Hey guys,
the right rear brake light on my 90 F150 has become a phantom light. Sometimes it works, mostly with engine running and lights on and sometimes it don't...like with the engine off. This occurs when I've got the trailer light T hooked up. Can't ever get it to light when the T is out. Both bulbs are good. Have everything else on that side...back up, taillight and blinker, just no brake light. How can I test the connections with a volt meter? Don't have a test light. The bulb socket and connector have two or three black wires and an orange wire and a green wire. Which is the brake light wire? Can I just probe the wire sockets to see if I'm getting juice...should it read 12 volts? Also, could it be the brake pedal light switch? This truck has over 150K on it. Is the pedal switch hard to change? Sorry for all the questions, but don't want a ticket and need to get this cleared up so I can haul the boat to the lake...summer is disappearing fast!

 

The wire to the right brake light at the rear socket should be orange with a blue stripe. It should have 12 volts to it when the brake pedal is pressed regardless of key position.
The right signal light uses the same ground and element in the light bulb. So if the right signal light flashes you can eliminate the bulb, socket, ground and wiring back to the turn signal switch.
The problem is likely the turn signal switch as the brake light circuit is routed through it. This enables the brake light to function when the turn signal is off but the turn signal has precedence over the brakelight. Meaning when the turn signal is on the brake circuit is interrupted for that side by the turn signal switch.
It is definitely not the brakelight switch as this supplies power to both brake lights. Actually it supplies power to the turn signal switch, and if the turn signals aren't selected on both brake lights are on.
Hope this makes sense.

Sparky

 

Thanks Sparky. So does this mean I should replace that friggin turn signal switch in the steering wheel? Is that the turn signal switch you are referring to? Did that about 4 months ago when one of the little nibs broke off...used an aftermarket replacement which amounted to a crescent moon shaped plastic piece. Guess I'll pull the wheel again and have a look see. Any other tips for this job?

 

Wow you only got 4 months out of the turn signal switch, that sucks.
The contacts for the brake circuit are all internal to the switch. I dont think you are supposed to be able to fix them, but I am sure there is a way. Just easier to get another switch.
You can verify the dead circuit by using an ohmmeter. Disconnect the steering column connector and on the side going to the turn signal switch measure between the following wires:

Light Green to Orange light/Blue stripe should be 0 ohms
(brake power in) (to Right hand brake light)
Turn switch has to be in centre off position

Light Green to Light Green/Orange stripe should be 0 ohms
(brake power in) (to left hand brake light)
Turn switch has to be in centre off position

Light Blue to Orange light/Blue stripe should be 0 ohms
(signal light (to right hand signal light)
power in) Turn switch to right turn

Light Blue to Light Green/Orange stripe should be 0 ohms
(signal light (to left hand signal light)
power in) Turn switch to left turn

For your problem you should have an open, or very high resistance for the first check.
Since the left and right brakelights share the same power feed wire(light green) and the right hand brake and turn signal share the same wiring (Orange/light blue stripe)to the rear bulb, as well as the same filament and ground. The only possible fault is an internal intermittent failure in the turn signal, like a bad contact.
Have fun

Sparky