1986 F350 - Connecting backup switch to main wiring harness

[ramses]

Hello. I have a 1986 F350 that I'm fixing up to use for work (welding). It is a regular cab with chassis 4x2, 460 7.5L, Warner T19 and Dana 70HD dually axle. Previous owners did a lot of patch work on electrical. Working on getting all tailights to work.

I'm only missing ho to connect the backup switch harness to the main wire harness that goes all the way back to the truck. There is a plastic (plug? M-11071) that has the purple/orange wire from Fuse 5, the pink/black wire that goes to the backup lights, and two (2) red/white wires on one side, and a small loop of red/gray wire and two other wires, which were cut or torn off, on the other side. My guess is these missing wires is where the backup switch harness should connect to. But dont know what to do next since there is not enough wire left to connect the backup switch harness. Is there a way around this?

[Gary Lewis]

What you have in your hand is C121 from the schematic below.  And the loop is the start circuit because you have a manual transmission.



But that connector also serves the backup light wiring, which is pink/orange to the backup light switch, and black/pink after the switch.

Does that help?

[ramses]

Sure helps. But since the two wires on the side with the loop are gone, how can I connect the backup switch to the circuit. Should (can) I replace C121? Or cut the purple/orange and pink/black from C121 and tie them straight to the backup switch harness?

[Gary Lewis]

Yes, you can eliminate C121, or at least the portion for the backup light circuit.  The connectors are there so that Ford could put one transmission on one truck and another in the next truck, plugging the right harness into those connectors.  So you don't really need them.  Just tap into the wires and extend them to the switch.

[ramses]

Sweet. Can I cut both red/white wires going into C121 and tie them together to take out C121 completely?

Thank you very much Gary

[Gary Lewis]

Yes, you can do that.  But, I hope you are doing a good job with the connectors.  Like soldering and heat shrink tubing.

[ramses]

Absolutely. Previous owner did a lot of hack jobs. With your input I can finish up the back end. Next I need to get to the turn signal switch, but the steering wheel is not original, the main nut is stuck and the shaft does not lock. Will figure it out. After that check headlights, wiper motor and fuel sending unit.

[Gary Lewis]

We are here to help, and we'll get through it together.