Mechanics

CHEVY 94 4X4: ONE HEADLIGHT IS BRI

1994 Chevrolet Silverado

Electrical problem
1994 Chevy Silverado V8 Four Wheel Drive Automatic 152000 miles

I thought I had a bulb problem, I installed new ones tonight. I still have my drivers low beam is dimmer than the passenger side. I tried 2 sets of new bullbs and even tried swapping sides with each one. I even tried a new headlight switch, dimmer switch, and a new ground to the dimmer side. It's frustrating the heck out of me. I am also not sure which way to turn the adjustment screws to adjust the beams.
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Geebi18
March 17, 2010.



Unplug the bright bulb. If the dim one goes out, you still have a bad ground, either in the socket, or more commonly where it bolts to the body. Current can't get to ground so it finds an alternate path through the high beam filament, then over to the high beam on the other side to ground.

A bad ground will usually also affect turn signals and running lights on the same corner. If they work fine, the problem is more than likely at the socket.

Caradiodoc

Caradiodoc
Mar 18, 2010.
To make sure we are on the same page here, the beams I am most concerned about is the low beams. When the lights are on dim the driver's side is dimmer than the passenger side.

Tiny
Geebi18
Mar 18, 2010.
Low beams are nice, but you have to use all the symptoms to figure out the cause of the problem.

If your truck uses separate bulbs for high and low beams, they still can share a common ground wire for the two sockets on one side. Did you try unplugging the bulbs on the right side to see what happens?

Caradiodoc

Caradiodoc
Mar 18, 2010.
So you think I need to unplu both bulbs on the passenger side and try the driver's side alone? I tried a new ground on the left side along with a new dimmer and headlight switch so something is resisting my volts. It still could be the ground as I have not thouroughly gone through it.

Tiny
Geebi18
Mar 18, 2010.
Unplugging the bulbs on the other side, whether head lights, brake lights, tail lights, etc. Is a quick, easy way to determine if they are involved in the lack of a good ground. If the left bulb is dim, it has to be getting voltage, but some of the 12 volts is being dropped across something undesirable leaving less than 12 volts for the bulb. That undesirable resistance can be other bulbs, resistance in a corroded connection at the socket, or resistance where the ground wire is bolted to the body.

Another way to approach this is to measure the voltages at the bulb. In the unlikely event the highest voltage is less than 12 volts, the problem is in the supply side. More commonly you will find a few volts on the ground wire where there should be 0 volts.

Caradiodoc

Caradiodoc
Mar 18, 2010.
Ok, after an extensive day of testing here is where I am at. I done everything step by step as you stated from removing bulbs and checking grounds. The passenger side which was the bright side had the common/shared ground, the driver's side low beam also had that shared ground. The Hi-Beam on the drivers side was a blue/white wire and went back into the firewall.

I took a digital voltmeter and checked both sides with the lights on low beam and the drivers side was 12.75V and the passenger side was 12.82V and that was with the engine running, not enough to make it as obvious as it is when you are pulled up next to a flat surface. With the engine off it checked 12.25 and 12.24. I ran a complete new ground from the dim side to the frame and also had no voltage leaks anywhere. I also tried one more set of bulbs with the same result.

I am really starting to think it is the headlight covers that are causing my problem. I am getting voltage reading within.08 every time I checked it. That is the only possible thing that I can think of remaining that could be causing it. What do you think? I am also going to donate more to you when I get it uploaded to my PayPal account. You have been very helpful.

Tiny
Geebi18
Mar 18, 2010.
I appreciate the donation, but think of it as a tip, not a requirement.

The supply voltage is only half of the story. You're right about the tiny difference side-to-side is irrelevent. Now measure the the voltages on two ground wires. I'm hoping you're going to find near 0 volts on the right and around 4 volts on the left.

Am I right in thinking you have two separate bulbs on each side? One for low beam and one for high beam? If so, do both sockets on one side have two wires each? Or does one socket have three wires?

I can't get into any of the online service manual web sites so I'm doing this from memory. Unfortunately, oldtimer's disease is starting to creep up!

Caradiodoc

Caradiodoc
Mar 18, 2010.
It does have 2 bulbs for each side. Also each socket has only 2 wires. So I should have some voltage on the ground side of the passenger side lights?

Tiny
Geebi18
Mar 19, 2010.
Nope. There should be 0 volts on the ground wire. I suspect you will find some voltage on the left ground wire. The higher the DIFFERENCE in voltage between those two wires, the brighter the bulb will be.

Caradiodoc

Caradiodoc
Mar 19, 2010.
Now should I ground my tester to the battery and then check for voltage on the ground side?

Tiny
Geebi18
Mar 19, 2010.
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