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My headlights will only light up on high beams.

The dimmer switch is new and I know that is often the source of headlight problems due to the way the circuit is wired. The same problem is exhibited with the original dimmer switch.

What's the likely source of this problem?

I intend to run some temporary jumper cable grounds all over the place to see if there is a grounding problem. Each headlights grounds are clean and on unpainted metal of the inner fenders.

The simplest, and most obvious answer is that both low beam filaments are burned out. Test each sealed beam individually to make sure that they burn on both high and low beams using jumper wires to a known good hot and ground circuit. If the bulbs are good, back up through the wiring harness from the headlight connectors to the dimmer switch and find the open circuit. If a faulty ground is the problem, it's likely that only one bulb will malfunction, since there's usually an individual ground wire at each headlight connector.
Jerry

Sorry I failed to mention that the bulbs are new, 6v units and I tested them out of the truck with jumpers to the battery and all was good.

Gotcha on the one bulb / ground connection. Makes sense.

Assuming stock headlight wiring which has a 3 position terminal board on the driver's side inner fender near the front ...

Attach a long wire to battery + with the other end stripped back just 1/4 inch or so. Touch one terminal at a time and see what happens on the front of the truck. One is high beam, one is low beam and one is for the parking light.

Because your high beams work, you have proven the circuit is good from the battery, through the headlight switch and down to the common terminal on the dimmer switch.

Thanks. I'll do that tomorrow. Yes, it's stock, 1950, 6v wiring.

Dimmer switch may be wired wrong. If you connect the lead from the light switch to the wrong terminal you could get the symptoms you see. The old one may have been wired incorrectly and if you wired the new one the same way ...

Interesting.

This truck hasn't had much done to it before I got to it. The wiring is still cloth insulated, etc and the dimmer looked like it had been there forever.

Should I shuffle the three wires around and see if the lights work? I suppose If they're wired wrong now I don't have anything to lose by hooking them up wrong a second time. If they are wired correctly now, can I do any harm by switching them as a test?

See this thread: https://www.stovebolt.com/ubbthreads/ubbthreads.php/topics/1279568/dimmer-switch-wiring.html

Looks like the middle terminal is the one that goes to the light switch.

Are you talking about the 3 terminals on the back of the headlight or the three terminals that Bill Hanlon mentioned?

The 3 terminals on the dimmer switch, and I'm kicking myself for not thinking of that.

I'm pretty sure the middle terminal goes to the light switch (as stated above). Doesn't make any difference which of the other 2 terminals is high and which is low.

Otto, you say you have checked each light and they both have Hi and Lo beams and work on the bench. You say the dimmer switch is new. At least one filament is lighting after installation so you know the ground path is good. The headlight switch is providing voltage to the lights. This is the most simple troubleshooting task you could possibly have.
As Jerry said follow the voltage, something is wired wrong or broken along the way. Check all wiring routing/hook up and condition of wiring regarding below list.

1. The ammeter provides power to the headlight switch. Check that
2. The headlight switch is wired to the center terminal on the dimmer. Check that
3. The other terminals on the dimmer can be hi or lo as Bill and Gord have said. Those wires go to the fender terminal blocks. Check that
4. The terminal blocks are to split the wires to left and right headlight plugs. Check that
5. The headlight plugs must be wired correctly also. Check that
6. I'm assuming you have regular H4 style sealed beam headlights with a three prong plug.

Following your advice, I connected a hot wire from the battery to each of the three terminals. The lights came on each time. I couldn't tell if the brightness changed between either terminal

Bartomos, I don't see anything that looks like a terminal block on either fender.

Ground wires from each lamp come out and screw to the fender.

Headlights are normal H4, 3 prong lamps. The original T3 lamps were still in place and connected so I don't think the lamps are wired incorrectly.

It's Bart A mos. You just told Bill Hanlon you tested that terminal block???????????? On the INNER fender.
Post a pic of the drivers side inner fender area.....and the ground wires you mention.

This is probably the place the miswire occurred.

I had to go back and look at the posts.

Bill at first was talking about the 3 terminals on the terminal block in your picture. Then he mentioned the three terminals on the dimmer switch. Those are the ones I tested.

I have no terminal block like the one in your picture on the inner fender. Someone must have monkeyed with things over the years.

WRONG!!!!! Bill said terminal board.

I think you are getting mixed up.

The dimmer switch has three "terminals".
A terminal block is an insulated phenolic board with three screws normally mounted to INNER fender.

This is where the screwup is probably. Someone bypassed the terminal board which is a good thing to get rid of unnecessary terminations, but it's where the screwup is. inside the tape.

Here's Bill's post.

Otto, you just posted at 11.23 with a Bill Hanlon quote. Read it.
Anyway let's move on

Ya, I posted the wrong quote.

I should have posted the second one, which I did above.

OK stand by for a minute for instructions.................

