No low beam in one headlight

[PamelaC]

The low beam only of a new driver-side headlight will not work in my 1991 Celica. I have checked and there is power all three of the openings into which the lamp plugs. I also have a passenger side bad tail light receptacle (corroded and a broken wire). Can these be related? If not what else do I look for to fix the headlight?

[PittsburghCeli]

If you have power at the headlight receptacle, then it must be your bulb. One of the connections is for ground and the others are for high and low beams. Measure voltage from a ground point on the frame to each wire. The ground wire should not have voltage and the wires for the high and low beams should have voltage. Then if you have voltage, it must be your bulb. If not, there could be something wrong with the drivers side headlight relay which is located inside the drivers side fender.

[PamelaC]

Thanks, I tested the bulb on the passengers side and both high and low beam worked. Power is also fine on the drivers side. So the problems with the backup light on the passengers side is not related? Is a relay easy to replace?

[new echo owner]

Have you check the fuse? There is a fuse for each individual head light. Voltage can some times back feed through the other headlight and shown as power at the terminals, but may not be sufficient voltage to light up the bulb. Both head lights are powered by the same relay, if the right side is working, then the relay is not the cause.

The backup lights are on a different circuit, so it probably won't cause the head light not to work.

N.E.O.

[PamelaC]

I re-checked the fuses: I am even more confused as to what the problem is:

• Both fuses in:
  • RH works high and low
  • LH works high only
• RH only fuse removed
  • RH works low only
  • LH works high and low
• LH only fuse removed
  • RH works high and low
  • LH does not work at all

[new echo owner]

Sound like you have a problem either with the head light switch or wiring. Do you have a meter that you can use to check the resistance?

N.E.O.

[PamelaC]

No, but my neighbor does. Unfortunately he is not home. What do we check and for what?

Sounds like driving with the RH fuse out would solve my low beam problem (just kidding) but it would save me from getting a ticket until I can figure this out.
PC

[new echo owner]

Originally Posted by PamelaC

No, but my neighbor does. Unfortunately he is not home. What do we check and for what?

Sounds like driving with the RH fuse out would solve my low beam problem (just kidding) but it would save me from getting a ticket until I can figure this out.
PC

Check for high resistance in the circuit. Are you checking the circuits with a test light then, since you don't have a meter?

N.E.O.

[PamelaC]

Yes, and hopefully I can ask him for help later today or tomorrow morning. I will post what we find.

[new echo owner]

A test light will work but not as accurate. You will need to disconnect both bulbs, and test the ground connections by hooking the test light to the positive side. Check the brightless of the light if it lights up, if it doesn't light up, you have a poor ground. The two power wire of course will not light up, and you do have to switch from low beam to high beam to check them. The power wire is red and red with yellow tracer. The other would be either high beam or low beam; Red with white tracer for low,and Red with blue tracer high.

N.E.O.

[PittsburghCeli]

Originally Posted by PamelaC

I re-checked the fuses: I am even more confused as to what the problem is:

• Both fuses in:
  • RH works high and low
  • LH works high only
• RH only fuse removed
  • RH works low only
  • LH works high and low
• LH only fuse removed
  • RH works high and low
  • LH does not work at all

After studying the wiring diagram for a while in the hayes manual...

Fix the corroded tail light wire first because it might be related and there might be some feedback.

If that doesn't work then..

I have it narrowed down to 1 of 2 wires that have to be causing your problem if it's not the corroded tail light wire. The LH red wire and the RH red-yellow wire going between the fuse and the bulb. I think one of them is bad in some way.

Run a new long test wire between the fuse and the bulb on one side and test, then try the other side. Remember to cut the existing LH red wire and the RH red-yellow wire out of the picture and not just splice into them. In other words cut the wire at the headlight and cut the correct side of the fuse wire. Then run a long wire from the fuse wire out the window and over the fender to directly behind the headlight wire just to test.

If something was wrong with the any of the other (ground) wires then either the light would never work or the light would always stay on. The other wires are negatively triggered which means, to turn them on, the switch takes the wire to ground, but ya never know with those weird symptoms, could be getting some feedback voltage. At least the test above would test 2 of the 6 wires.

Another thing I can think of would be to check for radiator electrolysis. To make sure your frame is not seeing any voltage because it can corrode the radiator. I am not sure exactly how to check but it's easy to find directions on the net and I think it's just a matter of check for voltage in one spot.

Also remember that basically everthing in a car should be 12volts DC if you ever get a voltmeter. Ohm meters can test the continuity of the wire to make sure it's not cut anywhere and should measure 0 ohms across a good wire and infinite Ohms across an open/cut wire. A digital multimeter measures Resistance in ohms(R), current in amps(I) and voltage in volts(V). I=V/R is the equation for current and P=VI is the equation for power which is measured in Watts. The good digital multimeters measure more current than others. Radio shack has them cheap.

That's my best guess. Good luck!