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Old 05-08-2019, 01:12 PM   #1
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Join Date: May 2015
Location: Toronto
Posts: 3
Headlights wiring issue

Hi everyone,

I have a problem with my headlights that I can't figure out! Hopefully, someone here is an electrical guru and can help!

This issue is the headlights won't turn on...
I can see that the HI and LO gets 12v when the bulbs are plugged in (from the H4 socket).

When there are no bulbs plugged in only the LO gets power (when LO is ON) and HI (when HI is ON). But as soon as I plug in one bulb in either side of the harness, the other side plug will flow 12v on both HI/LO connectors.

I tried troubleshooting with a multimeter, on the dimmer switch side it is ok (the switch command the right thing). Past the relays that is where the problem occur... only when a bulb is plugged in. If no bulb is in then HI is dead when LO is ON and vice-versa.

This is in a 51 3100. All the wiring harness and switches is brand new from Speedway, along with the HD headlight kit from LMC truck.

So I have my Hi and Lo wires hooked up to the HD harness with the relays.
I have taken apart the HD harness and routed everything, it looks like this:

The light actually worked a few times (1 out of 10) and HI/LO selection doesn't seem to change anything.
I tried switching bulbs (my bulbs are brand new 9003/H4 Syvania Silverstar) and the old sealed 7". Same story!

Anyone have some ideas?
89db is offline   Reply With Quote
Old 05-08-2019, 08:11 PM   #2
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Join Date: Nov 2009
Location: Sherman, ME
Posts: 2,060
Re: Headlights wiring issue

Double check your ground connections for each headlight bulb. And make sure whatever they're grounded to (like the grille or radiator support) in turn has a good connection to ground.

Each bulb has a common ground terminal that's connected to both the high & low filaments. If that common terminal is not grounded, you will see voltage present on the high terminal when power is applied to the low terminal and vice versa.
ray_mcavoy is offline   Reply With Quote
Old 05-08-2019, 09:07 PM   #3
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Join Date: May 2015
Location: Toronto
Posts: 3
Re: Headlights wiring issue

ray... you figured it out!

I cleaned up the ground off the headlight mount real good, and added an extra ground from the mount to the engine grounding strap.

The lights are real bright and everything works as expected.

Thanks again!
89db is offline   Reply With Quote


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