I have a friend with an '87 F150 4.9 who only has a brake light on one of his tail lights [I think it's the passenger side but not sure.] Anyway, the other light works just fine, so I'm sure this is a grounding problem- I just don't know where to look for the grounding points. Help... please?

Thanks so much!

 

There are two filaments in the rear tail lamp bulb.
One filament is used for the parking light.
The brake and turn signal share the other filament.

Not likely it's a ground problem because the tail lamps and license plate
lamp all share the same ground point which is G801. It's a Black wire bolted
to the inner side of the driver side front fender near the back of the headlight
area.

Assuming the parking light works and both the turn signal and brake light don't
work for that bulb, the filament could be burnt out or there is a wiring problem
from the turn signal switch to the bulb socket.


If the turn signal works and the brake light doesn't it's probably a bad contact in the
turn signal switch.

Here's the schematic for a 88. Color code of the wires may be different for a 87.

 

Thanks for the diagram/info- my chiltons has chassis wiring for the 87 in it but I couldn't find the ground point in it.

As to the bulbs, we already tried replacing both bulbs [as the back up lamp was out too] and that didn't do it, which I forgot to mention before, and is also why I figured the problem to be a grounding issue. I'm not sure if the turn signal is working correctly so I will check that out, but I don't believe the parking light is.

Far as I know, with a brand new bulb, the brake light is the only thing operational.

 

You need to verify what is not functioning.

Does the parking light work? You didn't say.
Does the turn signal work? Don't know.
Does the brake light work? You said no.

If all three of the items above don't work it probably
is a ground problem.

If any of the above items work, it can't be a ground problem.

Are both backup lamps not functioning or just one side?

Is this a new problem or was the truck purchased this way?

 

Does the parking light work? No.

Does the turn signal work? No.

Does the brake light work? Driver side only.

Are both backup lamps not functioning or just one side? Neither.

Is this a new problem or was the truck purchased this way? New problem, and I figured out why.

PO did a complete hack job wiring in the trailer harness. After getting underneath and checking things out, 2 original wires were broken and 1 was cut [that is, sheathing broken and wire partially broken]. From the looks of it, the PO's work with the cable ties failed, a part of the harness dropped, and then a rock or some other road debris nailed it, breaking the wires and cutting the other one.

Also after getting a closer look, the sockets [tail and b/u] are in pretty poor condition, so I just got back from the parts store with new sockets. I completely cut out the trailer harness as my friend isn't the type to need it anyway, and I was just double checking the location of that ground point so I could clean it anyway.

So, I'll get these new sockets installed and fix the other wiring and that should do it. We'll see. Thanks again.

 

Yeah it certainly seems that the hack wiring job was the issue. I've only gotten one side straightened out so far, new tail and b/u sockets installed and they're working properly. Just taking a break at the moment- it's a nice, sunny, humid day- I can only sweat for so long before I have to change a shirt.

Edit: Just confirming the bad trailer wiring was it. I just finished the job and now all the lights- parking, brake, turn, b/u- are working properly.

Thanks again for the wiring diagram and advice, F150XLT.