Headlight System and Troubleshooting

[RAD4Runner]

Also on Yotatech here, but for redundancy, in case one of the forums gets hit by EMP... posting on T4R as well
HOW HEADLIGHT SYSTEM WORKS.
(Schematic below)

When headlight switch is turned on, headlight relay in (engine compartment fuse block for first-gens 22RE) energizes, closes contact, sends +12V to both headlight fuses and to pin 3 of both bulbs.

Toyota uses switched-ground control; Dimmer-switch select which filament (high or low) of bulb to connect to ground.
When in low beam position, switch contact 6 (HL) of dimmer-combo switch connects pin1 (red with green stripes) to ground (G = zero volt, H is floating so there is 12Volt there.)

When in high-beam position, switch contact 5(FU) connects pin 2 (red with yellow stripe) of bulbs to ground (H = zero volt, G is floating so there will be 12V there. G is also connected to high-beam indicator so that 12V will turn it on.)


TROUBLESHOOTING: Voltage test points and expected voltage readings ON NORMALLY-FUNCTIONING SYSTEM.
To the best of my knowledge, the schematic and tests here apply to USA model (no DRL) 3rd-Gen and earlier 4Runners/Trucks.

(I verified below to apply to 1986 to 1988 22R-Eliable , based my own 1986 and pdf schematic of 1988).
This will determine if your wiring and power are good. If wiring and power are good, suspect your bulbs.
Other tests may be needed but these points will cover most troubleshooting scenarios.

Headlight relay and fuses are in fuse block in the engine compartment.

PERFORM THE CHECK BELOW WITH GOOD BULBS CONNECTED. PROBE THE BACK OF CONNECTOR OR PULL IT OUT JUST ENOUGH SO YOU CAN PROBE THE PIN BETWEEN THE GAP.
Multi-meter in Volts DC mode (usually 20-V range IF not auto-ranging).
Negative (black) probe on bare, shiny metal of negative battery post or bare, shiny metal of body ground.
Probe points A through H in Off, Low-beam, and High-beam modes.
Points E,F,G and H are on the H4 connector. Probe the back of the pins or Pull terminal out slightly to be able to probe the contact blades while lights are still operational.


EXPECT READINGS BELOW FOR NORMALLY-FUNCTIONING LIGHTS. Let us know what you see is different about your system..

H4 CONNECTOR PIN-OUTS (FEMALE SIDE ON HARNESS):

ALTERNATIVE METHOD OF VOLTAGE CHECK (across pins of the H4 connector):
NOTE: THIS SHOULD ALSO BE DONE WITH GOOD BULBS CONNECTED, otherwise you would not detect bad combo switch contacts. I ONLY PICTURE UNCONNECTED CONNECTORS TO HELP SHOW PIN-OUTS..

LOW-BEAM

HIGH-BEAM​

 

[RAD4Runner]

Symptoms: BOTH high beams do not work, High beam flash works AND low beams work.

Symptoms: BOTH high beams do not work, High beam flash works AND low beams work.

Troubleshooting:

Turn headlights on. Make sure low beams come on.
Switch to high beam. Still no light, of course?
Find Pins 5 (Red-Yel Wire) and 13 (wht-blk) of dimmer combo switch . Jump 5 to 13. Got highs?
If you get high beam, it's bad contact between those 2 pins. You may be able to clean or fix. However, the flaw in headlight wiring of that generation where current flows on thin wire over a convoluted path (ask 4crawler) and though the dimmer-combo switch will shorten it's life again.
If you still don't get highs, jump pin 5 directly to shiny, bare metal ground. Got highs? If yes, it's bad connection between pin 13 and ground.

(I have not actually done this work hands-on, so would appreciate if anybody could share pictures of switch and test points/pins.)