Heater Fan Motor

All photos from Clausager.

1962. On the face of it no white wires, but close inspection indicates the wires are wrapped in black tape, and there are traces of white at each end (arrowed), entering the motor near the top.

1964. Definitely two white wires entering the motor at one point, near the bottom.

1969, still two white wires, although one has sleeve on it, possibly indicating polarity! This wire does seem to be going to the coloured harness wire, with the other one going to the black.

1972. Now one coloured and one black wire, although it isn't clear where the two wires are going on the motor. Compare the depth of the motor bodies in the following pictures with those above.

1975, definitely one coloured and one black wire going to opposite sides of the motor. A clear difference in the shape and size of the flange on the motor body compared to the earlier one.

1980. Probably the same size motor (externally at any rate). Three wires coming from a 3-pin plug, two of them going into the heater casing plus the black earth wire going direct to the motor, and a third coloured wire coming out of the casing and going to the other side of the motor.

1968 MGC. This seems to have the early longer motor and the two white wires, ...

... but this 1969 MGC seems to have the later coloured wires and the shorter motor, even though it is still three years before the introduction of the V8. However this, and any of the photos, could be sporting a replacement heater unit or motor.

Dropper resistor for 77 and later 2-speed heater fan (circled) inside case (Photo by Andy Charman)

I didn't know the actual resistance and wattage values of the dropper resistor, but did some tests to see what sort of values they might be. I put a standard 3 ohm coil in series with the motor and it seemed to drop the speed by about half which would be about right. I then measured the voltage across it with the motor running at this speed and got about 5v, which at a system voltage of 12v (engine not running) is about half voltage which tends to confirm what I'd heard. This represents about 1.7 amps (voltage across the resistor divided by its resistance). Subsequently the manufacturer of the heater systems for the MGB and many other makes and models, Ashley Hinton, told me that they were 2.5 ohms so not a bad guess. Based on my test the motor has an equivalent 'resistance' of about 4 ohms, so with a 2.5 ohm resistor you would get about 2.2 amps, so about 5.5v across the resistor. Wattage is calculated by squaring the voltage and dividing by the resistance i.e. 5.5x5.5/2.53 which gives 12 watts. If you want to convert a single-speed system to a 2-speed the resistor would need to be greater than 12w to avoid burning it out, and probably screwed to a metal mass to aid cooling, making this type the best bet.

If you have a 2-speed you can measure the resistance by putting an ohmmeter between the green/yellow and green/brown wires with the fan switched off, and measure the voltage across the resistor by putting a voltmeter between those two wires with the ignition on and the fan switched to slow.

You can only calculate watts by measuring the actual resistance of and voltage across the resistor in this way as motor current varies with rotational speed, a stalled motor taking a lot more current than a freely running one. You can't measure the resistance of a stationary motor and use that in any calculations, except to see what the stalled current would be.