So a few months back I made a 12 gauge harness for the headlamp circuit in the miata. It has a ground-switched headlamp circuit like many cars. The bulb used is an h4 dual filament bulb. So when the high beams come on, the low beam filament turns off and vice versa. However, after my harness was completed and installed, everything worked great, except the low beams did NOT turn off when the high beams came on. Odd
but I remembered something that I had discovered earlier... Before I tore into the original harness, I removed both bulbs and then metered the sockets to confirm which wires were which. Upon doing this, I also noticed that the low beam circuit was NOT being interrupted once the high beam circuit was on. I'm thinking that the computer doesn't see a certain level of resistance without bulbs, so it just leaves the low beam circuit live. I'm thinking this is also now happening with my harness, since the low-draw relay doesn't have enough resistance to simulate a bulb. Doing research online, it seems that this can be remedied by placing a resistor or small wedge bulb (194/168) inline on the low beam trigger circuit to fool the computer into thinking there is a proper load on the circuit. The car is currently in storage elsewhere so I can't try this immediately, but I assume this makes sense and will work, no? I talked with Daniel Stern about this, and he suggested there may be some back feed on the circuit somehow. Either way, I think the bulb/resistor trick will work, I just want your opinions.
