The Stovebolt Site Forums The Shop Area The Electrical Bay High beam no low beams

New issue. Probably something silly that I can figure out, but might as well cheat and ask the experts first. Background is a 1951 truck that was long ago converted to 12v. Lights had worked last I knew.

Last night I had high beams but nothing at low. Something in the dimmer switch perhaps?

Hy JW51, the dimmer switch is a good suspect, but verify the headlight bulbs aren't burned out on the low beam side, good luck.

Unplug the wires from the dimmer switch and clean ALL the contacts. Take your air compressor and blow out any debris inside the switch contact area. Work the switch several times then plug the wires back in. Good luck.

The dimmer switch lives in a wet, dirty, hostile environment under the floorboard, and carries the full current needed to run the headlights. A new switch and a set of relays out on the radiator support close to the headlights makes a lot of sense. Then all the dimmer switch has to do is carry the small current needed to turn the relays on. That also protects the headlight switch from heavy current, and you get brighter headlights to boot!
Jerry

Quick check of the dimmer switch is to swap the 2 wires on the switch that go towards the headlights at the front of the truck. If the low beam now work and the high beams don't the switch is bad.

Particularly hostile in my truck. It’s gross under there. This rig eventually needs a full re-wire so I will keep the relay thing in mind.

And to the fella that suggested swapping the wires….brilliant! That might have eventually occurred to me after lots of futzing around. But no guarantees.

Just asking here but is there a connector on your firewall that connects the under dash harness to the engine compartment harness (with the headlight wires)? Sometimes if this plug gets loose it pulls out just enough to disconnect one of the wires for the headlights. I went through two dimmer switches adn two new headlight switches before I found the loose connector on the firewall (cab side not engine side) Just something to check.

Get a VOM (multimeter) or an ice pick style tester and start tracing. You just don't have enough info at this time.

Make your life a little easier. Instead of disconnecting wires and switching them, have a decent length wire with an alligator clip on each end. The amount of diagnosing alone you could do with that tool would just amaze you. Work smarter, not harder as they say. On the '51, battery hot from the alternator, and touch the other end to the headlight terminals on either inner fender. Verify the lights are good before assuming any of the switches are bad.

I can remember when I learned this lesson. It was on a Ferd station wagon. The last of their breed. Both low beams out. Ordered the switch, it came in and I proceeded to do battle with the steering column. Yes, late model column switch. Took me most of the evening with the newly returned lead mechanic telling me we weren't going to put in any overtime. We were averaging 3-4 hours nightly. He was back from cancer treatment and didn't have the energy to stay. So, if he went home, everyone went home! Anyway, got the switch done 30 minutes or so before end of shift. Still no low beams. Then it hit me. There is no way the driver would wait until both low beams were burned out, would he? He couldn't be that dumb! He was. I learned. Still got the job done by shift end but that was some unnecessary labor and parts thrown at it!!

Do the simple stuff first.

When rewiring my 50 chev pu 6v system I opted for the use of relays mounted on the firewall with the dimmer switch controlling power to the relays and the dash switch to control power to the dimmer switch. I used my own idea for a terminal block made of 1 inch thickness plastic which I modified. So I have been happy with the changes and encourage you to do the same. I like the idea of troubleshooting the problem with the least effort so no time and money is wasted. You will find the solution.....good luck

- The wiring diagram for a 1951 Chevy pickup is located at: http://chevy.oldcarmanualproject.com/electrical/wiring/pdf/51truck.pdf
- Other wiring diagrams are at: http://chevy.oldcarmanualproject.com/electrical/wiring/index.htm

Based on a quick view of the schematic, the dimmer switch could be the prime suspect (assuming the the wiring is still stock) since it is common to both functions. Try disconnecting the beam feed wires from the dimmer switch and manipulating them manually to the battery feed alternately.
- battery= black and red wire (you can leave connected)
- low beam feeder wire= Green wire (short to black and red wire for test, then disconnect)
- high beam feeder wire= Black wire (short to black and red wire for test, then disconnect))
If it is working manually, it is most likely the dimmer switch
don

I was out doing some other stuff on the truck a couple nights ago and now I have both hi and low beams but no hi beam indicator light. Not sure how to explain it “fixing itself” other than I have a loose connection somewhere. Maybe the indicator bulb just burned out or something but that’s weird as well.

Buy a spray can of electrical contact cleaner, and hose the dimmer switch down with it, then operate the switch repeatedly, lights on or off doesn't matter. Repeat as necessary until things either work right or the switch self-destructs. Something as essential to safety as the headlights deserves a new dimmer switch- - - -sort of cheap life insurance if you plan to drive at night.
Jerry

Oh yeah, will definitely fix before any night driving.