Weird Headlight Problem

Hello, everyone!

I have a 1986 C10 Silverado 1500.

When I purchased the truck, I knew the headlights needed work.

When turning on the headlight switch, the top driver's side lamp turns on, but not the passenger side. When hitting the dimmer switch on the column, the driver's side top lamp stayed on and the passenger side lower bulb turned on.

First troubleshooting step, I replaced the lamps in case of defect. At this time, when turning on the headlight switch, the driver's side top lamp turned on, but no other lamps. When hitting the dimmer switch on the column ALL FOUR lamps were off.

I assumed this meant either a bad ground or wire in the harness.

My next step was to clean the ground connections with a drill and wire brush, replace a damaged ground from the harness. I installed the heavy duty headlight harness from LMC truck.

After installation, the behavior returned to what I initially had (turning on the headlight switch, the top driver's side lamp turns on, but not the passenger side. When hitting the dimmer switch on the column, the driver's side top lamp stayed on and the passenger side lower bulb turned on).

This afternoon after work, I will replace the headlight switch but am not feeling confident. I think this will require pulling down the steering column to replace the dimmer switch.

Is there another possible solution that might not be as hard as that?

I seriously doubt it's either switch. And before I replaced one even if I suspected it I'd test it. Where I'd start is unplug all 4 headlights. With a multimeter check each headlight socket for ground.(in ohms mode ground 1 lead of your meter,touch the cavities in the headlIght socket with the other) each socket should have 1 socket that should make your meter read 0.00 ohms to 0.2 ohms. After that in volts mode again 1 lead grounded headlights on low position check your top 2 headlight sockets,should be 1 terminal in each reads close to battery voltage( check battery voltage with the meter) in high position check for 1 hot terminal at each socket. Post results. Just thought what I'd do before that. Park facing a wall,look at the wall while wiggling all the under hood wiring,watch for flickers on the wall.

Ricko1966 said:

I seriously doubt it's either switch. And before I replaced one even if I suspected it I'd test it. Where I'd start is unplug all 4 headlights. With a multimeter check each headlight socket for ground.(in ohms mode ground 1 lead of your meter,touch the cavities in the headlIght socket with the other) each socket should have 1 socket that should make your meter read 0.00 ohms to 0.2 ohms. After that in volts mode again 1 lead grounded headlights on low position check your top 2 headlight sockets,should be 1 terminal in each reads close to battery voltage( check battery voltage with the meter) in high position check for 1 hot terminal at each socket. Post results. Just thought what I'd do before that. Park facing a wall,look at the wall while wiggling all the under hood wiring,watch for flickers on the wall.

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Okay, here’s what I’m doing, per your instructions.:

Disconnect all four headlights but leave the battery connected (not a problem, since I am halfway through swapping out the headlight brackets and the grill is still off).

Set my meter to Ohms, Touch one lead to the ground point and check each of the sockets for a reading between 0 and 2.

After confirming that I have ground in each of the 4 sockets, switch to voltage mode and determine the current battery voltage.

Turn on the headlight switch and check the low beam sockets to see if one has battery voltage.

Next, engage the dimmer switch and check all 4 lamps to determine if I have voltage on a socket at each.

I’ll get to work on it in a few minutes and report my findings.

0 and .2 check battery voltage at the battery 1st so you know what battery voltage is.

Ricko1966 said:

0 and .2 check battery voltage at the battery 1st so you know what battery voltage is.

Click to expand...

Battery voltage is 12.62 vdc

Passenger top lamp:

.3 Ohm
12.62vdc

Passenger bottom lamp:

.4 Ohm
12.60 vdc

Driver top lamp:

.4 Ohm
12.45 vdc

Driver bottom lamp:

.4 Ohm
12.45vdc

The readings may be off a little. I had to use my cheap meter because my Fluke is dead as a doorbell, tried two sets of new batteries on it.

Also, a couple more points:

This occurred same pattern (no passenger high beam, no driver high beam) with the old headlights. I switched to LED and same results.

Today, I broke the ring off the ground wire from the harness and had to extend it, used the same gauge wire and this time grounded to the battery terminal. Got the same results as when grounded to the chassis.

DougC10 said:

Also, a couple more points:

This occurred same pattern (no passenger high beam, no driver high beam) with the old headlights. I switched to LED and same results.

Today, I broke the ring off the ground wire from the harness and had to extend it, used the same gauge wire and this time grounded to the battery terminal. Got the same results as when grounded to the chassis.

Click to expand...

