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Fog light wiring mystery

228 Views 3 Replies 3 Participants Last post by  KF4SQB
My neighbor has a 1999 Cherokee. Some years back his fog lights failed and a mechanic repaired it. Now they have failed again. I can't find an explanation for this set of observations:

- The bulbs are getting 9.3 volts, but not lighting at all. The bulbs work when tested directly connected to a 12 volt battery.

- Pulling the fog light fuse and relay does not interrupt current to the bulbs.

- Pulling the fuse for the parking lights does interrupt current to the bulbs.

I'm inclined to think the mechanic put the fog lights on the parking light circuit as a workaround to a wiring problem, except that I don't see how the fog light switch would still function, nor why the bulbs don't illuminate at least somewhat with 9 volts. And if I'm wrong about the wiring workaround, why the heck are the fuses behaving like that and why are the bulbs only getting 9.3 volts from a battery that's putting out 12?
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Bad grounds or faulty/corroded wires or wire connections can cause those symtoms. The logical solution is to trace all the fog lamp wiring, and repair or replace any defective wires or connections.

Be sure to use a fuse and a relay in the fog lamp circuit.

I would remove the old stuff and re-wire the whole thing from the firewall with fresh wires and parts.
Well, the lights started working again, at least for now.

The switch, which was replaced years ago by a mechanic, has a relay directly behind it. So, apparently, there are two relays on the circuit, which confuses me.
One relay prevents the fog lights from coming on unless the head and/or parking lights are on, and the other prevents them from coming on if the high beams are on.
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