I have an 85 corvette with left headlight is dim. I tried new light, cleaned ground, replaced plug, ran ground from the battery & still dim. Any ideas?

 

With the lo beams on, use a volt meter and poke the meter probe into the backside of the electrical connector Tan wire and the other probe to the Black wire. Compare the voltage you measure to the other headlight.

When you turn the hi beams on are both headlamps the same brightness?
The hi beams use the Light Green wire.

 

Often a bad ground will cause this.

 

With the highs on, only the right side works. When checking voltage with a test light, power is there on tan for low & with highs on power there on green wire. I will not be able to check voltage until this weekend.

 

Tested the low beam, tan wire, got 8 volts. Battery tested at 12 volts. Was not able to compare right side voltage yet. Tried again replacing the light & ran ground from the battery. NG. Could switch in dash or high beam switch be bad?

 

Here's the schematic.

First question is, have the headlights worked properly since you've owned the vehicle?

If not, it's possible Bubba Jr. may have modified the wiring.

Have you removed the wires from the left side headlight connector
and possibly reinstalled the wires into the connector in the wrong
position?


Both Lo beam or Hi beam receive 12 volts from the headlight switch.

When the headlights are in Lo mode you should measure 12 volts
on the Tan wire to ground at each headlight connector.

When the headlights are in Hi beam mode you should measure 12 volts on the Light Green wire to ground at each headlight connector.

 

Two measurements will tell you which wire going to the headlight is the cause of your low voltage at the headlight. Headlight on, measure voltage between the positive wire at the headlight and the positive terminal on the battery. Normally I would expect less than 0.2 volts. Do similar on the negative wire at the headlight, measure the voltage between the negative headlight wire and the negative terminal on the battery. Again, normally I would expect to see less than 0.2 volts. One or maybe both wires are going to show about 4 volts and the wire/s with the over 0.2 volts is the one you will have to examine to find where the voltage drop is occuring. Most likely it is the poor ground connection because the ground is a self tapping screw through metal which over time becomes rusty and rust doesn't conduct electricity. The positive wire has a couple of connectors and the connectors are suspect. If its the positive wire, start measuring along the positive wires closest to the battery and after each connector. One connector will likely have all of the voltage drop.
The socket that plugs into the headlamp is also suspect, those contacts become corroded over time and make poor electrical connection. A little sanding of the contacts and application of spray contact cleaner will fix that.