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12V Lights on a 24V Truck

24V Toyota Land Cruisers, at least in some parts of the world, have 12V headlights. On first glance they appear to be powered by a 12V centre tap. But they are a little more complicated, as Greg Bowie discovered.

There is a 12V headlight relay located on the pedal assembly bracket. It is powered by the tap off the negative pole on the driverís side battery. This relay also has a 24V hot lead. The 12V relay splits the 12V power feed and runs one line to the drivers side headlight, and powers it (the neutral goes to ground). The other 12V lead runs to the passenger side headlight as the neutral. The 24V power lead runs from the relay and goes to the positive pin on the passenger side headlight. Because 12V is supplied to the neutral and 24V to the positive pin on the passenger side, only 12V is available to the headlight. Therefore both headlights are 12V and the draw on the batteries is equal.

If the tap is disconnected from the driverís side battery and you turn on the headlights they will not come on as the relay needs 12V to power up. If the tap is connected and one headlight burns out the other will continue to work. This is Toyotaís safety margin to get you home. Itís not a good idea to run with one headlight for any length of time as the battery load is unequal.

If the headlights were wired in series and one light burnt out, both lights would be out as the circuit would be broken. So wiring lights in series is not a good idea, unless youíre prepared to change the bulb immediately when one blows. You can run two (or any even number) of lights safely off a centre tap, or wire them in similar fashion to the headlights. Or you can install a 24-12V converter.