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My brother-in-law is doing a Cummins conversion on his '72 F150 and called me in when he got in over his head with the wiring. He had an aftermarket GM wiring harness on hand that he had started installing before I came onboard. I used the Ford wiring diagram to tie everything in to the factory column and in dash hazard switch and for the most part everything is working. Where I run into the issue is when the hazards are on, if you step on the brake it overrides the hazards. I know this is correct for the back, but it overrides the front as well. Is this how they work from the factory or is something going wrong here?
As far as I can tell by the schematic and probing the terminals, the hazard switch simply connects all four legs together when activated. This sends power to the front signals and the dash indicators and back through the brake circuit to the column to power the rear lights. Problem is if you press the brake, since that circuit is used to activate the rear lights, it overrides the signal to all four. My simplified version of the factory harness. I used this to make my connections.
Last edited by shiftylilbastrd; Aug 6, 2022 at 10:59 PM.
Thanks for the reply. That’s the same diagram that I’ve been using to get everything wired up. I’ve never seen hazards function that way and have spent hours trying to troubleshoot what I had done “wrong”.
A closer look at the schematics reveals three things.
1. All four corner lamps (tail and front running) are grounded locally.
2. This eliminates the need to even consider an interruption in the ground side of the DC circuit. The interruption/take-over must be in the positive side of the DC current.
3. By depressing the brake we cause (via the brake switch) the voltage at the hazard flasher to be reduced enough to cause the hazard flasher to NOT be able to function, thus stopping the hazard flasher's operation. This is why the hazard flasher is bigger and more complicated than the other flasher; not only does it flash, but it has to recognize a drop in current and interrupt (open) the 12v DC circuit, causing the hazard switch to be isolated.
In simple terms, the hazard flasher and brake switch do basically the same thing. Both energize the bright side of each lamp.
........because of the hazard flasher, it is impossible for both circuits provide power at the same time. The Brakes will always work, and flashers will only work when the brake circuit is open,
