Blower motor stays on

[ohiahlberg]

Hi,
Sure can use your help. Anyone have this problem or can direct me. My blower / fan stays on. everything else works fine. temp control, heater / ac..
I love the 300m's but this is getting anoying. Thank You!!

[tinmanwpk]

Maybe try to recalibrate your ATC (Automatic Temperature Control unit - your heater/air cond control unit).

start car, leave it in park
set unit at 75 degrees
press at the same time the Floor, Mix (vent/defrost) and Defrost buttons
Your ATC will blink and throw a code number.
Release the buttons
1 or more of 16 codes will be there, but only one at a time between 23-51. To scroll thru press Panel Mode button on the ATC.

[odojak]

thanks

[tinmanwpk]

You're welcome. Hope it is the solution.

[kam300m]

Ok My brother had a 1999 Chrysler LHS that has the same electronic temp system in it and it always blew on high you could never turn it down, I did the thing with resetting it to get the codes and it told me code 36 and 26 which means that the ATC thermomic sensor is shot, well I took a hunch and replaced the blower motor power module. I replaced it and now it will turn off, it was only two bolts on the lower right of the passengers side. It is right by the blower motor and it had two plugins going into it! They are kinda hard to replace because there is no room all you need though is an 8mm socket! good luck!

[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).

[cocks12]

I think before it lands to more trouble. you can get the blower or fan replaced with a new one and check if it is working fine. if so the blower that you were using might have gone faulty.

[Bill Grissom]

Originally Posted by cocks12

I think before it lands to more trouble. you can get the blower or fan replaced with a new one and check if it is working fine. if so the blower that you were using might have gone faulty.

cocks12,
I never replaced the blower, but guess a multi-day job and >$100 parts, based on other cars. I doubt the blower is bad since the new "round pin" controller has been working 6 months now and that fixed the problem for others. 5 min fix, just spend time finding a good price.

[brian_qwerty]

Bill Grissom - Great post. After a few months of fan full on, one of the transistor legs was blown off this week, and the fan stopped. Just spent 30min digging the transistor out of the ATC. No number on the transistor so I Googled it and found your post. I won’t bother with a transistor now, just a replacement round pin module.

[Bill Grissom]

Update, since I saw Brian's post. My blower control is still working fine >2 yrs later with the round-pin module.

[FFShawn]

I have the same issue with the fan blowing on high. I replaced the control module and it was fixed for one day, it started again to blow on high. It may have been a defective part but I was wondering if there is something else to check or test before installing another module. I was told that the blower may be bad or maybe the relay is sticking. How would I test these?

[Bill Grissom]

FFShawn,

You give few details. By "same issue" do you mean you have the ATC digital control? Did you install the improved fan control module with the round pins?

Who told you the blower might be bad? That sounds bizarre since a bad motor would run slower or not at all.

Who told you a relay might be sticking? I wouldn't think there is a relay controlling motor speed. I expect the way the ATC works is that the user interface sends a low-level speed signal to the control module, probably a simple digital code on a few wires and the module does all the "heavy lifting". If a mechanic is suggesting these things, you should find a smarter mechanic.

[FFShawn]

Yes, I do have the ATC, I thought this whole discussion was about that. Yes, I did buy the new improved module everybody is talking about in this discussion. No, a mechanic didn't tell me about these solutions but other forums have mentioned solutions. They said that the blower fan can go bad and draw more amperage causing the module to overload. The relay feeds supplies power to the fan. All I wanted is somebody to tell me how to test these to make sure they are working correctly before I install a new one. Can you give me this information?

[Bill Grissom]

FFShawn,
Always helps to fully state all facts. I see endless run-arounds on the sites I frequent from people assuming others know what they mean by some brief statement.

I think the fan will draw more than normal current only if it slows way down (bad bearings, dragging cage). When the motor spins fast (your problem), it generates "back emf" that reduces the current draw. You can read up on wikipedia and such. Hopefully, you can return the failed modules, though some stores don't warrant electrical parts.

The easiest way to test a relay is to just swap them and see if the problem follows the relay. You need a wiring diagram to locate the correct one unless they are labelled (engine bay box ones are). I think it unlikely that a relay is the problem. That is just a power feed to the whole climate system and maybe other things. Measure with a multimeter to see if you are getting the full battery voltage. In new cars, you usually must stick a pin in a wire to probe since the connectors are all sealed.

[FFShawn]

Thanks for the info. The store has a life time warranty on the module and they already ordered another to replace the bad one. Even though the fan runs on high it seems like it varies in speed, I think I'll replace it just to be sure. I would hate to keep replacing the module.