GMC Headlights (4 bulbs) all dead or dims only.

[WorkinLonghorn]

I can only run with dims (outboard ones). If I need high beam all are dark. I replaced the dimmer switch with a known good one, no change. What would cause this? Thanks. - Brian

[VetteVet]

The power wire for the headlights is the blue wire at the dimmer switch terminal. It connects to the brown wire for low beams and the light green wire for the high beams.
I would try jumping the blue wire and the lt green wire to see if the high beams come on.
If they don't then you might have a break in the light green wire between the dimmer switch and the left high beam head light. Check the dimmer terminal plug for corrosion.
You may also have relays in the wiring if a previous owner had installed them and the high beam relay may have failed.
It"s also possible that all four high beam elements have burned out. Since your two outside bulbs work on low beam I would rule out bad grounds.

['68OrangeSunshine]

Check ground leads from frame to radiator bulkhead.
Check each bulb for continuity.
Check voltage to headlight plugs.
Check regulator and alternator output.

[michael bustamante]

definatly check your grounds at the radiator support

[WorkinLonghorn]

Turned out to be a defective brand new ACDelco dimmer switch. I tested the continuity through the switch and only one position of the switch worked. Returned it for another and problem solved. Thanks for the replies. I should have tested first but I'm used to new things actually working. -BA

['68OrangeSunshine]

Since AC Delco went to Mexico, I don't trust their quality control.
I now use Denso spark plugs for that reason. I hear they're made in Battle Creek, MI.

I have had a defective 'new' boot-tap Hi-Lo switch, but it was a cheap Checker Auto ''house brand'', not an AC Delco.

The issue I found on my '71 Jimmy, was all 4 elements on all 4 bulbs burned out. This was caused by a runaway EVR [external voltage regulator]. When I replaced the EVR, the front engine wiring harness, and the bulbs, and grounded the Lights harness at the passenger side radiator wall under the Batt Box, [where the PO had hidden a cold splice - no solder, no crimp, just twisted wires and e-tape ]
it has all worked great ever since.
Sylvania replaced all my halogen lights under warranty, but their engineering section required me to submit milliAmmeter readings on each lamp. Had to build a special harness and buy a surplus Ammeter to get the readings. But they delivered, after all that.