Headlight

Tiny
PICKUPGUY1
  • MEMBER
  • 2004 PONTIAC SUNFIRE
  • 100,000 MILES
The low beam on the drivers side is very dull when they are turned on. Same result with a new bulb
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Friday, March 25th, 2016 AT 5:03 PM

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Tiny
CARADIODOC
  • EXPERT
You should find the high beam is also dim. That is a real common symptom of a bad ground, however, GM has a rather bizarre circuit for their daytime running lamps, so there may be a computer involved with this problem. I'm an electrical specialist, but even I get dizzy trying to follow their wiring.

To verify a ground problem, unplug the right headlamp bulb, (both of them if there are two), then the left one will go off completely. That's because with a bad ground, current goes through the left bulb, can't get to the ground return path, so it goes over to the high beam on the other side, then through that bulb to ground. The right high beam will be on dim too but if it's in the same bulb as the low beam, you won't see it because the low beam is on bright already. You might see the high beam indicator on the dash is on dim when it shouldn't be, and on some cars a bad headlight ground will make the turn signal indicators on the dash light up dimly all the time.

This applies to all other car brands. Yours might act differently due to the weird circuit.
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Friday, March 25th, 2016 AT 8:17 PM
Tiny
PICKUPGUY1
  • MEMBER
Thank's for the reply. So how can I trouble shoot this?
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Saturday, March 26th, 2016 AT 7:00 AM
Tiny
CARADIODOC
  • EXPERT
Where I would start is by measuring the voltages on the bulb's socket, but to be valid, that will need to be done with the bulb plugged in.

You must find 12 volts on one wire and 0 volts on the other wire. If you have a double-filament bulb, don't worry about the third wire. What we're concerned with is a 12 volt and 0 volt wire.

I suspect you're going to find around 3 6 volts on one wire and 12 volts on another one. If you do find 12 volts, ground the other one. If that makes the bulb get full brightness, that ground wire is suspect.

Do you have a single bulb with the high and low beam filaments in it or do you have two separate bulbs, one high beam and one low beam?

I'm having internet trouble tonight and can't get into the wiring diagrams. Instead, tell me if you have one or two bulbs on each side, and what you have for voltages and the wire colors they correspond to. When I can look at a diagram, I'll be able to figure out where to go next. You may need to unplug the Body Computer, then the headlights should work like on any other car without daytime running lamps. That will make diagnosis easier.
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Saturday, March 26th, 2016 AT 9:21 PM

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