Automatic HVAC blower module fix

[Dr.Fiero]

For most, just yank the module out and for $40-50 replace it. Done.

I just can't leave well enough alone though, and need to see WHY something failed! So, when the blower locked on full blast even with the key off (pull the J fuse it shuts off), out the module came.

Took it apart, and figured out it's a MOSFET in it that gets shorted out. You can still get them off AliExpress but the actual part was discontinued. It's an IRFBA1405P. Super TO220 case (looks like a typical TO220, just thicker). I think it was about $1.50 each in 10's. Pretty easy to change out.

Just poke through the sticker on the module and remove the Torx screw. Then you can pull off the heat sink. Pull back the outer edge of the foam seal. 2 or 3 thin flat blades through the now exposed gap, and work the 5 lock tabs until the cover starts to lift. the 3 tabs of the one connector, and the 2 large motor wires need to be wiggled through it. Don't mangle the 3 tabs of the connector.

The TO220 is now right there, and is a simple job to swap out. It's even surface mount pads, so no through hole mucking about. Reassemble is reverse, making sure to replace the now destroyed heat pad with your fav' heat sink compound.

Theory of operation is pretty simple. Power goes in through the 3 wire. Gets tapped to run the internal stuff, and passes right back out to the motor. The control circuit inside (normally under control of the BCM - until the FET shorts!) diddles the 1405P to pulse the ground which also comes in through the 3 pin, and out the other motor wire. The 3rd pin of the 3 pin is what connects the module to the BCM.

In the attached pic, you can see the old MOSFET just sitting there, and the two wires I attached that go to a switch so I can at least turn the fan on/off until the new module arrives in 4 days (it's getting major cold out and I need heat! Can't wait 4-6 weeks for the components to arrive). Note at the edge of the board it says "GMT800" - clearly the same module {board wise at least} used in the earlier big trucks.

Hope someone finds this useful.

 

[Mooseman]

An old post by @the roadie where he found another part number that is still available at Mouser and Digikey. It's IRF1404LPBF.

Another issue I had encountered with my 02 TB was a burnt out/overheated connector contact. I think it was the one to the motor but the one going into the module could also be affected.

 

[Mooseman]

Added this thread to the Technical FAQ since it's a shorter read and and still refers to the original thread.

 

[santon]

I repaired several of these modules so maybe somebody will find my experience useful. The original IRFBA1405P MOSFET is rated for 174A of continuous drain current at 10V and 25 degrees of C, and about 123A at 10V and 100 degrees of C. The IRF1404LPBF will also work here, although it is less robust (the corresponding rated current values are 162A and 115A). In any case, the rated current of these MOSFETs are much higher than the actual current draw of the blower motor - I believe it should be not more than 25-30A. So, the MOSFET could fail because of two possible reasons: 1) an excessive current draw by the aged, defective blower motor; 2) bad cooling of the MOSFET itself. The MOSFET gets quite hot and needs a good cooling. The original heatsink is rather small. Also, the MOSFET only touches the heatsink. The heatsink is mounted on the cover of the module and simply pressed against the MOSFET, and the cover is secured with just one screw. I wanted to improve the thermal contact of the MOSFET with the heatsink. If the IRF1404LPBF is used, this MOSFET has a regular TO-220 packaging with a small opening for mounting the MOSFET on a heatsink with a screw, however, it seems impossible to use a screw in this situation. So, I had some thin (1.5 mm thickness ) PCB material without the foil that I used for making my own circuit boards for different projects. This material is similar to the PCB of the module and can withstand high temperatures. I cut a small square of this material and used a Superglue to attach it to the PCB under the MOSFET. This provided the MOSFET some elevation from the board, shortened the distance between the MOSFET and the heatsink and improved the thermal contact with the heatsink. Of course, a thin layer of a thermal paste should be applied to the place of the thermal contact. I believe this method can improve the cooling of the MOSFET.
The design of these original GM modules is bad. I know people that keep replacing them all the time; although if the MOSFET fails repeatedly, I would consider looking at the blower motor. Bad blower motor could be responsible for the melted contacts of the connector too. BTW, I came across and disassembled a non-original module with a gray enclosure, probably similar to this. Surprisingly, the design of this module is completely different from the original one. This module utilize the MOSFET in TO-220 package, and the MOSFET is mounted directly on the heatsink with a screw.