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I am working on a custom a/c installation for my '70 El Camino, using parts from several kits that I have at my dealership. I would like to know if a stock GM blower motor could be made to be infinitly variable in speed, similar to how a dimmer switch works on a light? If so, what kind of a rheostat would I need to regulate the voltage? if not, why, and what might my options be? This is the only part that I have not figured out. Thanks.

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"SSuper Dave" Palmer
A.C.E.S. #1483
Team Chevelle #32 Gold
1968 Chevelle
1988 Trans Am
1970 El Camino (electrical system operating, Vortec engine starts and runs at last!Bodywork is continuing.
 

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Dave,
I don't see any reason why your idea would not work. The two step resistor that normally controls fan speed is just that two step not variable.
The only problem I could forsee would be a slower than "normal" speeds may not be good for the motor.
As far as specifications for a variable resistor you will need to match that up with the Amps/Current and Voltage (12v) specs of the motor. To small of a resistor will burn out from the load.
Once you have the specs check with a local Electronics supply store. Some of the guys that work in these stores are fairly sharp. However they may get stumped when you tell them it's for a blower motor for a car.
Radio Shacks are very helpful but may not be able to adapt ther knowledge for your needs. Seek out an independant non-chain electronics supply. Look in the yellow pages.
Good luck.

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FastSS396Man
Brian

69' SS396 Chevelle
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Dave

The blower speed is directly proportional to voltage. The lower speed requires higher voltage drop across the series resistor. The higher the voltage drop the more heat will be generated. What I’m leading up to is it will require a pretty hefty variable resistor and it will heat up. Likely more heat than you want in the tight space behind the dash. As you know the stock resistors are installed in the duct they need the airflow to keep them form burning up.

Additional information- the A/C blower moves more air than the heat only blower. The motor is physically larger and draws more current. On the factory system the high speed uses a relay that bypasses the under dash wiring. If you do use a variable resistor something else to consider is the load on the #10 wire supplying under dash power.
 

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Typical current draw for an AC blower motor would be 20 amps at full speed. At lower speeds the current would be less. Anyway, the rheostat you would need would be large. Maybe someone knows of a DC circuit that uses a power transistor to handle the current load to the motor. Kinda like the variable speed circuit on electric drills, but for DC.
 

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Discussion Starter #5
Thanks for the advice. This may not be the way to go after all. Anyone know if there is anything solid state that will work? Some of the GM climate control systems that we have now use an infinitly adjustible blower and no rheostat.

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"SSuper Dave" Palmer
A.C.E.S. #1483
Team Chevelle #32 Gold
1968 Chevelle
1988 Trans Am
1970 El Camino (electrical system operating, Vortec engine starts and runs at last!Bodywork is continuing.
 

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The only way to do true variable control on a DC motor of that size is with PWM (pulse width modulation). The circuit is actually very simple for something electronic. It's basically just a sawtooth generator, comparitor and a big FET. Since the FET is only on or off it doesn't generate the heat you'll get otherwise.

You can do a linear (transistor or resistor) circuit but it could easily be generating 50watts or more at part speeds.

Peter
 

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Didn't the Ford T-birds from the late 80's-early 90's use a variable speed blower? I remember renting one of these cars around that time frame and noticed this feature. Maybe you could hijack one of the rheostat type switches from one of these cars in a junk yard. I would imagine the blower motors would be similar in current draw (you may want to check first)

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