1991 PA headlight electrical problem diving me insane.

[BPL3]

Hello again kind Buick enthusiasts.​

Where to begin such a long and convoluted story? A year or so ago, my low-beams stopped working. Tested headlights, worked fine, so due to my natural tendency to underestimate problems and overestimate my intelligence, I found out just how much of a pain it is to replace the dimmer switch on the column. Did not make a lick of difference, so I saved the old switch and switched the high and low-beam connectors(slight modification needed).

Last week, the high-beam circuit quit working, as well. Failed the same way as the low-beams; after the first time they died, a day or so afterward, they would still come on intermittently. So I began to check everything I could. There is 12 volts coming out of the light control unit to the yellow wire of the high/low-beam switch. There is 12 volts at the headlight socket, until you plug a bulb in, then it drops to zero. Thinking bad ground, I run a ground straight to the plug, no luck. However, direct wiring the positive lights them up. Disconnect yellow wire from switch and give it 12 volts from a hot terminal on the back of the fuse panel, no luck.

So I am thinking a bad splice somewhere. Disconnected the battery, began unwrapping all wire bundles in engine compartment, soldering every splice I find. Do the entire length of the driver's side run and all but the very front of the passenger side. No luck. begin removing more of dashboard, for the dual purpose of tracking wires and hoping to run a couple of new wires through the firewall. Realize that I was about to really have my car in pieces, which I may or may not be able to reassemble properly, stop, put dash back together.

Reconnect the battery and find the high and low-beams functional. An hour later, on the way to work, the high-beams go out again, leaving me with the low-beams, which had gone out a year prior to the high-beams. A week later the high-beams go back out. Time for drastic measures.

Buy the relays, 12 gauge wires, connectors, fuses, new high temp sockets. Soldered every last joint, ran the wires from the relays on the battery side inside the frame supporting the radiator. Wire it all up and turn it on, bingo! Switch to high-beams, they don't come on. Switch back to low-beams, they don't come back on. Momentarily short terminal 30 with 85 on the relay(giving 12 volts positive to the terminal that the old positive headlight wire is connected to, bingo! Back on. Of course, this is not a tolerable situation for my car to be in, it is hardly convenient to pull off the headlight to short the relay on every time I need headlights.

Go to take the truck to work, instead. Won't start, battery dead. Charge it up,still won't start. Already late for work, I move all my needed work items back into the Buick, planning to short the terminals just to get me to work. Turn it over, the twilight sensor turns on the headlights and they work! Cool, save me some time, off to work...except now, it won't idle, just dies the second I let off the gas.

I made it to work, accelerating at all times, because otherwise I was coasting until a good time to stop and start it again. Just finished typing all the above and now I ask for a miracle answer from one of you. Like,change fuse 11 to a 15 amp and it will fix everything, plus give me magical powers. I am not too optimistic about that,so any advice given, will be appreciated.​

 

[ryenie]

There looks to be circuit breakers, one built in to the headlamp switch, and one in the i\p fuse box for the lighting control module. This might explain intermittent issues, instead of a fuse blowing forcing you to replace it to make it work, after the circuit breaker resets, it works again (i think it's just a bi-metallic switch, not like a house breaker). But after tripping so many times it may need to be replaced (like a house breaker). Pins H and G (orange and red wire to the LCM), are the power feeds for the LCM, the orange wire is powered by fuse 14 (15amp) (hot at all times), and the red is powered by 20amp breaker (also hot at all times). It might help to check the voltage at the red wire when the issue is present, no voltage would indicate breaker is tripped.

 

[BPL3]

Thank you, will do.The odd thing is, that if I disconnect the LCM from the high/low-beam switch and hook it to positive, they still don't work. That is why I keep thinking it is a splice somewhere. Will check what you suggested in a few minutes.

 

[ryenie]

Thank you, will do.The odd thing is, that if I disconnect the LCM from the high/low-beam switch and hook it to positive, they still don't work. That is why I keep thinking it is a splice somewhere. Will check what you suggested in a few minutes.

Theres also an adaptive light monitor module after the dimmer switch and before the bulbs. It looks like the last module before the light bulbs. It controls both the high beams and low beams. It could be keeping the voltage from the dimmer switch from reaching the bulbs. From the wiring diagram, it appears that there are resistors in the circuitry so applying full voltage beyond this point may cause the bulbs to get overly bright or hot, not sure, but as a word of caution.

 

[ryenie]

Theres also an adaptive light monitor module after the dimmer switch and before the bulbs.

I believe this is the bulb out indicator. Not 100% but a tan and light green wire go in from the dimmer switch, and tan\white, lgt grn\blk come out and go to the bulbs. There are 4 splices i see in the diagram. After the dimmer switch, One before the almm, on the tan wire (low beams) for the drl resistor. One on the lgt grn wire that goes to the gauge cluster for the high beam indicator. And after the monitor a splice in each wire to provide the battery+ to the left and right bulbs.