All the below assumes you have a 24V truck that was designed to run 12V headlights, using the evil center tap system.
All the wiring codes in my post are for a 1981-84 BJ42, however the principle should be the same for any 24V Toyota truck that has the 12V center tap to run 12V headlights...
24V 1984BJ42...Stock headlights.
Evil center tap supplies 12V to the headlight dimmer relay. The headlight dimmer relay is a 12V relay. It also has a hot 24V lead that comes into the relay (this is the beginning of the equal draw system). Power is split to whatever headlight wire needs 12V or 24V, depending on the headlight on/off switch, and high/low beam toggle.
Headlights off should show no voltage to any of the LH side harness female pins, and 12V to Red/B and 12V to the Red/Y headlight harness female pins on the RS. Red/B is the nuetral pin, and the other pin showing voltage is the highbeam pin, so they cancel each other out.
Headlights on lowbeam should show 12V to the lowbeam pin on the LS. The RS should show now show 24V to the low beam pin and 12V to the neutral pin.
Headlights on high beam should show 12V to the highbeam pin on the LS. The RS should now show 24V to the highbeam pin and continue to show 12V on the neutral pin.
So, a constant 12V supply is given to the neutral pin on the right side headlight (from the tap on the negative pole on the highside battery), and the high/low poles run 24V, feeding off the highside battery positive pole, through the headlight dimmer relay. This same tap supplies 12V to the dimmer switch, which routes 12V's to the high/low on the drivers side headlight, with 0V neutral. Hence, both batteries have equal draw.
The 12V's available to neutral pin and the 12V available to the high beam pin on the right side headlight side cancel each other out when the headlights are off.
Need some help guys... I've got a '79 BJ40 24v cdn model with the 12v centertap headlights.
I've got the drivers side headlight operating fine.
Nothing at all from the passenger headlight.
Tested the voltage at the passenger bulb with the bulb unplugged and it measures exactly the same as the drivers side both off, low beam, and high beam.
However, when the passenger bulb (checked, works!) gets plugged back in, I get zero volts on high beam and low beam leads.
Where is the +12v disappearing to on this passenger bulb once the bulb is present in the circuit?
The drivers bulb always works fine low and high regardless.
Tested the switch, relay(dimmer), wiring harness, different bulbs - all test ok.
WTF is going on??? Any ideas??
Thx! Driving me crazy...
Fuses good? Same problem if you switch bulb sides?
You should not get the same voltages on the left and right side headlight pins. Read the above info in post #2 thoroughly for pin readings with headlights off, on low, on high. Left side is the 12V side. Right side is the 24V side. If you are not getting 24V on the correct pins on the right side but are showing 12V neutral...that would be your problem (24V wires not working).
The headlight dimmer relay has a 24V lead into it, as well as the 12V center tap. Check that the 24V side is working. If it is supplying 24V to the dimmer and no 24V coming out to the two lines that need it (high and low when selected) then sounds like the dimmer relay is not working on the 24V side.