Headlight Wiring Help

[Motorhead-47]

I bought a 67 C10 last weekend and I need some headlight wiring help from the experts. Background....the previous owner put a new Painless Performance harness in the truck. On high beams I get bright lights and a good high beam indicator on the dash. On low beam, the lights are very yellow/dim and the high beam indicator on the dash glows dim. Both the headlight switch and dimmer switch are new.

OK, here is what I see with a multimeter. With the lights off, all three terminals at the harness at the headlight plug show continuity to ground. With high beams on, I see 13v on the high beam terminal at the headlight, ground at the low beam, and 13v on the ground wire. On low beam, I see 13v on the high beam, ground on the low beam, and 7 volts on the ground wire.

This drawing is from looking at the back of the headlight. Hopefully I have that right.






Anyone have an idea what is wired wrong where?

[Oldtruckfanatic]

The ground is supposed to go right to.a ring terminal at the core support. If you disconnect the headlights do you still show 13 volts at the ground wire? Or is it showing 13 volts because its hunting for a ground. It should be fairly simple. Power from the headlight switch to the dimmer switch and power from the dimmer switch to the headlights and the ground to the core support.

[Oldtruckfanatic]

Here is a stock headlight harness plug. The ring terminal goes to the ground side of the plug.

[Oldtruckfanatic]

Not sure why it posted sideways

[Motorhead-47]

I verified that the ground wire on each headlight is grounded to the core support. With both headlights unplugged, looking face on to the connector, on low beam I get 13v on the far left plug (I thought that was the high wire?), the top terminal is to ground, and the far right terminal is to ground. With both headlights unplugged I also now no longer see the dim glowing high beam indictor on the dash.

On high beam, I get....nothing (no voltage no continuity) on the far left terminal, the top terminal I get continuity to ground, and the far right I get 13v.

[Motorhead-47]

OK...I just took a look at your image and realized the ground terminal on both sides is in the top/center lug on my truck. Top center is supposed to be the low beam and not the ground right?

[Oldtruckfanatic]

If you're looking at the plug the ground should be the right side. If you're looking at the back.of the headlight it should be the left side

[Motorhead-47]

Originally Posted by Oldtruckfanatic

If you're looking at the plug the ground should be the right side. If you're looking at the back.of the headlight it should be the left side

Got it....not sure if the connectors came preinstalled with this harness or if the previous owner installed them but they are definitely in the wrong slots.

Thanks....I'll fix this tomorrow.....beer owed!

[Oldtruckfanatic]

It should be just like your drawing

[RichardJ]

Continuity checks will serve little purpose and if you aren't careful, send 12 volts into your ohm meter and you could screw it up.

Clip one volt meter lead on the battery Neg terminal and leave it there. Check for voltage on the ground terminals at the light sockets. It wouldn't hurt to check ground side of parking lights. You need to find 0.0 volts. A couple of tenths is acceptable.
You make these checks with the lights on. The little battery in your ohm meter does not provide enough current to properly check these ground circuits.

In your first post you said you measured 7 Volts on one of the socket grounds, but weren't clear on where you were measuring from. At any rate, voltage on a ground indicates a bad ground.
Start from the light socket and work back until you find a good ground. The radiator core support sits on rubber, but it is bolted to the fenders. The fenders are bolted to the body. Neg bat to body should be 0.0 V. Neg bat to engine should be 0.0 V. Neg Bat to frame should be 0.0V.

With a bad or no ground on the headlight socket, current from the low beam side of the dimmer switch will flow through the low beam element, through the high beam element, back througth the high beam wire to the high beam indicator and find its ground there. Resistance will be high so the high beam light will not light at all and the high beam indicator will only glow.

[Stocker]

Assuming the headlamp plugs are wired correctly, which you said you'll check / fix tomorrow....

You also said each plug is grounded to the core support, but you should make sure that the core support itself is grounded too.

[Motorhead-47]

Wired the headlamp connectors correctly today and both high and low beams now work as they should. Amazing!...

The funny thing is the previous owner drove it around for about 300 miles with the headlights wired all goofy. Guess he never drove it after the sun sat.