I need help!
'94 cherokee, 4.0 HO. just bought a few months ago. truck needs work.
OK so here is the history. i read thru many pages of heater info.
I had a heater motor that would only run on high speed. it squealed badly.
pulled the resistor. it was rusted.plug was OK. replaced resistor. motor ran on all speeds after resistor replacement,but still squealed, & resistor started to smoke. shut down quickly.
pulled heater motor. it would barely spin by hand. replaced. tested motor & resistor prior to full install,motor spins great, runs on all speeds, but after a minute on low fan speed resistor heats up & starts to smoke.
So my question is - after replacing bad resistor & motor(not installed, just plugged in), what would cause the resistance to heat up & burn out resistor. again resistor plug looks great. I may be having a ground issue, new wires from Jon (5-90) are on order. thanks for your help.

 

anyone?

 

High resistance in the circuit from corrosion/connections (check resistance with no power applied) or back feed through ground. Is the dash switch and/or connector burned up?

 

Yeah sounds like something is drawing too much current and making it burn up. Not sure what these block resistors are rated for amp-wise but it won't take much over their limit to burn a resistor up. (Do it all the time at work)

Like Turbo X_J said, probably some corrosion causing it. I'm reiterating this also, make sure there is no power applied when checking resistance. You can mess some other stuff up as well. Including the multimeter you are using.

x2 on the switch. If the terminals on the switch have burn marks or are corroded then that would be part of your problem. Burned terminals almost always mean something is drawing too much current somewhere.

 

Mine is a '94 also...

How is the heater blower switch, and the connector on the back of it ? Maybe the switch & connector are fried enough to have problems.(??)

Seems like these parts don't hold up very good if the blower motor begins to draw more power due to age & wear.

I had to replace the switch, the resistor, and the blower motor. Couldn't locate a connector/pigtail for the switch, and used spade connectors on the wires for the new switch.
I bought the switch & resistor online. The resistor came with a new connector, but didn't need it.

Let us know what you find out....

 

Let me see if I understand timeline:

1. replaced resistor

2. bad motor, new resistor got hot.

3. replaced bad motor, but not the resistor you just replaced that had gotten hot.

4. Sounds like you have good motor now, but newish resistor got fried by old bad motor? Might want to replace resistor (again) and see if everything is OK now.

 

Firestorm has the timeline correct.
I did not think the switch &/or the switch plug would be shot since the plug at the resistor was great.
I did shut the power down before the resistor fried, i think.
I will replace the resistor again, & i guess i will have to check the switch & plug.
I will keep u all updated.