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Weird Wiring- Possessed Tail lights- 1991 F150

I recently bought an old farm truck with lots of minor problems. I figured it was kept running well enough, considering the chassis has 300k on it. It is a 3rd vehicle for the sometimes heavy-duty jobs that just can’t be done with a compact.

My last safety concern related job involves fixing the taillights. One running/brake light worked when I bought it. I took the housings out and discovered that the original wiring was cut and new wires were run to both tail lights. The reverse lights were never replaced. I tried reverting back to the original wires; they don’t work. I reconnected the replacement wires on both tail lights and now the running light,brakes and turn signal work… except for one thing. When I depress the brake, the turn signal is interrupted! I thought it was supposed to work the other way around.

What can I do to make the turn signal work when I am pushing on the brake?

You’re missing a ground connection/wire.

Sounds like the new brake wires just go from the brake switch straight to the lights, bypassing the logic module that integrates the brakes and turn signals. You need to get a wiring diagram and resurrect the original set-up.

Do you have a factory service manual, or at least a Chilton or Haynes manual?

If you want to start from scratch, here’s some good info:

Brake light schematic

Thanks everyone! I have a Haynes on order, but your links, pics and suggestions will give me a good place to start in the mean time.

The logic modules that I am familar with are wired into vehicles that have seperate turn and brake lights. That old Ford interrupts the brake light circuit and replaces it with the turn signal input when turning. Most trailer lights are wired for the old system and the modules simplify wiring in a trailer.

If I had this problem first thing I’d do is measure the resistance between point c279 and point “Y” with the left turn switch on. It should be a high resistance. Likewise c279 and point “Z” with the right turn switch on.

Most problems with tail light wiring on trucks begins with people adding trailer lights. Removing all remnants of a botched effort to wire in a trailer light connecion might be the best place to start.

When I look at the diagram supplied by @knfenimore, there is no logic module. Ignoring the section of the turn signal switch that sends power to the dashboard indicator lights, the turning signal switch is nothing more than two single pole, double throw switches–one switch for the left side and one switch for the right side. Take the left side. When the turning signal switch is not set for a turn, the power to the filament comes from the brake light switch. The brake/turning signal filament is lit when the brake pedal is depressed activating the brake light switch. When the turning signal switch is set for a left turn, the power for the filament comes from the turning signal flasher instead. This causes the left side filament to blink on and off. However, the right side filament receives power from the brake light switch. When the brake is depressed, the right side filament will glow constantly, but the left side will blink because it is not receiving power from the brake light. The same reasoning applies when you signal a right turn. I think that the new wires run are not connected through the turning signal switch as they should be.
I had to install turning signals on my first car–a 1947 Pontiac. Turning signals were an option on the Pontiac in 1947 and my Pontiac didn’t have that options. I had to connect the brake light switch through the turning signal switch. There was only one wire running to the back of the car for the brake lights and the left rear light was connected to the right rear light. I had to break this connection and run a separate wire to the back for one of the turning signals. The diagram that knfenimore gave you is much like the diagram that I followed to install the turning signals on the 1947 Pontiac back in 1961.
Trace through that diagram and you will understand how the signals and brake lights function.