Trueno headlight issue ...

[Ivan141]

Looking at the wiring diagram, I cannot find any evil sourcery on toyota's part in the headlight circuit, save the common source with switches in ground construction.

Each bulb originally has 1 12v wire, and 2 ground wires (high and low beam coil). The ground wires are paired up between both headlights, so both high beam coils share the same ground, and the low beams have their own ground. Each ground is wired to the switch, which grounds the appropriate one, and leaves the other one floating (open contact).

Are you sure you havent inadvertantly made a connection between the high-beam ground and the low-beam ground?
Try removing the relays and bulbs and measure the resistance between the low- and high-beam grounds...shoud be infinite, otherwise you have a shortcircuit or a bad switch.

Another thing that might cause problems is the type of relay you are using. If it's a breaker type it would switch the lows on whenever they are not grounded...but they'd still turn off because of the main headlight relay.