The Stovebolt Site Forums The Shop Area The Electrical Bay Hi Beam Indicator Light

I am finishing the last of outstanding items to complete the restoration of my '53 GMC short bed, keeping the original 6 volt, positive ground circuitry. The previous owner had purchased a pre-made wiring harness which when installed presented no significant problems (although the wiring color codes did not always match original) but the hi beam indicator light stays on when switching from hi to lo beam. As part of the restore I installed a new dimmer switch and replaced both headlights. I have reviewed the threads I could find on this problem and tried the suggestions provided but the problem persists regardless of what I tried, even trying a new dimmer switch. My last attempt has been isolating the left light from the main wiring harness, with jumpers to the headlight pigtail that attaches to the left fender terminal strip, including a jumper to the positive ground frame connection. Switching from lo to hi and back, the indicator light does dim slightly but is always on. The voltages measured at the terminals on the dimmer switch clearly shows why the light stays on but I am baffled as to why there is voltage on each of the hi and lo beam terminals for the headlight harness.
Battery voltage is 6.2 volts. Voltage from the light switch is 4.9 volts when switched to hi beam and the hi beam terminal is 4.8 volts and lo 4.7 volts. Switching to Lo beam, the
voltage from the light switch increases to 5.1 volts, the hi beam terminal measures 3.1 volts and lo beam terminal 5.1 volts. I did check to be sure there is no contunuity between the two terminals on the terminal strip, it is also new. The wiring harness and connections conform to the schematic in the GMC Trucks Maintenance Manual except for one other mystery. The harness from the dimmer to the terminal strip for the hi beam is a blue wire. Another same color blue wire in the harness, connects the voltage regulator to the armature terminal on the generator and there is continuity between the two wires, which does not conform to the manual schematic, but disconnecting the wire to the generator does not change the voltages measured on the dimmer switch terminals. I assumed from the schematic that the hi and lo beams were powered by two circuits from the dimmer. I was told that is not the case, that the three terminals on the back of the headlight are all at the same voltage and the circuits are made when the dimmer switch changes one to ground to complete the circuit. If that is the case where in the circuit does the connection to ground take place? The ground wire from the headlight bucket attached to the fender does break the circuit when lifted, so it is part of the circuit. As I said, I am baffled, it can't be this complicated.

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Hello Bill,
IT IS, it is real simple. Not going to get involved with wire colors or positive ground or your ohm/volt testing or what the previous owner did.

Here's the scoop:
1. The dimmer switch has three terminals, one is a hot feed from the headlight switch, one goes to low beam prong on headlight, one goes to high beam prong on headlight. (connectors/terminal blocks probably used along the way)
2. The headlight has three prongs. One is ground, one is high from the dimmer as mentioned, one is low from dimmer as mentioned.
3. The hi beam indicator bulb hot terminal is wired to the hi beam terminal on dimmer. The bulb will need a good ground to it's case.
4. So............ wire it that way, and no other wires, no other way, and it will work.

The two headlight wires usually run down the left inner fender to the left headlight, then the hi and lo are spliced and cross over radiator/grill area to the right headlight. The headlight grounds are short wires from headlight prong to a nearby body, shinny, clean bare metal spot.
This bundle can also have horn and park light wires included.
The dimmer is a simple single pole/double throw switch. One input from headlight switch and two outputs to either Hi or Lo filament in headlight bulb.
FYI: It does not matter which output terminal on the dimmer is used for high or low. Just so you know which one's you choose.

hi BillK
are you sure you are looking at the high beam indicator bulb, because they all look the same when you take them out of the back of the speedo, sometimes when i plug the three bulbs back in they end up in the wrong places
the high beam indicator fits just like the speedometer back lights
-s

Very good point. Also, what if the hi beam bulb is burnt out? You would see shine from a gauge bulb thru the Hi beam lens. Sorry Bill, don't mean to drive you crazy.

oh while we on the subject
the high beam bulb is suppose to be a 51 not a 55 like the rest of the instrument cluster
many trucks got a 55 in the high beam indicator and the extra heat damages the tiny colored plastic
-s

It sounds like one or both of the headlight connectors is mis-wired, using the ground pin of the 3-prong connectors as either high or low beam. That would allow voltage to back up through the indicator light circuit and keep the light on all the time, while still allowing the headlights to function almost correctly. I've seen this happen occasionally when a new wiring harness was installed by someone without a lot of experience at rewiring a vehicle.
Jerry

Looking at back of H4 bulb with middle prong at top: left prong is ground, Middle is Lo, right is Hi

It is a known fact that dash lights/gauge lights will shine thru the beam lens and make it seem like it's on. The fact you say it's "dim" sometimes could be a tell tale sign of that.

Did we lose Bill? Are we talking to ourselves again?

I"m not lost and want to express my sincere gratitude to all who responded and especially Hotrod and Bartamos. Albert Einstein is widely credited with saying, “The definition of insanity is doing the same thing over and over again, but expecting different results.” That is where I was at a few days ago, so after talking it over with a couple of knowledgeable friends, they were convinced it was a grounding problem. So I removed all the ground wires in the lighting circuit, including the ground strap to the truck frame, cleaned the surfaces, scrubbed the connectors with emery paper, rechecked the wiring diagrams and harness connection instructions by the harness mfger. All were correct and the circuits checked out until I plugged the headlights into their sockets. The split voltages re-appeared and the red light stayed on. So I bought a new pigtail, hooked it to my spare dimmer switch and with the headlight hooked up got the same results. I don't expect I will be driving the truck much at night so reluctantly decided to lift the lo beam wire so only one element works and signed on today to let you all know. Then I read your last response Bartamos and then understood what Hotrod was saying. I knew immediately that the wiring from the headlight pigtail was not terminated correctly. So, now the headlights work as designed and I can move on with the remaining punchlist items to complete the truck, but expect that it is never going to be "complete". It has been a memorable project though and this is just one more for the memory book. Thanks again for your help.

I mentioned in my first post that the circuits were terminated as diagramed in the GMC Trucks Maintenance Manual schematic, with the Hi and Lo wiring terminated on the sides of the socket picture and ground between the two connections in the middle of the picture. There is no mention in the Lighting System section of the correct wiring for the headlight socket and it wasn't specified in the harness manufacturer instructions either. So a heads up for anyone who may take things literally as I did.

Nice to see someone who enjoys this stuff as much as we do. Restoring is fixing, learning and fixing and learning. Helping is learning for me. Let us know when the next thing happens. Don't worry, it won't be long. It's really fun to bring back an old truck someone didn't want. Glad the lights are sorted. I kind of like the electrical part as much as the mechanical part of old trucks.
Hot Rod Jerry called this one.