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Thread: Repairing complicated blower fan circuits. A tip for 1 of the many issues.

  1. #1
    Join Date
    Oct 2004
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    Lou. Ky.
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    87-635 '78-R80/7 '96-960

    Repairing complicated blower fan circuits. A tip for 1 of the many issues.

    The troubleshooting of both blower fans is a huge PITA and can have you going in many directions and replacing parts that don't fix the problem.

    Just last week I replaced a sticking AC switch. Would engage fine, but to disengage was getting difficult.

    The problem, I installed a different AC switch and all the sudden the variable function on the blower speed switch quit working on both the heater and AC fans. However both fans still worked on high speed. The little click you feel when turned all the way. I replaced the switch behind the blower switch and the problem is fixed. Going through all the troubleshooting posts can get you going in many different directions.

    So in my case the variable quit working. But both fans would work on full speed. Both fans operated exactly the same. The blower fan circuit is complicated if you've ever studied the ETM. You can get lost in it real quick. Each fan has its own circuit. Just throwing this tip out there. Hope it will help somebody out sometime. It can get expensive replacing the wrong parts.

  2. #2
    Join Date
    Oct 2005
    Location
    Boalsburg, PA
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    3,806
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    '83 633csi
    Having never really dealt with the HVAC on these cars, I had a look at the diagrams!

    HOLY MOLY

    Ive got access to AllData these days, and I can get diagrams for these cars!

    Typically, in all the modern cars I deal with, anytime you loose the lower speed functions of the blower motor, its the the resistor that is bad. All it really does is reduce voltage to the motor, at the price of getting the resistor really hot over the years and eventually cook.

    Im guessing the switch you replaced has a variable resistor built in it? The diagram im looking at doesnt show any real resistor symbols, but labels it as a Solid State Blower Speed Control.

    Glad you got it fixed!
    1983 633csi a.k.a. Wolfie - M30B35 Swap - Getrag 265 - 3.73LSD

  3. #3
    Join Date
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    Hi Jeff, the board behind the switch has 3 components to it. A resistor, transistor and a potentiometer. The Pot. Is actuated by the blower fan speed knob. The resistor is just a regular ol type. I did not ohm it out, but I will and report back if indeed it is. I just figured it was either the Pot or the transistor. None of the components looked burnt or fried. Fortunately I had more than 1 of the blower speed boards. The 1st one smoked the potentiometer immediately. Even shot out a little flame like a very small flare. That one definitely got smoked. Thank God I didn't do that to an expensive brand new one.

    All I can think of is the pot may have been past its normal travel when positioned to the knob. It has extra travel at both ends when not connected to the knob . Before and after the stops turning the knob..I was testing before I put the climate control all back together.

    Driving today I have noticed that the AC fan is now running at a constant very low speed in position 2 and running. That's without AC being turned on. It was not doing that while testing with everything apart, before reassembly.

  4. #4
    Join Date
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    And to add. There is a micro switch with 2 wires at the full blast position. That is the little click you feel when turned full right. And what kicks the fans on full speed. I suppose it bypass's the potentiometer circuit. Not looking at the ETM right now.

  5. #5
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    Actually I don't know if it is the AC fan that is now running at very low speed without AC being engaged. It could possibly be the fresh air/heater fan. Whatever, it's one of the 2. I'll find out.

  6. #6
    Join Date
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    87-635 '78-R80/7 '96-960
    Found out that it is the outside fresh air/heater fan that is now running full time while running. Don't really like that as it will just cause the bearings in the fan to wear quicker. Right now fan is in good shape, but running full time at even a low speed it will cause it to start to wear. I may wire up a switch to shut it down from running continuously.

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