect. Also changed the turn signal selector and everything still behaves as the old one. no brake lights but T/S and running lights work. Also changed the flasher tonight but no luck. BTW - flashers don't work either but I can't swear they worked before the brake light issue. Never use em.

Looking for more suggestions........any help is appreciate

 

if you disconnect the wire to the lights and substitute your test light at the switch will it light?

 

OK - So good news - All is working again. Between my last post and now I ran outside and decided to re-clean the connectors on fuse#4 and the ends of the fuse itself. Put it all back in with a little dielectric grease this time and now everything works. Very strange the way it was behaving. Shame on me for not cleaning it really well 2 days ago. I should know better. Thank you for the suggestions.

 

awesome news.
Yeah, had 12 volts but no amps to go with it! Glad you’re up and running.