headlight wiring question for 740 1987

Here's the question. I upgraded to e-codes, and did the following - but
still get a bulb failure light.

I used the passenger side wiring as the main run from the headlight
switch. Instead of just cutting off the driver's side, I bought extra
wire and ran a length from the high and low beam across the car and
spliced each wire respectively with it's match on the passenger side.

In simple terms, the two lows are now joined and the two highs are now
joined and each pair goes into the respective high/low relay.

Why do I still get a bulb failure light? I thought this would only come
on if I disconnected one side. I plan on just pulling the dash bulb -
but need to make sure that's OK

 

I just remembered that my running lights and turn signals have exposed
wiring. Would this trigger the light on the dash?

 

OK, let's assume that the green wire drives the low beam on the left and
the green white wire drives the low beam on the right. As long as the
two separate wire are running the lights then the current required for
each wire is approximately the same. Inside the bulb integrity relay the
current on each wire is compared by running it through each wire's own
personal coil that is wrapped around a normally open reed switch. Each
coil is wound in the opposite direction of the other. When the current
flow is close to identical the magnetic field induced by each coil is
cancelled by the other; the reed switch remains open and the bulb
failure indicator light stays unlit. In your system one coil is
unpowered and the switch closes showing a false positive.

The same thing applies for the two bulbs in parallel on the white-blue
wire on the left and the white-red wire on the right for the lows beams.
The relay is biased internally to compensate for the extra load from two
filaments.

Bob

 

Aha! Makes perfect sense.

Gracias Amigo.

 

Did the bulb failure light come on before you did the work? I wired mine
similarly and the indicator works properly, it's actually very nice to
have since it tells you if you have a rear light out. They can be
finicky when they get old though, dirty contacts in the taillights, bad
ground connection, and the solder joints within the sensor itself crack
sometimes.

 

It did come on with no bulbs in the headlights and only the running and
turn lights in. The turn and running lights had bare wire. I applied
liquid electric tape, it still came on, so I pulled the bulb from the
dash.

 

The turn and running lights are not monitored by that sensor, if it
comes on with the headlights unplugged then the problem is with the
taillights. Bad grounds, dirty sockets, mismatched bulbs, those are the
usual problems.

 

Interesting, thanks. I need to inspect those, check the brake light on
the back dash and see if I have an issue there.

Thanks!