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Old 09-15-2007, 01:16 PM
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Blower motor resistor explain please.

Hey I was wondering if someone could tell my how a blower motor resistor and switch is supposed to be wired up.

A basic schematic would be great.

I understand that the resistor is to be located in the ductwork but what exactly does it do? Burn off some of the volts by heating a coil of wire, and the longer the wire the lower the fan speed?

Am I correct in this assumption? Or am I way off base?

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Old 09-15-2007, 02:18 PM
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Quote:
Originally Posted by Holder350
Hey I was wondering if someone could tell my how a blower motor resistor and switch is supposed to be wired up.

A basic schematic would be great.

I understand that the resistor is to be located in the ductwork but what exactly does it do? Burn off some of the volts by heating a coil of wire, and the longer the wire the lower the fan speed?

Am I correct in this assumption? Or am I way off base?
I don't have a diagram for you but the resistor is what gives your blower fan it's various speeds. Generally when these fail, you'll only have high speed or everything except high speed. There's quite a bit of load on these so make sure your connections, both at the switch and at the resistor, are clean and snug.
On newer vehicles, these have generally been replaced by a module which takes care of this and other functions in your HVAC system including control of actuators for coolant flow, ducts, blend doors and etc.
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Old 09-15-2007, 07:06 PM
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thanks I am actually I'm having to rewire part of my factory harness and was having a hard time understanding how the resistor works/what it actually does.

trying to simplify things in the cab and wireing up a basic selector switch for the fan.
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Old 09-15-2007, 07:47 PM
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Think of a resistor as a control valve. What it actually does is limit the amount of current flowing through it to a device. Resistors come in various values to obtain just the right amount of current needed for an application.

Vince
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Old 09-15-2007, 07:56 PM
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look at the coils on the turning part of a headlight switch, there controls for the dimming of the gauges. blower motor resistor does same thing, if you think on how a TP sensor works. it's all relative.
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Old 09-16-2007, 09:49 PM
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On most vehicles, the high speed fan uses a direct 12 volts to the blower motor. Lower speeds switch the circuit to go through various combinations of resistors and reduce the voltage to the blower motor.

The resistor pack is inside the air plenum because moving air helps keep the resistors cool. Resistors fail when they get too hot and burn up or break, and that's when you lose the lower speeds on the blower.

I've had them fail on a couple of my newer cars, and the resistors I took out seemed to be very light-weight and brittle. However, the resistors on my '75 pickup are very heavy duty wire and it would take a lot for them to burn up or break from vibration.

Bruce
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Old 09-17-2007, 02:02 PM
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A blower motor resistor pack changes the voltage the motor sees and thus changes the motor speed. When you set your fan on high, the current flows straight from the fan speed switch to the blower motor, which runs at full speed. When you select medium speed, the current is sent through a very small resistor, usually 0.5 - 1.0ohms, which reduces the voltage to the blower motor and it runs slower. When you select the low fan speed, the current is sent through a higher value, usually 1.0 - 2.0ohms, which further reduces the voltage the motor sees and it runs even slower. Reducing that voltage to a high current device such as motor creates a lot of heat, which is why the resistor pack is installed in the heater duct.

If you want to wire up your own speed selector, err on the safe side and get at least a 20A switch, or a relay setup. You should be able to measure the resistances and figure out which speed switch output goes to which input on the resistor board. Remember, no resistance = high speed, mid resistance = mid speed, high resistance = low speed.

75gmck25, the reason your resistors are so large is because the fan on those old trucks was set to run whenever the truck was on. Modern cars have drafts built in to vent outside air, and they love making things as cheaply as possible, so they just print resistive material onto a circuit board. Those things overheat and burn out all the time.
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Old 09-17-2007, 07:07 PM
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thanks all I got it figured out.....

Ended up using one out of a 86' chevette that I had setting in the field.......

Now I need to find a 4 position panel mount fan switch. Cant find one to my liking from work....

I remember having a van with an aftermarket rear air system that had a fan knob in the dash.....Wish I could find one of those...
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