High Beams Go On When Pulling Fog Light Switch
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Joined: Aug 2022
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From: CA
High Beams Go On When Pulling Fog Light Switch
Howdy all,
It was a rather foggy morning here in Idaho and I tried to use my fog lights for the first time, however it appears that when I pull the tab out to activate them, my high beams also turn on. I did some digging on old forum posts and it seems it might be something to do with the fact I have aftermarket LED bulbs? I'm running single light headlamps with 9007 bulbs and the fog lights are definitely aftermarket but were installed by a previous owner. I had the fast blinker issue due to the lower voltage on the LEDs which I fixed by picking up a LED flasher relay so I'm assuming the issue is with the resistance/voltage from the LEDs in the fog lights. Would simply adding resistors on the fog lights themselves work? Or is there perhaps something else at play? I doubt its the grounds as I cleaned and reinstalled the entire wiring harness a couple of years ago when I put a new engine in.
Thanks!
It was a rather foggy morning here in Idaho and I tried to use my fog lights for the first time, however it appears that when I pull the tab out to activate them, my high beams also turn on. I did some digging on old forum posts and it seems it might be something to do with the fact I have aftermarket LED bulbs? I'm running single light headlamps with 9007 bulbs and the fog lights are definitely aftermarket but were installed by a previous owner. I had the fast blinker issue due to the lower voltage on the LEDs which I fixed by picking up a LED flasher relay so I'm assuming the issue is with the resistance/voltage from the LEDs in the fog lights. Would simply adding resistors on the fog lights themselves work? Or is there perhaps something else at play? I doubt its the grounds as I cleaned and reinstalled the entire wiring harness a couple of years ago when I put a new engine in.
Thanks!
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Joined: Apr 2010
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From: Clayton MI
I doubt it has to do with them being LED's..... More likely how they are wired. The fogs are supposed to turn OFF with the high beams...... As they are not stock, gonna have to trace the wiring, and see what the previous owner did.
Record Breaker
Joined: Dec 2018
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The correct headlight bulb for a non-sport truck is 9004. 9007 does not interchange, see here: https://sealight-led.com/blog/9004-vs-9007-bulb.html LED bulbs contain electronic circuitry and the fact that later years of the second gens use ground switched lights often confuses the circuitry. If you look at the wiring diagram the fog light relay gets grounded when the fog light switch is turned on. With the low beams on the relay receives its power from the power wire that goes to the headlight lamp, through the high beam filament (which doesn't light up because the relay coil has a higher resistance) and to the relay. When the high beams are turned the high beam filaments get connected directly to ground via the multi-function switch. Now the relay has ground on both sides and turns off the fog lights. Yes they were smoking some good stuff in Chrysler engineering in the late 90s .... 
Obviously the LED bulbs don't do the "pass-through" of 12V like a filament does so the relay doesn't turn on *but* the LED bulb now sees a ground via the relay coil when the fog light switch is turned on. This cause the high beam LEDs to come on.
Obviously the LED bulbs don't do the "pass-through" of 12V like a filament does so the relay doesn't turn on *but* the LED bulb now sees a ground via the relay coil when the fog light switch is turned on. This cause the high beam LEDs to come on.