When I purchased the truck I got it home and was testing what worked and what not and the heater switch just burned up when i turned it on. I replaced the switch with a brand new one and it did the same. Any idea what is causing this on my 96 Jeep Cherokee?

 

what are you calling the heater switch? Fan speed selector? Switch that selects A/C, vent, heat, defrost?

 

fan speed selector!

 

I should have asked the first time: do you mean literally smoking and melting etc.

The switch selects the appropriate coil(s) on the resistor pack ("resistor") and the thermal fuse on the resistor is what usually blows if the fan is beginning to fail and draws too much current. In this case the fan will run on High speed only until the fuse or entire resistor is replaced.

I'm not absolutely sure where the resistor is located on the '96. On the '93 it is in front of the passenger's knee and reached from below. At some point they moved it up behind the glove box. You can test it with an ohm meter for continuity across the fuse which looks like a diode. It should test 0 ohms if good.

You can check the fan by unplugging the connector in the engine compartment and running it directly of of a power source. You might check first with your meter to make sure it doesn't have a dead short.

 

It melted the switch like almost caught fire. I replaced the switch and wire leading to it and it did the same again.

 

same thing on my 95, looking to see ideas.

 

This happens when there's too much current flowing through the circuit. The blower motor itself is the usual culprit of this.

 

How do you check the current draw through the fan motor? By draw do you mean amps or volts?

 

Your checking Amps. Hook up a meter in series from the fan power wire.