Your first guess is more likely to be right. Chrysler has a very long history of innovation and being the first with developments that actually benefited car owners. Like all other car brands today though, they are following them and adding an unnecessary and unreliable computer to systems that never needed a computer before. We used to have a simple, reliable switch to turn the heater fan on at one of three or four speeds, but the engineers have convinced us it is critically important that we have a continuously-variable fan speed control, and that requires another computer. Since computer circuits can't handle switching high-current loads like blower motors on and off, a separate power module is needed. Now you have a very trouble-prone computer, a high-failure power module, and all the additional wiring connector terminals that are prone to overheating.
Any of these things can cause intermittent operation, but as a general rule, the power module will fail and never work intermittently again. Of all the problems that occur with the computer, failure to turn on the fan is not one of the common ones. There really aren't more wires in the high-current part of the system than before, so while a bad connection is still more likely to occur than on older systems, it's still a better guess the fan motor is just wearing out.
After many years of use the brushes in the motor wear away and will eventually make intermittent contact. Once the motor is jarred enough to get it started, it will stay running until the next time it is turned off and back on, then it may or may not start up again. When it doesn't run, try banging on the motor or heater box under the dash. If that gets it going, that's a pretty good indicator to suspect the motor. You can also measure the voltage across the two motor wires. If you find any voltage there, the motor should be running, and that shows the computer and power module are working.
To check for that voltage, you're much better off with a test light than with a digital voltmeter. The motor speed is controlled by using "pulse-width modulation", (PWM). That means turning the 12 volts on and off hundreds of times per second. The ratio of on to off time is varied to vary the motor speed. The AVERAGE voltage changes but that isn't what a voltmeter sees. They take a reading, analyze it, then display it while they take the next reading. One reading might be when the voltage is 12 volts, and the next one might be when it is 0 volts. That makes the readings on the display bounce around all over. A test light will smooth that out and you'll see its brightness go up and down smoothly just like the motor speed is supposed to do.
There were some base heater systems at first that still used a four-position switch and no power module. Those had a resistor assembly bolted to the heater box. They have a thermal fuse built in. Those fuses burn open from too much current flow due to tight bearings in the motor, or too much heat in the heater box due to restricted air flow. They will never be intermittent, but you can have intermittent fan operation due to overheated terminals on the electrical connector. If you have that resistor assembly with four or five wires, check the connector body for signs of melting and the terminals for any that are blackened.
Ignition switches take a beating too on the part that turns on the accessory circuits, when you have that resistor assembly in the heater box. The biggest thing you can do to reduce the number of failures of heater fan switches and ignition switches is to avoid using the heater on the higher speeds. My daily driver yet, in the middle of Wisconsin, the road salt capital of the world, is an '88 Grand Caravan. Since it was new, the heater has been on the highest speed less than 20 minutes in those 25 years. Nothing in the entire heater system has ever needed to be replaced.
Tuesday, January 7th, 2014 AT 12:06 AM