AN ELECTRICAL LIGHT MYSTERY. 1978 F100

[JBMCQ]

Hoping someone has had this occur so as to point me in the correct direction to fix it.
I have a background in electric troubleshooting and repair.
Have not as yet pulled out the multimeter to test anything.
I am in the swim pool industry, working on everything from motors to

CPU based controls using relays to activate the various components.
I am having a hard time visualizing what could cause this.
1978 Ford F100 2wd
The Problem: Right rear brake light / during daylight hours both
brake lights function, although it seems the right rear is somewhat dimmer.
When I turn on the headlights/parking lights and apply the brakes
the passenger side (right rear) light goes out completely.
Foot off brake light back on / Have not tested to see which filament
in the bulb is lit. Break light bulb has 2 filiments one for tail light/one
for turn, brake.
Suspect it is a short in the harness wishing its not. So as I said
hoping for someone who had this occur to shed some light on the
subject, (pun intended) or someone to help me with procedural steps
to test + determine causes.
Thanks in advance for your help,
Jim M

[cep62]

Ground problem, maybe a bad socket. These old plastic ones tend to go bad.
try touching a known good ground to the base of the bulb.
I like to use a long wire and clip it to the negative post of the battery. (for testing only)

[forrestbump]

First of all...

I am NO electrical expert by any means...

I had the same problem as you in my '65 F250 and it turned out to be a broken ground wire. I know it's labor intense, but tracing and testing wires is the only sane way to determine what's wrong.

Hope you don't take my answer as dismissive to your question... just want to prevent you from spending money on parts that you may not need.

[fordman]

i agree with the ground issuse of the socket probably or the socket going bad. or even the bulb contact to be worn down and not contacting the flat spring inside of the socket

[basketcase0302]

Agreed with above on possible ground issue. Check the main ground to engine / engine to frame / engine to cab grounds.
Then I'd put the voltage meter on the L and R brake light leads where they exit the cab heading toward the rear of the truck.

[JBMCQ]

Hey,
Thanks for the input , exactly what I was hoping for when posted. Bad ground makes sense.
Will check and post results.
Thanks for the replies.
Jim M
If you ever have a pool question just let me know , When asked how long Iv'e been working in that buisiness
the standard answer is too DAMN long ! HA. Not a lot I havn't seen.

[BRUTUS_T_HOG]

sounds like a bad ground to me. a short between the switch and the socket would blow fuses. run a jumper wire from the battery negative to the light socket ground and see if it disappears.

you can also use a multimeter attached between a good clean ground and the light socket ground set to volts.. if more than .5V you have a bad ground