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MikeD', Technician
Category: Car
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Experience:  ASE Cert, AS Degree in Automotive Diagnostics
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2001 Pontiac grand Prix GT, (3800) When I turn the blower

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2001 Pontiac grand Prix GT, (3800) When I turn the blower motor for A/C to the lower settings 1-4, it does not come on. it does work on 5. The odd thing is, when set to 1-4 the automatic headlights go out as well. If the fan comes on in 1-4, which they sometimes do in the lower settings, the lights begin working again... The headlights work fine when using the manual switch and the blower motor set on 1-4 does not cause the lights to go out. In the beginning, the blower would go out while in 1-4 for just 3 seconds. Then it changed, when turned on, it would take a few seconds to begin working. Now, they will not work at all in 1-4, but headlights go out when set to 1-4. I do hear a clicking sound under the driver side dash when set to 1-4.. Is this something you have seen befor? Mike

You most likely have a Logic blower control module going bad. Your high blower has a bypass and sends full current to the blower motor, but the other speeds are pulsewidth modulated through the module. This module is behind the dash on the HVAC box. A skilled technician usually can get it out without pulling the dash. I have provided the schematic to show you how the system works.

MikeD', Technician
Category: Car
Satisfied Customers: 696
Experience: ASE Cert, AS Degree in Automotive Diagnostics
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Customer: replied 8 years ago.
Where exactly is this module? I am Skilled, not a certified technition though. I always fix all my own issues. I am 100% confident I can replace this without the assistance of a cert. tech. Does the power need to be disconmnected while replacing this module? Do you have any idea of the price range for the part we are looking to replace. Is this the most common cause of this problem?

Here are the instructions. It is the most common problem with this complaint. I can not say definitely because I do not have the vehicle in to diagnose the problem I usually use an oscilloscope to check the voltage signal to and from the module.

The heater fan is located in the passenger footwell:

Remove the carpeted panel under the passenger side dash, held in place with 2 Christmas tree type holders
Remove the light fixture from the panel and set aside

Here you can see the fan:

The resistor is between the fan and the firewall.
After removing the fan you will se the resistor:

With the seat all the way back look up under the dash in front of the fan you should see a small screw 7/32" there are 2 more closer to the floor carpet.
Remove the screw closest to the fan and loosen but do not remove the 2 closest to the floor carpet. If you can't get a wrench on the back 2 screws you may want to try and break the old resistor out just be careful, and then you will have more access to the back 2 screws.
Place the new resistor in place using the back 2 screws as a guide, tighten the front screw, the front screw alone is good enough to hold it in place.
Reattach the 2 wire connectors

Customer: replied 8 years ago.
Is this resistor the same thing as the Logic blower control module which you said in your first reply that I most likely have going bad? I am nder the assumption that if the resistor burns out on speed one-four it will not work again. This problem was an on and off issue to begin with and now is a constant. I did not think once a resistor went out it would come back as it burns the wire on the resistor. Correct me if Im wrong..
Do you have any idea of the cost range to replace this item?

This is a resistor module, not a standard resistor, It can have spurts of intermittent operation before failing. But like I stated earlier, I like to test input vs output with an oscilloscope or factory scan tool before replacing parts. There could be a wiring problem or a problem with the control head also, although the resistor module is the most common it is not the only thing that can cause this.

Customer: replied 8 years ago.
Going with this scenario, can this also be the cause of the automatic headlights going out at the same time as the 1-4 settings on the fan?
Ok, I have reviewed all the schematics and find nothing in common with either circuit except the body harness that runs behind the dash. You may have a pinched wire that pinches between the wire that goes to the headlight switch and the wire that goes from the ac control head to the resistor module. I think you have a short causing feedback in both circuits. I would get it properly diagnosed before replacing any parts.

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