Trueno headlight issue ...

[Red]

Thanks, Ivan. It should be that simple--but it isn't.

Assuming the schematic you are looking at is for a front end the same as the US GT-S (only two headlamps, and pop-ups) and not four a model with four fixed headlamps.

The schematic is in several ways wrong. It shows the headlamp wiring with "pin 3" in the center, at the shared terminal between two filaments. (If we have the same schematic.) But if you jump up to the front pages of the manual, "pin 3" is actually to one side of the headlamp--and harness plug--and not in the center. That's incorrectly diagrammed according to standards for schematics.

I mention this only because it confirms the schematic is problematic, no poetry intended.<G> There also appears to be a common point within the headlamp highbeam stalk switch, and the master lighting switch on the dashboard could be another mystery.

I have not made any inadvertent connections, I tired a number of combinations, used a test meter, try diode isolation, etc., but the left and right headlamps are interconnected and apparently in more than one place/way in the circuitry.

The relays are installed in a very simple manner: When the low beam circuit is closed, the relay filament receives the power that the headlight filament would normally receive, i.e. directly form the socket, using the same two pins (1 and 3) that the low beam headlight would use, and verified against the colors of the wires going into the socket--not just against the pinout diagram, to make sure it is the correct wires.

If I do that, all I have to do is connect a high beam bulb across pins 2-3 (the factory standard, again verified by wire colors) and then the low beams GO OUT when the high beams come on. And the high beam stays on, regardless of the high beam stalk and indicator!

After several days, spread out over most of two weeks, the degreed EE at Sylvania could only reach the smae conclusion I did: The relay coils present a larger resistance than a light bulb filament does, and this is tricking the Toyota into thinking "the low beam is out, I'll engage the marker filament instead". ("Engage" probably being an artifact of a voltage divide in the circuit, not a physical switch or relay.)

If for some reason you have an H6054 bulb there--so you see something else odd about it. You cna light the bulb by powering up ANY two pins from it, i.e. high beam to common, low beam to common, OR high beam to low beam. There's some question about how the bulb is made internally (i.e. if the filament is center-tapped for common, or if the low beam represents "the first half" versus 'all" of it).

I didn't try a ground-to-ground check on those two pins, that would make sense. But I couldn't (haha) have a short circuit or bad swtich, or else my normal headlight wouldn't have been working properly, would they? The headlamps work perfectly well with stock headlamps--they just don't like seeing relay coils instead of lamp filaments.

Offhand I don't know the relay coil resistance, these are standard Bosch 20/30 amp relays, certainly much higher than the "near zero" ohms of a good filament under power.