[Bill Grissom]

This info relates to the electronically controlled blowers, which are part of the Automatic Temperature Control (ATC) units in some Chrysler vehicles, usually in later models and higher end cars. Mine is a 2002 Town & Country Limited AWD. If you have an ATC, you will have digital LED's for the temperature settings and the blower speed knob is small and has LED's that light at each rotary position. If you have 2 large knobs with a blue-to-red graphic, that is the analog system which simply switches a few resistors in series with the motor, as in 1960's cars.

The part that fails and causes the blower to stay on high is the motor control module. It is behind the glove box. The failed control has flat wedge-shaped heat sink fins. You can find posts with photos that lead you thru replacing it. Some call it a "resistor pack", but actually an electronic unit that regulates motor current with a MOSFET transistor. However, it serves a similar function to the "resistor pack" in the analog systems and is even in the same location in ~2001 up minivans. Note that ~2000 minivans have the resistor pack on the firewall (engine side) and ~1996 ones have it far up under the dash.

I got a replacement motor control on ebay. I recall $25, but could have been more. The new one was a different design, with round pin heat sink "fins". Replacement takes 5 min and it worked fine. Before that I had ordered replacement transistors, because I thought a simple fix and I am an engineer who must tinker. As most know, when transistors fail they usually fail shorted, which is why the motor stays on high.

I bought 2 HUF75343P3, which I recall was the original transistor and a HUF75345P3, which seemed better since 0.007 ohm ON resistance vs 0.009 ohm. I installed the later. After a few months running on the new control, I swapped in the repaired original to see if it would last thru a Sacramento summer. After ~4 months, it failed on a cool September day (blower stayed high again). I had cut slots across the fins to try to improve heat transfer and replaced the cabin air filter (still looks clean).

Going further to re-engineer the "flat fin" design would be quite an R&D effort. From what I read, MOSFET's don't usually throttle a current in a steady manner as a resistor, but rather are usually switched on and off at high speed (~40 kHz) to time-proportion the current. They can also be used to convert a DC source into variable frequency AC that drives a "brushless" motor, but unlikely here since usually more costly plus I expect requires multiple transistors. Either way, the transistor failure could involve much more than simply ON resistance, things like electrical resonance with the motor. Therefore, it would be hard to select a "better MOSFET" just from a spec table. In addition it turned out harder than expected to replace the transistor. You must uncrimp the heat sink and it was hard cleaning solder from the holes to get the new one in. While the transistor costs ~$1, shipping is high (why I bought several). I bought from Newark. Digikey or Mouser are others. Hopefully, Chrysler or their supplier did the R&D mentioned above and the new control will last much longer. We will know in a few years. If someone finds a simple circuit fix that lasts longer than mine, please let us know.

For those who need an executive summary:

1. Insure your replacement control module is the new design with round pins.

2. The original design (flat fins) doesn't fail just because the air filter clogs, as many have suggested.

3. Replacing the transistor in the original design doesn't cure the problem for long. It appears a fundamental design issue, either with the control circuit, the heat sink, or both. Perhaps as the blower motor ages it changes enough to fail the transistor (more current draw, sparking brushes, etc).