Question: How can I test a DC car blower motor with an external resistor? (note that there is an existing question about testing DC motors, but the assumption there is that it has an internal resistor, and it also assumes the tester has access to a "good" motor or a schematic, which I do not)

The motor has two terminals on it and the associated resistor has 4 different labeled resistance levels: R8K, R40K, R50K, and R70K.

Is testing the motor as simple as hooking a car battery to a high wattage 8000 Ohm resistor and then connecting the leads to the two spade terminals on the motor, or is it more complex than that?

[![enter image description here](https://i.stack.imgur.com/wWXyQ.png)](https://i.stack.imgur.com/wWXyQ.png)

[![enter image description here](https://i.stack.imgur.com/YUh3w.png)](https://i.stack.imgur.com/YUh3w.png)

[![enter image description here](https://i.stack.imgur.com/ustty.png)](https://i.stack.imgur.com/ustty.png)

Answer: 
The resistor is used for voltage reduction to the DC motor in your car fan. The power dissipated in the resistor can be calculated from \$ P = \frac {V^2}{R} \$ or \$ P = I^2 R \$. The resistor needs to be able to radiate this heat away without getting so hot that it melts nearby plastic components or setting itself on fire - hence the size.

The fans in your house are AC and so a capacitor can be used to reduce the voltage without any power being dissipated. A capacitor has an impedance (the resistance to AC) given by \$ Z = \frac {1}{\omega C} = \frac {1}{2 \Pi f C}\$. The current through the capacitor is 90° out of phase with the voltage so the power dissipated is zero.
If that was only one effective resistor it would only need two wires. 

It has several connections matched to the number of speeds available for the heater, 3 or possibly 4, which is why it is so large also due to the current it has to pass.