A common fault on the MG-Rover 200/400/MGF/25/45/ZR/ZS models is failure of the heater fan speeds, usually on positions 1 and 2.
The speed control is achieved by using different value resistors switched in series with the motor, a three resistor pack is mounted in the fan housing behind the glovebox. Position 4 bypasses the resistor pack and allows the fan to run at full speed, this is a useful test as it proves that the Fuse, Relay and Fan Motor are working fine
The original pack used a special resistance wire called Constantan wound into small coils to make up the individual resistors, a combination of metal fatigue caused by heat and vehicle vibration and the wire being too thin results in the coils breaking.
A genuine replacement resistor pack (original part number: JGM100050) will cost £30 and upwards, however the part still has the inherent weaknesses mentioned above and will fail again in time.
A simple solution to repair the failed pack is to replace the coiled resistors with high power ceramic resistors. The original MG-R values were 0.27 ohms, 0.75 ohms and 1.48 ohms.
Because the resistors reduce the voltage to the fan (thus reducing it's speed) the "wasted" voltage is converted to heat - the resitors get hot in the process!
Therefore the resistors have to have a minimum "Wattage" rating to enable them to pass the current without failing and still be able to operate within their design ratings, the minimum wattage should be 11 watts. Anything lower will burn out quite quickly! It is feasible to use larger wattage resistors ie. 25 watts, but their use will be limited due to their larger physical size.
Try to avoid using the same value resistor in all three positions, ie. 3 x 1.5 ohm resistors, as the speed on position 1 (and possibly 2) will be too low. It is also tempting to use this value as it is readily available in an 11 watt resistor, but due to the value of current flowing in the fan circuit it will exceed the 11 watt rating.
If you cannot find values close to the original mentioned above it is quite acceptable to use two resistors in parallel to achive one of the desired values, as an example...
We need an 0.27 ohm 11 watt (or higher) resistor, if you connect two 0.47 ohm 7 watt resistors in parallel you have a 0.23 ohm 14 watt resistor - extremely close to the original.
Similar combinations can be used for the other resistors. Remember when you connect resistors of equal value in parallel the total resistance halves, but the power disippation (wattage) doubles.
I hope this has been somewhat informative!