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 V70 is 25+ year-old design so it uses fairly primitive methods. A *series* resistor (switched for selection) is an acceptable way if controlling speed of a brushed DC motor driving a blower load. The worst case dissipation of such a resistor is half the motor full power consumption, but the designers also have to consider the possibility of the motor stalling due to worn-out bearings or debris sucked into the fan (or possibly critters). 

You will note the one-time thermal cutoff (fuse) mounted at one end to deal with over temperature. 

The large physical size results from the required worst-case power dissipation and the requirement that the part should not run (say) red hot for safety and reliability reasons. The resistances are relatively low and the voltage does not much exceed 12V.

More modern cars may use a large MOSFET and pulse-width modulation to control the blower speed. AC fans such as ceiling fans use different methods to control speed.
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.