Hot headlight switch

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Oct 20, 2013 | 08:35 PM

Whiskey Bent

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Hot headlight switch

'78 F-350 headlight switch get very hot after about 5 minutes. I changed switches but not better. The big thick black wire with yellow stripe is also hot. Any body else have this problem?

Oct 20, 2013 | 08:38 PM

montana_highboy

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From: Big Sky Country

FMC will assist you shortly....

Oct 20, 2013 | 09:04 PM

fmc400

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From: Austin, TX

How hot is hot? Warm to the touch, or so hot you can smell it? Any additional lighting, hacked wiring, or modifications we should know about?

Oct 20, 2013 | 09:41 PM

Whiskey Bent

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Hot enuff that I cant touch it for more than a few seconds. After about a half hour the all the lights except the head lights cut in and out and I can hear the relay in the switch click. The wiring is stock and seems ok.

Oct 20, 2013 | 10:16 PM

wyckedcombo17

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From: Suburbs of Chicago

Sounds like you're dropping voltage pretty good across the switch. When you replaced it, did any of the connectors appear burned up?

Oct 20, 2013 | 10:25 PM

Whiskey Bent

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It all looked ok but it did smell burnt. Where does that thick black wire with yellow stripe go? It seems something with this wire. I don't want to take wire harness all apart if I don't have to.

Oct 21, 2013 | 07:16 AM

fmc400

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From: Austin, TX

Quote:

Originally Posted by Whiskey Bent

It seems something with this wire.

Not really. Something downstream of the switch is drawing excess current. This is corroborated by the fact that it happens with more than one headlight switch, and the breaker interrupts. The main feed going IN to the switch will be hot for the same reason that the switch itself is hot.

There are two possibilities for this happening:

1) Excess loading from incorrectly installed aftermarket lighting, OR
2) Pinched wiring or frayed insulation downstream of the headlight switch.

I'll take your word for it that #1 doesn't apply to this case, so #2 must be it. Inspect the wiring very carefully starting at the headlights, parking lights and side markers, all the way up to the switch. Look for wiring mashed in between sheetmetal or cut in some way. If that all looks fine, start over at the tail lights and work your way forward.

I know you said the wiring looks OK - but if the wiring really was OK, then you wouldn't be having this problem. That, or the wiring is in fact not stock. They can't both be true.