Thanks to all who have contributed to this thread - I have read through it, and others, and have some understanding of the issue, but my lack of electrical enginering knowledge means I still have questions.
I understand that the HBI is positively switched from the LO (dipped) light circuit when the headlight relay is triggered by the headlight switch, so that even when the dimmer switch is in the HI (or flash) position, current still flows somehow through the LO filaments in the bulbs/sealed beams (see circuit diagram below, or the schematic on Page 1 of this thread) to the HBI and dashboard ground (brown wire).
When the aftermarket harnesses are used, the LO bulb filaments are replaced by the solenoid coils in relays, which have a much lower resistance, and somehow this stops the HBI bulb from illuminating fully.
My questions must sound very naïve to someone who really understands circuits, but I'd really like to understand what's going on here. My questions:
How come the LO lights are not always on when the headlights are switched on, since there is always a circuit through the HBI? Does the tiny 1.2W bulb control the current in the circuit when the switch is in the HI (main) position, therefore never allowing the LO filaments to illuminate noticeably?
Will the HBI to ground circuit not trigger the LO relay solenoid when the headlight switch is in the HI position?
A separate question is regarding the ARB harness with an extra relay. Is this just fixing the problem by putting an extra solenoid (in a third relay) across the LO/COM (common/ground) terminals in the unused factory headlight connector?
I would really appreciate if anyone can explain the above questions!
Thanks
EO