On many cold winter days, I have wished that the blower motor on my MG would run at a higher speed to keep up with the air being sucked out through the many cracks and crevices in the cockpit.
While replacing the fan, I noticed that the motor ran much faster being connected directly to the brown circuit at the fusebox. I decided to do some voltage and current measurements on the factory wiring harness and found that the original wires are of insufficient size to carry the necessary current to the fan motor. With the motor in operation, voltage fell from 12 volts to 9.7 volts and current was only 2.6 amps. Hooking the motor directly to the brown circuit at the fusebox and bypassing the smaller gauge wires resulted in a voltage drop of only one volt and a higher current supply to the motor of 3.5 amps.
To remedy the problem of the slow running fan motor, I installed a relay in the fan motor circuit, and tapped voltage from the purple (fused) side of the fusebox. On the MGA, I used the same method, but tapped fan motor voltage from the brown/green (horn) side of the fusebox.
The relay, along with 12 gauge wire, supplies the motor with the current it needs, without the large voltage drop associated with the smaller wires, and greatly increases the amount of air delivered by the heater.
While replacing the fan, I noticed that the motor ran much faster being connected directly to the brown circuit at the fusebox. I decided to do some voltage and current measurements on the factory wiring harness and found that the original wires are of insufficient size to carry the necessary current to the fan motor. With the motor in operation, voltage fell from 12 volts to 9.7 volts and current was only 2.6 amps. Hooking the motor directly to the brown circuit at the fusebox and bypassing the smaller gauge wires resulted in a voltage drop of only one volt and a higher current supply to the motor of 3.5 amps. 