The MGA With An Attitude

At 01:04 AM 1/21/03 -0700, Bruce Vay wrote:
>>".... I did try to rebuild the fan motor but mine is running poorly and almost stops. It just ran slow before. ...."

Oh-oh. I hope you didn't mess up something, like the wiring sequence.

Be sure it turns freely with just a little nudge of the finger tip. If not, check alignment and fit of the bearings/bushings, put a drop of oil in the bearings. Also check the condition of the carbon brushes. They should slide freely in the holders, and the spring force should hold them firmly in intimate contact with the commutator. The copper segments of the commutator should be smooth, clean and bright. You may need to file or sand the copper bars to get good electrical contact with the brushes. That and/or new brushes will usually get the motor running. Also see my information on rebuilding the heater motor.

Check the voltage at the feed wires when the motor is running (or trying to run). It ain't goin nowhere on low voltage. A low voltage problem might be from loose or corroded connectors in the wiring, or from a bad fan control switch. If it gets good feed voltage and still doesn't run well, then there's something amiss inside of the motor. When in doubt, hook the motor directly to the battery power to see if it runs okay. If that works, then look for the problem in the wiring harness or the switch.

>>".... The local Brit shop sold me a used motor which is different than the midget one probably from a 1500 or a B."

If the shoe fits, wear it. A heater motor is an independent device. Give it 12 volts and it should do it's own thing, regardless of what car it's installed in. Of course it does have to physically fit the cabinet.

Do pay attention to the direction of rotation. The blades of a squirrel cage fan are curved like the top of an airplane wing. The outer sharp edge of the blade should be leading in the direction of rotation. Direction of rotation should also go towards the larger outlet end of the air chamber in the cabinet. So first you need the squirrel cage that is designed to run the right way for your particular heater box. Then you need a motor turning the right direction. You might use the fan from your old unit. If the new motor turns the wrong way, first try switching the feed wires. Some of the newer motors will run backwards when you change polarity. The older ones will always run in one direction regardless. In that case you have to swap a few wires inside to reverse the polarity of the field windings to make it run the other way. See rebuilding the heater motor.

>>"I picked up a motor from a 1970-80's Chevy and Impala I think and it is about the same size as the Lucas but faster and has a larger cage. Oddly enough with a little metal nibbling I can fit it to the heater can and the cage fits fine."

I would usually prefer to keep it original for future maintenance reasons. If you do find some advantage in making such a modification, be sure to keep good records of the change and the parts used so you (or anyone else) can service it in the future.

>>"I could also add a motor speed control and have a multiple speed motor. Have you ever done this?"

No. With my MGA I have never had any reason to slow the speed of the heater motor.

I think every heater motor I have ever seen has only two wires. Give it 12 volts and it will run at top speed. To make it run slower you have to switch in some resistance to drop the voltage. This requires a multiple position motor control switch. The more resistance you put in series with it the lower the voltage at the motor, and the slower it runs. This works similar to a ballast resistor in the ignition system of later cars. A resistor passing a current will get hot (wasted energy), and you have to allow for the heat to escape so it doesn't burn something.

If the heater motor draws 5 amps at 12 volts, the resistance is 2.4 ohms, and the power is 60 watts. If you put a 2.4 ohm resistor in series with it, the voltage at the motor is cut in half, the current is cut in half, and the power is cut in half. Then it will be using 30 watts total, half in the motor and half in the resistor. When the motor is getting only 1/2 the prior voltage and half the prior current, it will have only 1/4 of the original power and it will run MUCH slower. The other half of the power will be heating up the resistor. The resistor has to be able to dissipate 15 watts of power (heat), and you might burn your fingers on it when it does that. This is not good energy conservation, but this is the way cars have been built for as long as they have had multi-speed heaters. In many new (newer) cars the speed control resistor is installed inside the heater box for protection and air cooling of the resistor element.

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