Are headlight and blower motor electrical systems related?

ARWOLKOWSKI

MEMBER
1992 BUICK LESABRE
150,000 MILES

Or run off the same system, e.G, body control module? How to troubleshoot?

My mother's car ('92 Buick Lesabre) has two issues;
1) Blower motor works only sometimes (on all speeds). She had a "mechanic" look at it, but he couldn't find the problem. A second shop had a look at it (blower was working before entering shop) and said the switch, relay and resistor all check out OK. The motor is new and tests OK. They couldn't find any problem. It all worked fine on the way home and then started working intermittently again, the next day! Note: When the blower motor doesn't work, the indicator lights on the heater control don't work either.

2) Headlights, dash lights & interior lights won't shut off unless battery is disconnected. The second shop said she needs to replace the "headlight control module", but they didn't have one in stock to replace it. Do you agree with the diagnosis? If so, where is it located so I can get one from a salvage yard?

Are these two issues related to the same problem, e.G, bad BCM? Can you provide a systematic troubleshooting sequence for me to rectify both problems? Please provide wiring diagrams for both electrical systems?

5 Replies

RIVERMIKERAT

MECHANIC
6,110 POSTS

It could be a loose ground

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Thursday, March 29th, 2012 AT 1:33 AM

ARWOLKOWSKI

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6 POSTS

? Would there be a common ground for both the blower motor and lighting system that would cause these problems. Or are you saying the loose ground is for the blower motor problem?

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Thursday, March 29th, 2012 AT 2:59 PM

RIVERMIKERAT

MECHANIC
6,110 POSTS

The blower issue could be a ground problem.

Reading you question again, if the issue was just the instrument lights and headlights, I'd say headlight switch. If it was just the interior light(s), I'd say that at least one of the door switches or wires is grounded. The wiring schematics I have access to don't do a very good job of showing how any of these circuits are wired. It very well could be a bad BCM. The only way to check would be to see if you can get one from a junkyard that will allow you to return it if it doesn't fix the problem.

What about the marker lamps? They get power from the lighting control module.
I found a diagram of sorts.
Look behind the left side of the dash, left of the steering column. The lighting control module is there. Look for the yellow wire and the purple wire. The yellow wire goes to the beam selector switch. See if it has power with everything off. The purple wire appears to come from the headlight switch. See if it has power. There is a light blue switch. Check it for power, as well. But I don't see anything for the interior lights. If the purple or blue wires don't have power, but the yellow does, the LCM is bad.

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Thursday, March 29th, 2012 AT 8:01 PM

ARWOLKOWSKI

MEMBER
6 POSTS

I don't have a wiring schematic for the blower, but I've read that there is always power to the blower and the blower control switch supplies the ground. Is this true? If so, how do I go about checking to see if there is a proper ground. Is the blower also controlled by some sort of module as well?
I'm a long distance away from where the vehicle is at this time so I can't check the wires coming from the LCM at this time. I'll make a call this evening to find out if the marker lights were also coming on. At this point I think I'll at least get a LCM from the wreckers before I head out to where the car is. Where is the BCM located on the car? My dad said he already changed the "computer", so I'm assuming he changed the BCM.
Thanks for your help. I'll keep you posted.

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Thursday, March 29th, 2012 AT 8:28 PM

RIVERMIKERAT

MECHANIC
6,110 POSTS

No. The blower has constant ground and switched power. That's how the speed is controlled. The ground is usually a single black wire with a spade connector. My diagram shows a ground wire from the blower and one from the blower control module together going to ground. Both black. Power is a purple wire.

My reference doesn't show a body control module. Just the lighting control module on or near the steering column on the left.

Going through our reference and what you've written, it looks like you have multiple problems. Our reference doesn't list whether wiring for all of these circuits is in the same bundle. If they are, what you might have is a short between multiple circuits.