Is it possible the fourth terminal isn't used? There might be a blank spot in the electrical connector where one wire is missing. Changing the resistor shouldn't be necessary when adding or deleting air conditioning. The resistor must be matched to the number of settings on the switch. If you end up with a different switch because it is part of the heater / ac controls, then you WILL have to change the resistor to get the correct number of speeds.
There are two different ways the resistor can be wired by the manufacturer, and depending on how they do it, there will be different symptoms. The most common way is to put all of the resistors in the assembly in series. On the second highest setting, one resistor is switched into the circuit to lower the current flow. On the next lower setting, a second resistor is switched in to add to the first one, thereby lower the current flow even more. In this configuration, a burned switch contact will cause the fan to revert to its lowest speed when the dead speed is selected. All other speeds will work properly. It is possible for all of the resistors in the assembly to be the same size and value. This is what GM used so the fan would always when the switch was turned off, as you have noticed.
The second method is to use resistors in parallel that are different values for the different speeds. A wire from each switch terminal feeds a different resistor. With this configuration, a burned switch contact or defective one resistor in the assembly will cause that one speed to be dead, but all other speeds will work normally.
The number of terminals will match the number of speeds on the switch. The highest fan speed terminal on the switch goes to the relay. The total of the number of lower speeds, (usually 2), means each of those has a wire that goes to a terminal on the resistor assembly, and the third wire from the resistor goes to the motor.
The fan will always run on the lowest speed when the ignition switch is turned on, unless you select a higher speed. That was a GM thing. They called it "Flow Through Ventilation".
Friday, September 17th, 2010 AT 8:49 PM