LED that's a new part of the equation,LEDs anyway with those readings you should have all 4 headlights,plug your old headlights back in,wiggle the connectors,where they go on the headlights I suspect corrosion in the sockets. This is very unlikely but I want to rule it out so leave the leds out of the equation at this point test your old sealed beams with the battery and some jumper leads. LEDs are polarity specific,if one of tge headlight sockets is wired incorrectly,the LED won't light,if the LED is wired incorrectly it won't light.

Ricko1966 said:

LED that's a new part of the equation,LEDs anyway with those readings you should have all 4 headlights,plug your old headlights back in,wiggle the connectors,where they go on the headlights I suspect corrosion in the sockets. This is very unlikely but I want to rule it out so leave the leds out of the equation at this point test your old sealed beams with the battery and some jumper leads. LEDs are polarity specific,if one of tge headlight sockets is wired incorrectly,the LED won't light,if the LED is wired incorrectly it won't light.

Click to expand...

Just want to point out that the original glass headlights did exactly the same thing. I replaced them to see if they were defective.

I can go buy another set to test with, but that will cost me about $70 without telling us anything new.

Regarding the sockets, wiggling doesn't change anything. I plugged and unplugged several times, as well.

Please recall that I plugged the LMC harness into the first socket in series, which seemed okay and now the headlights are plugged into brand new sockets from that harness.

I can get another socket, cut the original one off, and wire the new one in to test, but, with the same behavior I had with original harness and original lamps, I am not confident that this will change the behavior.

Okay. I didn't realize you threw your old headlights out. With those readings you should have 4 working headlights. Take an incandescent test light hook it to the ground and powers you listed,it should light,in all 4 holes. Verify. Post back. As for what you've changed or what I recall,what ever. I don't change parts to see if they are defective. I test,then replace what's bad. Sometimes your new part is bad and I've seen plenty of posts about lmc headlight wiring being incorrect. Anyways throwing new parts in very often just complicates the trouble shooting.

Check your wiring, too, against a wiring diagram. When I installed an LMC headlight relay harness, their pinouts on the headlight sockets were wrong.

Ricko1966 said:

Okay. I didn't realize you threw your old headlights out. With those readings you should have 4 working headlights. Take an incandescent test light hook it to the ground and powers you listed,it should light,in all 4 holes. Verify. Post back. As for what you've changed or what I recall,what ever. I don't change parts to see if they are defective. I test,then replace what's bad. Sometimes your new part is bad and I've seen plenty of posts about lmc headlight wiring being incorrect. Anyways throwing new parts in very often just complicates the trouble shooting.

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I retested and the results were very close, except this time both of the passenger side sockets and the driver side top were at .3 ohms.

Just to get myself on to the next problem, I switched the sockets for the lower and upper passenger side lamps. When I hit the dimming switch, I have two headlights on the top, so that's how I'll run until I can get together with one of my friends or, failing that, put it in the shop.

DougC10 said:

I retested and the results were very close, except this time both of the passenger side sockets and the driver side top were at .3 ohms.

Just to get myself on to the next problem, I switched the sockets for the lower and upper passenger side lamps. When I hit the dimming switch, I have two headlights on the top, so that's how I'll run until I can get together with one of my friends or, failing that, put it in the shop.

Click to expand...

Slow down and go back to what you changed. You have anew headlight harness? A relay kit? Doesn’t matter.
Make sure the wires are pinned out correctly like the guy above said.
The relay kit I bought for my 86 wasn’t pinned right either. Same symptoms.
Likely your problem. Did they work before the harness?

Grit dog said:

Slow down and go back to what you changed. You have anew headlight harness? A relay kit? Doesn’t matter.
Make sure the wires are pinned out correctly like the guy above said.
The relay kit I bought for my 86 wasn’t pinned right either. Same symptoms.
And stop buying new headlight bulbs until you figure out what’s wrong.

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If it has exactly the same behavior as the prior bulbs and wiring, then the changes I made are not the issue, are they?

I appreciate everyone's suggestions, but this isn't productive. I'm testing everything out along the switch, fuse panel, and harness to find where the issue is.

DougC10 said:

If it has exactly the same behavior as the prior bulbs and wiring, then the changes I made are not the issue, are they?

I appreciate everyone's suggestions, but this isn't productive. I'm testing everything out along the switch, fuse panel, and harness to find where the issue is.

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Well the headlights don't have a fuse or go through the fuse panel. The headlight switch wont just power some headlights but not others Good luck.

Very true, but I’ve got 12vdc and ground at very headlight socket and brand new bulbs, but still the same behavior. I’m testing everything because what’s there doesn’t make sense.