Summary of the situation:

I have a 1975 Ford truck that suffered from the “Ford flashing headlight problem”.

The factory wiring uses 18ga wires over a very long path and through an under-rated switch to power the headlights. When someone “upgrades” the headlights to halogens, the increased load overheats the switch causing its internal breaker to trip.

The fix is insert a relay and use heavier wires (14ga) to carry the power to the headlights over a shorter trip. You can read about it here:

Original wiring:

Revised wiring:

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Is a 20A fuse correct here? It seems too high for 14ga wire.

Should there be a fuse on the control line for the relay (contact #85)?

(A couple months ago my first Garage SE question would have been “why do the headlights on my truck flash?”)


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asked Mar 9 ’11 at 0:44

As far as the fuses are concerned, it’s never a bad idea to add a fuse to a particular section of a circuit that you’re concerned about. For the high-beam fuses, in particular, I would start conservatively. Your average high beam (for a normal headlight) is 55 or 65 watts. In a 12 V circuit, that’s less than 6 amps nominally. If you put in a 10 amp fuse, you should be ok. I have looked at some amperage vs wire diameter charts and 20 amps seems a bit too close to the 24 amp maximum for my comfort levels. To reiterate, it’s never an issue to start conservatively on these things, and I think 10 amps will suit you just fine.

Wire Gauge vs amperage diagram

answered Mar 13 ’11 at 20:22


20 amps is more than enough for each circuit – you can probably run a 10 or 15amp fuse without issues. You’re not going to cook the wiring with 20 amps…

And you should definitely fuse your control wiring as well (Contact 85) – That would only require a 3 amp fuse at most.

answered Mar 14 ’11 at 3:34

Since the vehicle already has a fuse (the original) for the headlights it could be replaced with the 3 amp fuse to protect the input (pin 85) of the relays.

answered Dec 31 ’17 at 4:38

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