The Stovebolt Site Forums The Parking Lot '60-'66 Trucks Headlight question 61 C10

I only have high beams on my 61. Thought it would be easiest to parts shoot, and have changed the dimmer switch, as well as the headlamp switch on the dash. The dimmer switch (new and old) seems to toggle from high beam to an off state. So other than having two new switches, nothing has changed.

My guess would be bad ground to the low beam, but have not tried to chase it. Previous owner wired in a toggle switch for the dome light, so it no longer operates through the headlamp switch.

I hate the thought of an electrical problem, and it's clear some things have been spliced behind the dash.

If it were your truck, would the next logical thing be to replace the Under Dash harness? That would get me a new fuse block, connectors for headlamp switch, ignition switch, dimmer switch and firewall connectors. If that takes care of the high beam, it should also take care of the dome light.

Thanks!

Chasing shorts and bad grounds can make a person take up drinking. Odds say it is something very simple. If your plan is to keep the truck for many years then a new under dash harness might just make sense. If you plan to have fun for a couple of years and pass it on, then find and fix what you’ve got.🛠

Maybe the low side of the head lamps are burned out?

All four lights come on in high beam - when I click the floor switch all lights are out again. Is there a dual filament involved, or are the 4 lights just on/off state? I agree, not a fan of loose grounds or shorts. Had a bad smoke check in the cab of a '53 many years ago. I'd like to fix it right once if possible!

Well should be easy to figure this out. I will assume you have a foot dimmer switch. I will assume you have four headlights. Two of them light and two do not. I assume you know which set of lamps are low and which are high. (inners and outers)...So... You have a connector on the dimmer switch. The switch has three contacts. The center is a 12V input. The other two are Low and High beam. Turn on the main headlight switch. Click dimmer until a set of lights come on. Using a ice pic style test light, clip the clip to a good clean bare metal place near the switch. Touch the ice pick end into the center contact on the switch with connector still plugged to it. You may have to pull connector out a tad. Test light will light. Touch the other contacts to see which one also lights the test light. Then click the dimmer once and see if the other switch contact lights the test light. If that checks out, then you know the trouble is beyond the dimmer. Like the dimmer connector itself, the wiring, the bulb connectors, the bulbs, the ground.
You can use this same method on the actual headlight sealed beam connectors, especially on the set that does not light. It would be good to know if your high beam indicator on the dash is lit.

Many variations to these tests. Like using little jumper wires to see if you can get the other bulb set to light from the switch or run a test lead from the battery to each bulb with connectors taken off. This bypasses everything just to test bulb. A million ways to trouble shoot, but you can see the idea. Which is, 1. See where 12V is and where is isn't. 2. To test bulbs with direct power. 3. Isolating the components and wires to see who is the bad guy. Isolating is bypassing existing circuits and switches and act as the switch yourself and using temporary replacement wiring. Bla Bla Bla....

It took me so long to type, there are two more responses, WOW! Anyway, now I see that you have it wired wrong at the dimmer. or wired wrong to bulbs. Read my other post and see what's up. Look at the wiring diagram for your truck to see how switch is wired to the bulbs. Check the dimmer to see if it's feeding one contact, then the other contact per my post. No dual filaments. Just two on/four on.
See Geoge's post below.

Thanks for the tips! I hope to spend some time crawling about and getting it figured out! Will update when I have good news.

There are dual filaments in the low beams. (outside lights.) On low beam the low side of the outer lights come on. On high beam the outer lights switch to high beam & the inner lights come on also. You will notice the outer lights have 3 terms. & the inner ones just 2. Like Bart says, get a test light & follow the circuit.

George

Thanks George, I had not suspected the bulbs as they seem to have both gone out at the same time. Cold front blew through, but hope to get out to the auto hobby shot tomorrow!

Thanks George. I stand corrected. Dual filaments on outers. I even have a 1960 GMC but it's all apart.

Cathy: I just checked the 1960 wiring diagram. Hope you have one for 1961.
Just as George says. 3 pins on outer bulbs, 2 pins on inner. The ground is done by wiring the two driver side bulbs together and to ground, and the two passenger side bulbs together and to ground.
The high beam is done the same way, as driver and passenger pairs. The diagram shows one high beam wire and one low beam wire coming from the dimmer to a dash connector (I believe you might call this the turn signal connector), then to a smaller 2-pin connector, out of that connector/block with 4 wires to bulbs. (If GMC wiring is the same as Chevy)

I'm guessing that there is a miswire from that last 2-pin connector or in the "chain wiring" of bulbs.

When one head light goes out it doesn't get noticed some times. Then when the other one goes out, it immediately gets your attention.

Indications show that all grounds are good and:
1. That BOTH outer bulbs low beam filaments are out. (blown or not receiving voltage)
or
2. The low beam circuit has a disconnect/broken wire/short. Starting at dimmer switch low beam output contact and checking voltage all the way to bulb.
or
3. 1 and 2 at the same time...or something causing 2. to blow 1.

Thanks - that was my thought on the grounds - but would not rule out a broken wire or short. Have pushed the trip to the auto hobby shop into next week. Gives me a good place to start and I appreciate the input!

Pretty easy to test bulbs. Take out the outer bulbs from the front. Observe which contact is ground. Take to the bench. Use a battery or a battery charger as a power supply. Attach ground lead to proper bulb terminal and touch each of the other two contacts/filaments, one at a time, to see if they light. As in: A bulb test.

Bulbs checked, but behind the bulbs, the headlight harness is a mess. Looks like someone had some "spare wire" but couldn't be bothered with proper splices. Rather than chase trouble, I went by Bowtie Bits and picked up a new harness (only $30.00 before tax). Will head out to the base on Wed and spend some time in the heated auto hobby shop. Your tips were spot on and appreciate your input.

Nice to have High and Low beams - the new harness went in without a hitch. Next question - should I trash the old harness, or wold any of the connectors be helpful to anyone?