The current drawn by brush-type blower motors is extraordinarily high when operated at maximum speed. IIRC, XM blower pulls around 25A at maximum . . . so about 300W. 25A is the penalty paid for operating such a motor at 12v.
Given this, it's pretty inexcusable, design-wise, to control the fan speed with a series resistance, when electronic control (eg PWM) would generate far less heat.
Worst case, however, is when the blower is set to "near max speed", and so the series resistance has to dissipate considerable heat.
One thing owners can do is to avoid leaving or switching blowers to max speed as soon as the car is started, and for two reasons:
- immediately after an engine start, the alternator output will be very high (to recharge a part-depleted battery), and this raises system voltage, which includes the supply to the blower.
- the starting-up current for these motors is even higher than their running current, and this further aggravates the initial current inrush into the motor, further shortening.
This prolongs motor life.
Best, therefore, to
- avoid leaving blower on max speed
- avoid running blower at 'near max' speed, when a large running current will be being fed through a small section of the resistor
It's under this last condition that maximum resistor heating will occur.
However, nothing excuses the primitive and cheap design of both blower motors and resistive speed control. A prime example of bean-counters at work, resulting in vehicles literally going up in smoke.
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Toyota have one of the largest recent historical recalls, iirc. But at least they put their hands up early on, and put things right. In a way, this can encourage some customer loyalty. It seemed to take forever before Peugeot admitted that some of their engines could suddenly lose all power, and Renault that Clio bonnets could blow open.
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