GMC. Hi beam light question

[57rookie]

So, I am continuing to work on my 1971 GMC project.
The hi beams dont work.
It is a GMC and when I push the hi beam floor switch (which is brand new) I get no headlights with one push and the inside two headlights when I push it again.
The GMC has 4 headlights so I think two inside ones are low beam and all four would be hi beam but not sure.
Harnesses to the new headlight lamps look fine (someone else put them in), I replaced the floor hi beam switch with no luck.
I dont see a fuse for headlights at all. Dash headlight knob works fine. Can’t seem to get the power to all four of the headlights.
I am stumped.
Any Ideas???????

[Willys_MB]

I'm no expert, but I don't think it's wired from the factory to have all four headlights on. I think it's supposed to switch from low beams to high beams; however, you CAN wire it in such a way that all four come on.

Dumb question, but have you checked for continuity in your bulbs?

[Stocker]

The two inside lamps are high beam only unless someone has monkeyed with the wiring. Look at the rear of those lamps -- there should be two prongs on each connector, one for 12v high beam and one for ground.

Looking at the rear of the outer lamps, each has a three prong connector. One is 12v for low beam, one is 12v high beam, and the third is ground.

It's possible the connector to your floor-mounted dimmer switch is the culprit -- that wiring is prone to failure.

The odd thing is your inside lamps coming on without the outer lamps coming on at the same time, since high beam operates all four together. You may have grounding issues on the outer lamps.

Really need to start chasing voltage & ground with a test lamp or voltmeter to figure out where the problem is.

[57rookie]

That’s some good advise. I will check the connections to the lamps again and start with ground to the three prong outer ones. Thanks

[El Dorado Jim]

like Stocker said, all four lights should come on when you have the high beams on,check all connections, if you cant figure it out, try asking this question in the Electrical section

[mrein3]

999 out of 1000 times there is a hi-beam/low-beam issue with this model truck (67-72 GMC and Chevy versions) it is a furry connector on the foot mounted dimmer switch.

Before you take the front clip off, disconnect the floor switch, spray the connector out real good with a contact cleaner, spray off the prongs on the switch itself, reconnect, and see what you got.

If you still have issues, jump connections on the floor switch connector. All that switch does is send 12 volts from the headlight switch to either the low-beam circuit or the high-beam circuit. You can't screw it up by jumping any of those three wires to any one of the other wires in that plug.

If still no hi/low function, THEN you need to look further.

[StingRay]

Outside low only. Hi beam all 4.

[57rookie]

Well I wanted to install my new ignition switch which went perfectly today but I did first pull the harness to the floor Mtd dimmer switch and cleaned the harness connections. The floor switch is new. No luck still only getting only inside two headlights to work or no headlights when I toggle it. I tried jumping the harness with no luck.
I will now pull the bezels and headlights and check with a test light to see why no power to outer headlights.
I am not sure how they are suppose to be grounded (where the ground wire actually is grounding in the three prong harness but I have to keep working on it. I suspect it must be a ground issue at this point.
I highly doubt that I could have two bad headlights at the same time and the look to be very new from previous owner probably chasing the same issue...
Thanks for any advise.

[BROWN 70]

Hi guys, I have a 70 gmc. just recently I was checking the head lights on mine also. only the lows would work. when I switched to high beams there were none. so im about ready to go buy some high beams, and I thought pull the bezels off and check power. the power was there and all I had to do was wiggle the connector and clean the brass connection, bingo all four light up. in my case just a little cleaning and wiggling did it. BROWN 70

[Stocker]

Originally Posted by BROWN 70

Hi guys, I have a 70 gmc. just recently I was checking the head lights on mine also. only the lows would work. when I switched to high beams there were none. so im about ready to go buy some high beams, and I thought pull the bezels off and check power. the power was there and all I had to do was wiggle the connector and clean the brass connection, bingo all four light up. in my case just a little cleaning and wiggling did it. BROWN 70

Good deal, always better imo to troubleshoot than just buy new parts without knowing the problem.

A tip for electrical issues: after cleaning the connectors, smear on a little dielectric grease to help prevent future corrosion.

[mrein3]

Originally Posted by 57rookie

Well I wanted to install my new ignition switch which went perfectly today but I did first pull the harness to the floor Mtd dimmer switch and cleaned the harness connections. The floor switch is new. No luck still only getting only inside two headlights to work or no headlights when I toggle it. I tried jumping the harness with no luck.
I will now pull the bezels and headlights and check with a test light to see why no power to outer headlights.
I am not sure how they are suppose to be grounded (where the ground wire actually is grounding in the three prong harness but I have to keep working on it. I suspect it must be a ground issue at this point.
I highly doubt that I could have two bad headlights at the same time and the look to be very new from previous owner probably chasing the same issue...
Thanks for any advise.

At this point, you have to think it through.

You know the headlight switch on the dash works because the inside lights work.

What you don't know is:
Does the dimmer switch work?
Do you have two burned out headlights?

I would make a jumper wire to eliminate the dimmer switch. I believe 12V "in" to the dimmer switch is the top wire, not the two next to each other. Jump from the "in" wire to one of the two bottom wires you get lows, jump to the other and you get highs.

Unplug dimmer, turn headlight switch on, figure out which wire turns on the inside lights, then move the jumper to the other wire.

Then go under the hood and test for voltage at the non-working lights.

If you have voltage there you still don't know if the ground wire by the light is bad or the bulb is burnt out. Your test light has it's own return path to the battery. If you move from the headlight wire with voltage to the other wire, you are giving that wire a known working path back to the battery. If the headlight turns on, fix the ground wire. If it doesn't light, you have a burned out bulb.

Repeat on the other side with headlight switch still on and jumper wire still in place.

After you get them working, search these pages for "Headlight Relay Upgrade" and do it. Stock wiring to the headlights is about 20 feet of 16 or so gauge wire. Stock headlights have about 10.5 volts at them and are quite yellow. Turn your headlight switch into a headlight relay controller (follow instructions in headlight relay upgrade) and you'll be amazed at what 13+ volts does to light the road at night verses 10.5.

[VetteVet]

Here is a relay kit for the GM trucks with the four headlights. It shows how the low beams and high beams are wired with the two dimmer switch wires Light brown (low beam ) and light green (high beam) and the two inside lights are just jumped to the high beam wires on the relays. Without the relays the two inside lights are just jumped to the outside lights to the light green wires.

[Steeveedee]

Yes! That headlight relay upgrade made a huge difference on my truck.

http://67-72chevytrucks.com/vboard/s...d.php?t=782071

[57rookie]

Update - actually had time to go thru the wiring today.
Cleaning the harness on the LH side fixed the LH outside light.
Moving to the RH side I actually had two issues. Harness was loose so I cut the 3 wires and soldered in a new headlight wire/Harness that I had from another previous project. Then still no LH outer light.
3 lights out of 4, I was making progress.
Because all 4 headlights looked to be fairly new from the original owner I never it took me a while to thing - what if the bulb is bad...
I took the bad light and plugged it into the LH side that was working and no go.
I went and bought a new one and bingo, all 4 headlights working!
I also used a test light for power checks during the above scenario, handy tool that I forget to use more often. Thanks for the advice!

[Stocker]

Good deal, glad you figured it out & got 'em working! It pays to never assume anything, eh? Thanks for the update!

[Grumpy old man]