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Discussion Starter #1 (Edited)
A fairly common problem with the heating, ventilation, air-conditioning system (HVAC) in our cars is that the fan won't run, or it runs full blast all the time. Fan speed is controlled by a power transistor module in series with the fan. It regulates current to the fan. If your HVAC fan runs full-speed all the time, the most likely problem is that the power MOSFET in the module is shorted. You'll either have to replace the module, or find a suitable substitute N-channel power MOSFET. This thead is aimed more toward the folks that have no fan; that is, the power transistor module is open. If you have:

  • no fan and no air flow from the HVAC vents
  • access to the helm manual
  • have already determined that the power transistor module is bad
  • have basic soldering tools and skills
  • want to save ~$50 by repairing the module
welcome :wavey: .

The helm manual has good info on troubleshooting the HVAC in chapter 21. You should be able to isolate the power transistor module as the problem. Nobody will blame you if you just go to the dealer and buy a new module. They aren't expensive (about $50), but if you have soldering skills, you can fix it for nearly free, and it's easier than going to the dealer.

The best way to access the module is through the panel under the glove box--the same panel you remove to get to the ECU (see pg 11-4 of the helm). The power transistor module is easy to spot, because it's black on a white blower duct assy. (see pg 21-5). It looks like this once you have it out:





This is what the power transistor module looks like with the black cover removed:



If your fan won't run, and you've determined that the power transistor module is bad, the most likely problem with the module is just a blown thermal cutoff. A thermal cutoff is like a fuse, except that it opens at a certain temperature instead of a certain current. The thermal cutoff is attached to the heatsink, and is in series with the gate of the power MOSFET. To test to see if the thermal cutoff is blown, use an ohmmeter and measure the resistance between the two small solder blobs on the right of the printed circuit board (shown circled in red):



If the resistance between the two circled points is open, then the thermal cutoff is blown. Lucky for you. :) All you have to do is bypass the thermal cutoff by shorting it. Just solder a short jumper between the two points.

If you prefer to replace, rather than bypass, the thermal cutoff, or you need access to the power MOSFET; de-solder the three big solder blobs in addition to the two circled, and the printed circuit board will lift from the heatsink. I was unable to cross-reference or google the part number of the N-channel power MOSFET, but I'd bet that any Radio Shack N-channel power MOSFET with the same case and pinout will work.

The thermal cutoff can be replaced if you like. Mouser.com has the part for less than 20 cents, but the shipping will run you more than $6.00. The part number is printed clearly on the cutoff. IMO, the thermal cutoff makes the module less reliable. It's just another part to fail. If the module should overheat, it's cheaper and easier for me to buy another Radio Shack power MOSFET than to order another thermal cutoff.

I bypassed the blown thermal cutoff in my own RSX about 3 years ago, and the power transistor module has worked reliably ever since.

I hope this post has been a help.
 

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Excellent writeup!!

This saved me a bunch of money today. The thermal cutoff blew on my car and I had a tough time in this cold weather for the past few days. I would have got as far as finding that the Power Transistor module was faulty (using the Helms) but would very likely bought a new one.

I vote for this post to be a sticky.
 

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I just took my 02 type s to the dealership for this problem. For those of you who would rather ,it took about 3hrs and only cost 117.00...........to let everyone know:)
 

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Just bringing this back from the dead......


curious, what is the part number of the Power Transistor Module? I think i might buy it, but i don't want to spend like 4 days explaining to the guy in the parts dept what i need.

Thanks in advance :D
 

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If you can solder, you spend $0 and 30 minutes of your time.
Im actually working on someone that is parting out their car, if they can send it to me it will be only fifteen dollars - shipped.

fifteen dollars is a good price for something i would probably just fuck up anyhow.
 
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Whoa... This is what I was looking for. I want to make sure I understand this completely. If my fan is blowing hot air 100% of the time (as in, need to keep the round vents closed in order to keep from cooking yourself), it is this transistor? I did a motor swap on the car last month and it never had this symptom before, but now the dash is like a blast furnace.
 

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Contrary to what erik loza states, my fan out of nowhere just wouldn't run. The lights for recirculate and air conditioning lit up, but when I turned up the fan to whatever position it wouldn't blow. The next day it worked fine. Just to ensure it won't happen again, most likely I should examine this transistor?
 

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Whoa... This is what I was looking for. I want to make sure I understand this completely. If my fan is blowing hot air 100% of the time (as in, need to keep the round vents closed in order to keep from cooking yourself), it is this transistor? I did a motor swap on the car last month and it never had this symptom before, but now the dash is like a blast furnace.
I honestly don't think so but it's not hard to diagnose this problem... do you have a voltmeter? Just take the transister out and test it. I had a bad transistor and I still had hot/cold air when driving fast depending on where I turn the nob, just that the blower didn't work whatsoever.
 

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I've isolated the problem and successfully bypassed the thermal cutoff.... I guess I should have asked this question before i did it, but does anyone know what the potential risks are of actually bypassing the thermal cutoff?
 

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Not sure honestly but congrats on getting it done, I gave up and just replaced mine with one from the dealer.
 

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Discussion Starter #20 (Edited)
I've isolated the problem and successfully bypassed the thermal cutoff.... I guess I should have asked this question before i did it, but does anyone know what the potential risks are of actually bypassing the thermal cutoff?
35mpg, I'm pretty sure that the thermal cutoff is there to save Acura money when they rebuild the modules. If the transistor module overheats and the thermal cutoff blows and saves the MOSFET from failing, that probably saves the rebuild shop $5 for the new MOSFET compared to 20 cents for the thermal cutoff.

The economics are different for the car owner, however. In the first place, the thermal cutoff is something that can fail on its own, so it makes the module less reliable. No doubt lots of modules are failing, without overheating, just because the thermal cutoff goes bad. In the 2nd place, even if the module overheats and destroys the MOSFET, it would be cheaper for me to go buy a $5 MOSFET at radio shack than it would to mail order a 20 cent thermal cutoff and pay $8 shipping from Mouser.com.

In any case, the first step to troubleshooting a fan that won't run is to measure across the thermal cutoff on the transistor module. If it's open, go no further. You've found the problem. How you decide to fix it is up to you.

My fix has been reliable for 3+ years.
 
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