Welcome to our knowledge base of totally random and probably obscure stuff relating to 1973-1991 GM C/K/R/V Body Headlights. This includes Suburban, Blazer, 10, 20, and 30 series trucks among others.

We've recorded these notes and observations not only for ourselves (because we forget stuff over time), but also in case anyone else can find it useful. If you find something incorrect or wish to contribute something, just let us know here.
 GM Headlights Configuration and wiring
 GM Headlights Common failures and issues


Configuration and wiring

The high and low beams are not fused. The park lamp circuit has a dedicated fuse, and the main headlights rely on a circuit breaker integral to the switch for overload protection.

The quad headlamp system is similar to the single headlamp, with the main difference being both high and low beams contained in one headlamp unit or separate units. The low beam lamps remain on when the high beams are activated.
square body headlight wiring diagram
The switch is a combination unit. Turning the knob activates the instrument dimmer function, while pulling out one stop on the knob turns on the parking lamp circuit. Pulling out two stops on the knob activates both parking and main headlamp circuits. The high/low beams are activated by a separate, simple on/off switch either on the floor or steering column stalk.
square body headlight switch wiring square body dimmer switch wiring
The high/low beams are activated by a separate, simple on/off switch either on the floor or steering column stalk. The steering column mounted switch is shown below: dimmer switch

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Common failures and issues

It is important to note that the high and low beams are not fused. The park lamp circuit has a dedicated fuse but the main headlamps draw power straight from the battery through the switch. There are no relays used in any of the circuits.

Headlamps only flickering: Modern headlamp units can draw more amperage than older units. While they put out more light, they also draw more current than the circuits were designed for. If too high a draw occurs, the thermal circuit breaker in the switch opens and the headlamps will shut off until the thermal breaker cools enough to close again. This can often be seen as a fault in the switch, but the headlamp units are actually the problem in this case. The main symptom is the headlamps blinking or flickering while no other lights are affected, mainly after several minutes of operation (once the heat in the circuit builds enough to trigger the breaker).
Solution: The obvious and easiest solution is to replace high amperage draw units with lower current ones. But this may reduce the light output, which is the primary advantage of larger lamps. A better solution is to install a relay system to separate the switch part of the circuit with its lower amperage wires, from the battery circuit. Then when the switch is activated it uses low current to activate a relay to switch on the high current part of the circuit. This reduces the high amperage section of the circuit and larger diameter wires may be used in these sections. This may be wired according to the diagram below, or plug-in kits may be purchased from companies such as LMC Truck.
square body headlight revised wiring diagram

Entire light circuit including dash lights flicker or go out: This issue is defined by all the lights failing at once, or random ones failing without seeming cause. Fuses will be unaffected, and sometimes jogging the switch will temporarily fix the issue.
Solution: This issue is usually one of the switches. The switch units are sealed mechanical switch, and the contacts can wear. This may result in a bad connection under certain circumstances, or no connection at all inside the switch. Under high amperage conditions the plastic may melt or the contacts overheat, as seen below on this high/low dimmer switch: overheated dimmer switch

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