12V Question


Old 10-07-05, 12:06 PM
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12V Question

Hi, The risistor that controls blower motor speed on my car has died. The resisitor is removed now. So, I spliced into the hot wire that went to the resistor and fed it directly to the blower motor via a toggle switch. I installed an inline 30A fuse, but the wire still gets very hot. If I run the correct size wire directly from the battery instead of splicing into the existing one will this eliminate the wire getting hot? thank you in advance.
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Old 10-07-05, 12:30 PM
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No, it won't, assuming the wire is the correct size now. Heat is caused by current. The current won't change if it comes from a different location in the system.
Old 10-07-05, 01:06 PM
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What you probably removed was a potentiometer (pot) which is a type of variable resistor usually connected to a knob. Is this the case?

The circuit was designed to function with a certain range of resistances in series with the motor. If you have reduced or eliminated this built-in resistance, the motor will get too much current and burn up. You need to find out what the appropriate resistance is in the circuit and replace it.
Old 10-07-05, 02:48 PM
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You may want to check in on the automotive site.
If you are referring to the resistor stack, usually mounted on the blower or on the duct work, Why not just replace it It is inexpensive and usually very esy to swap out.
Old 10-07-05, 03:46 PM
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Assuming the resistor was used for fan speed control only, and NOT to allow an 8 volt motor to be used on a 12 volt system.
Some reasons wire will get hot, Bad motor, wire size to small in diameter or more then likely your connections are poor and they are creating heat from the resistance due to insufficient contact area.

The size of the fuse is to protect the wire, if the fuse is to large it may not blow and the wire can turn into a heater and catch fire.

I may be wrong, as I recall the largest car fan motor that I came across pulled about 15 amps.
I don't think a 30 amp fuse is going to protect your wiring.

With out knowing the current your motor needs, start with a 10 amp fuse at-at the Battery, Then say 12 gauge wire to your switch and 12 gauge wire to the motor.
I would not use more then a 20 amp fuse with 12 ga wire.
Don't let this line back feed power into the old existing line it may power something you don't want.

If the 12 ga. wire gets hot, your motor is likely bad and that's what made the resistor open up.
Old 10-08-05, 07:25 AM
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The blower motor is in a volvo. In the fuse box it is protected by a 30A fuse oem with #12 wire. The cover of the box also shows the diagram with a 30A. That is why I used an inline 30A fuse. With the toggle switch which is also a 30A switch, the blower motor works great, but naturally only on high speed because of the speed controlling resistor which is now disconnected because it is bad. It cost $150.oo to replace. (ouch) Not made aftermarket. Only available from Volvo. . Would the #12 wire spliced together with wire nuts be a poor connection? Because that is how I connected them. Would it be better to undo those and solder? I put a 20A fuse in the link and it blew after 5 minutes of operation. So, I replaced it with the original 30A and it runs but the wire gets hot. Is there some type of generic type of resistor that I can install inline to keep the wire cool that is cheaper? I don't mind having the blower work only on the one speed with my toggle switch.. THANK YOU.
Old 10-08-05, 09:55 AM
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Once again, it is not the connection, it is the missing resistor pack that is causing the overheat and fuse blowing. You could put a high wattage resistor in series with the motor to limit the current, but I would not have any idea about what resistance value to get. Maybe you could still measure a value from the dead unit?
Old 10-10-05, 11:16 PM
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I assumed that you spliced a wire across the old resistor and that short length of wire was getting hot.
which would indicate the connections are bad.

If you have, say over 2 ft of wire and the whole length is getting hot then you will need to increase the diameter of the wire.
The larger diameter will have less resistance and will run cooler.

or like you asked to add resistor. a resistor will decrease the total current in the circuit.
The draw back with resistors is,
They get hot, and that's wasted power.
I believe the fan resistors in cars are placed so the fan blows air across them to keep them cool. this allows the resistor to be smaller.

With out the motor I can only do some rough math. Assuming DC 14 volt motor that pulls 25 amps.
14 volts is about the cars charging voltage.
You would need a resistor 0.25 ohms to 0.36 ohms but the resistor will need to dissipate 100 watts.
You would need to find the next largest size resistor 150 watts

Using a 0.36 ohm resistor with that motor should make the total circuit currant about 15.2 amps.
The motor will get about 8.5 volts lowering its speed.

a generic 100 watt resistor can get as hot as a 100 watt light bulb.
Most but not all resistors are rated for use without a fan so a 150 watt cheap resistor may be 8 " long to dissipate heat without a fan.

If you can find a 100 watt 6 volt light bulb it will do the same thing as the resistor, it will get hot and light up wasting power.

Your best bet is to find a used resistor from a junk yard.

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