After you have wired the dimmer correctly FOR SURE.

1. Unplug each headlight plug. Let them hang.
2. Use a ice pick test light.
3. Attach test light alligator clip to a good ground place.
4. Turn on headlight switch.
5. Probe each hanging plug and see if you get voltage on the same plug contact on each plug. One matched set of plug contacts should show voltage. Then click dimmer and another matched set should show voltage.

You are testing the disconnected plug, not the light bulb.

One of the tests will show nothing because it's the ground contact. Look at the pic I posted for ref. Note it's the back side of bulb. But you just need to know that the voltage is at the same contact on each plug. At each click of the dimmer.

If not, that needs fixed.

If the terminal block has been deleted by PO, Then he probably spliced those wires from the dimmer so as to split each wire to left and right. That is where you need to un-tape/unsplice and fix.

Post any questions at any time.

Not really anything to do with this problem, but the output wire coming off the back of your alternator is really wimpy.

Here are the results of the ice pick test.

This was with the dimmer switch on high beam.

No light on the ice pick in any of the 6 holes when the dimmer switch is on low beam.

Just FYI, The dash lights work including the high beam indicator light.

All screwed up. Rewire.

I gathered that. How do I know which wire is supposed to go where?

I assume hi beam wires from plugs meat up somewhere and then go to hi beam terminal on dimmer switch, etc?

Which ones are which?

Stand by for a minute......

Assuming your pic is showing the contact side of the plug............
You can do a proper splice to split each dimmer wire for left and right or you can install a new terminal block and split them that way.

The colors I used don't indicate positive and negative. Wire per the description.

When you get around to rewiring, DO NOT solder or tape the wires, Just twist them together for a test. Don't remove any terminals or wires from the plugs.

My drawing shows the plugs looking at the truck from the front as you would see them when installing the lampsOn the dimmer terminal, is the center terminal the switch and left and right are hi and lo?

The center terminal of the dimmer is wired to the headlight switch, yes.

The other two are either Hi or Lo. Makes no difference as long and when they split, they go to the same plug contact.

Read my post again, I think I was still typing when you just posted.

What gauge wire should be used with 6v light system?

They used 12 ga for headlight to dimmer and dimmer to Hi beam. They used 14 ga for dimmer to lo beam.
By looking at the bulb contact rear face I posted you can figure out which is Hi and Lo............or use 12 ga for all of the wires we are talking about.

Low beams could also just be burned out if never drove on highway much?

Ed

Bulbs are new. Bench tested and working on hi and lo

......and it's wired incorrectly

Do any of the restoration companies sell a good headlight only harness with the headlamp plugs (1950 6v system)? I just need go from the headlamps to the dimmer switch. Headlight switch, brake lights, taillights, and running lights all work properly.

The previous owner re-wired this truck, incorrectly it seems, and there are so many splices wrapped in electrical tape from stem to stern that trying to undo the mistakes will take forever and who knows if it would be successful. He also used the same color wire for everything which makes it very difficult to trace.

As if that weren't enough, it appears that 16 gauge wire was used for the ground and lo beams (assuming the wire is actually connected to the lo beam side).

So, instead of buying 12 gauge wire and running new lines individually, I thought it might make more sense to buy a complete , properly made kit if one is available.

NO. Don't need that. Otto, you are making things more difficult than is necessary and it takes too long to fix your problems.

1. Buy two headlight plugs at Oreillys.. part no. S526 $10 each. They have a pigtail.
2. Buy a 12 foot roll (or two) of 12 ga black wire at Oreilly's for $10. ea. OR..........one 12 ft 12ga and one 20 foot 14 ga. $10 ea.
3. Crimp on ring tongue terminals on the dimmer end of new black wires and attach to dimmer screws.
4. Run those two wires down the left inner fender. Leave a loop (slack) near middle/front of inner fender in each wire and continue wires to each plug and butt spice to plug pigtail. Same place for each plug as per previous info. The slack loop is to have excess wire for splitting/splicing/terminating second wire.
5. Splice in a wire at each slack loop, or install a terminal block as a splitter. Run the additional wires to each plug and butt splice at same plug pigtail per previous info.
6. Install (butt splice) ground wire for each plug to a clean metal spot.
7. Shrink sleeve and liquid tape all splices and crimps. Route and tie wires to clear obstacles and heat sources. Add split loom.

Test your wiring'lights by just twisting wires together before terminating, crimping or shrink sleeving.

This is cheap for two.

Terminal block

Yes, I'm going to order new sockets for the lamps. There isn't really enough of the original cloth wrapped pigtail to work effectively. I also found some crappy splices that were disconnected under the black tape.

Your lights will be burning holes in the pavement

What is the 3rd terminal on the terminal block for?

Obviously one is for both hi beams and one is for both lo beams. The grounds go directly to the fender. Is the third on for the parking lights?

Yes, parking lights