I have 91 civic. I replaced the heater blower motor, but will only work on 3 and 4 speeds. I realize that this may be the resistor which I have not tested yet. However...The fan will not turn on by itself at the high speeds. I have to poke a stick into the housing to turn the fan a little and then turn the fan on 3 or 4. Does anyone know why this is?

 

First probe with a voltmeter the two pins in the fan plug with the fan still plugged in, key on, and switch on 4. If you have 12 volts at the fan plug but the motor doesn't start and run, the motor is bad. Even though you just replaced it, doesn't mean it can't be bad.

Not running on 1 and 2 is almost always the resistor. Unplug it and you should have continuity (a few ohms resistance) between every terminal and every other one. If not, replace it.

If you don't have power to the motor on 4, see if the positive or negative side is open. The circuit works by feeding power to the motor first, then the circuit to ground is completed through the switch and resistor. If you have 12 volts from both pins to ground, the power is coming in but the switch is not closed. Most likely the switch itself.

If it's zero volts on both pins, the fuse holder might not be making contact. I think the fuse is under the hood. I don't think the '91 has a fan relay, the power comes from the key switch to the fuse then to the motor.