Headlight Relays

[Schurkey]

I searched this forum for info on adding headlight relays to the GMT400 trucks, and got a pageful of hits none of which seemed to be relevant.

This is how I added relays to brighten the headlights on my '88 K1500, and also modified the system to keep the low-beam lights lit when the high beams are selected. None of this is "new" or invented by me. There's instructions and wiring diagrams all over the internet. I did "correct" some shortcomings of the wiring systems pioneered by others, primarily by retaining OEM-style protection via two self-resetting circuit breakers instead of a single (or double) "fuse" as recommended elsewhere. With self-resetting circuit breakers, the lights may flash on and off if there's a fault, but they don't go out completely until a fuse is replaced. (Automakers install self-resetting circuit breakers, often right inside the headlight switch--but those are bypassed when the relays are added.) With two circuit breakers, a fault in one system (Low-beam, for example) won't kill the high-beam headlights.)

Adding relays is NOT appropriate for vehicles with DAMNED DAYTIME RUNNING LIGHTS (DDRLs) that operate by turning on the headlights. The low-voltage DDRL signal may simply activate the relay, which turns on the headlights at full brightness. If you have DDRLs, there's instructions on the internet that can allow you to disable them. I have disabled the DDRLs on all my vehicles that came with them. DDRLs that operate lights other than the headlights can use relays on the headlights.

Note that it is REALLY EASY to provide TOO MUCH voltage to your headlights when using the relays. In Europe, headlights are rated for brightness at 12.8 volts. They're rated for lifespan at 13.2 volts. In America, lifespan seems to be rated at 14 volts. Industry sources say that increasing the voltage 5--6 percent above the "lifespan" voltage will brighten the filaments about 20%, but cut the life of the bulb in half. Full alternator voltage is likely to be higher than the headlights are rated for.

First up is to inspect your headlights and the connectors that the headlight bulbs plug into. My truck uses four sealed-beams, and while none of them were pristine, one high-beam unit was especially bad. The reflector had deteriorated so much that lots of light was "leaking out" the back where it doesn't do any good. Lens condition, reflector condition, and bulb/filament condition will be important whether you have two headlights or four, sealed-beam or "composite" assemblies. My headlight wiring connectors were fine.

Rotted reflector leaking light


My low-beam lights are rated for 55 watts each. 55 x 2 = 110 watts total. About 9 amps of current draw, worst-case. I used a 10-amp Type 1 self-resetting circuit breaker to protect the low-beam system. My high-beam lights are rated for 65 watts. About 11 amps, worst-case. The next-largest circuit breaker in the ATO-ATC style is 15 amps, and that's what I used on the high-beams.

My truck has the headlight wiring running in a bundle along the left fender, under the washer bottle. Low-beam wire is tan 18 gauge, high-beam wire is green 16 gauge. Measuring the alternator voltage with the truck running, I got 14.6 volts. Yet I had only about 12 volts at the headlights, a loss of about two and a half volts through all the wiring, connections, and switches in the headlight circuit. I measured almost no voltage (about 1/10 of a volt) on the ground wire with the headlights lit, indicating that the headlight grounds were already acceptably good.

First, I pulled the tan and green wires from the harness, and clipped them at a suitable location.


I installed a factory-style Weatherpack connector. This will plug into the relay system, but--if I ever remove the relays--I can simply re-connect this connector to return my headlights to the original configuration.


I pulled power for the relays from the power-distribution studs at the passenger-side firewall. ATC-ATO fuseholders were used, 10- and 15-amp circuit breakers installed, and then spliced to 14-gauge wire routed to the relays. More-powerful bulbs may need heavier wire and higher-amp circuit breakers.


Relays mounted on the inner fender area with two sheet-metal screws. One screw had the area around the hole stripped of paint, because that's the ground location for the relays. I used a star-washer between the inner fender and the ring-terminal of the ground wire termination to help assure a good ground.



(My wire colors may be different from yours.)

Parts needed:
2 "Bosch" 4- or 5-pin relays of 30/40 amp rating. Only four pins are used, but 5-pin relays seem more common.
2 4- or 5-wire relay sockets with "pigtail" harness. Only four wires are used, but 5-wire sockets seem more common. I removed the unused wire in each socket.
Note that some folks prefer "weatherproof" sealed relays and sockets.
2 ATO-ATC fuse holders
1 10-amp Type 1 Self-resetting ATO-ATC style circuit breaker
1 15-amp Type 1 Self-resetting ATO-ATC style circuit breaker
2 pair of Weatherpack two-pin connectors with pin sizes appropriate to your wire gauge
1 Diode (optional) to keep low-beams lit when selecting high-beams.
2 Sheet-metal screws, #8
1 Star washer suitable for #8 sheet-metal screw

Miscellaneous supplies: heat shrink tubing, ring terminals, solder or crimp splices, several feet of 14 or 16-gauge wire, several feet of corrugated plastic wire loom, good beer, etc.

Once you're this far into the project, it's easy and cheap to add a simple diode between the relay trigger wires to keep the low-beams lit when the high-beams are selected.
Note that the leads on this diode should have been insulated before I took the photo.


Relay wire guide by connector number:
86 = relay trigger. White wire in my photos at relay, connects to tan (low beam) or green (high beam) wire coming from firewall.
85 = relay coil ground. Connects to grounding screw. Black or blue wire in my photo.
86 and 85 are interchangeable, as long as one of 'em is grounded and the other is connected to the headlight wire from the firewall.

30 = Power input to relay, route power through fuse holder and circuit breaker at selected power source so the entire wire is protected by the circuit breaker.

87 = Relay output to headlight bulbs Yellow and Green wires at relays in my photos, connects to original headlight wiring (tan and green) going to headlights.

87A = fifth terminal, not used in this application.



Voltage at my headlights improved from 11.9--12.1V to 13.8V, near-perfect for brightening the lights while preserving bulb life.

Now that the headlights are acceptably bright, I need to properly align/aim the headlights.

[fast68chevy]

more alternator output and better wiring grounds for H/L's i like to swap on 90s early 2000s ford taurus 3.)L alternator onto my serpentine chevies because cheap and easy and over 14 volts ouput even at idle,.,. makes for bright lights and quick easy engine starts always.. just drug home a chjeap 88 C1500 pi kup the other day i need one of these alts for it too now,, and one for the '75 C20 daily driver work truck too..