Blower Motor Resistor HELP

jafo2424

Original poster
Member
Dec 6, 2011
22
I have an '03 with the Automatic Climate control. HVAC Blower just completely quit. (All speeds). Rear blower/controls still work. Searched around it did some fixes. Replaced the #35 fuse. (still didnt work) Pulled the blower and benched tested and it worked. Ordered a replacement MOSFET and replaced it in the Module. (Pic of the old and new below)

Just hooked everything back up. Now it works only the highest speed. (which I believe is off the #35 fuse correct?)

So my question is what next? Is the resistor Still Bad ???? Since its working on #5 is their any chance it could be the head unit ??

As always, thanks for all the advise!
 

Attachments

  • FET.jpg
    FET.jpg
    58.8 KB · Views: 12

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Just a bit of terminology. The resistor pack is only for the 5-speed manual HVAC system. The automatic system you have has a speed control module (with the FET inside) and more than 5 speeds. Only the resistor pack design uses a different fuse for speed 5 compared to 1-4.

So in your case, if it runs at all, the fuse is good and most of the wiring is good, and it's only the speed control function that has a problem. The speed control is handled by a control wire from the head unit to the speed control module that ends up on the "gate" of the FET. If you had an oscilloscope or used your DMM on the AC function, you could see what is happening on the gate of the FET.

Think of the FET as a relay that can turn on and off very quickly. The control wire turns on and off, and the PROPORTION of on to off time is what delivers more or less energy to the blower. The blower is actually fed power either all on or all off, but faster than the blower can react so it appears to be a smooth speed variation. Like a lamp dimmer. They are typically either all on or all off, but at a switching speed faster than the filament can change intensity or your eye can detect.

My best bet is that the FET you have is bad, and its DRAIN and SOURCE are conducting when they shouldn't be, like a shorted relay. Or other circuits in the speed control module that connect the head unit to the FET GATE are bad. If you have a scope or DMM, it's a few minute job. If not, I'd buy a new speed control module. Statistically, it's almost never the head unit. If you had a local buddy to swap with, there's another troubleshooting tactic.
 

jafo2424

Original poster
Member
Dec 6, 2011
22
Thanks Roadie,

So just so I am correct, is this the Speed Control Module you are referring to ??? (Photo)

Also, I have seen AC Delco Part #15-81728 or AC Delco #15-80567 both listed as a fit. Can I use either one??
 

Attachments

  • 15-80567.jpg
    15-80567.jpg
    8.6 KB · Views: 70

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
That's the unit. A finned heat sink in the airflow, not a bunch of cheap looking resistors. And a lower number of wires in the connector harness. Both of those Delco part numbers will work, along with MANY GM part number equivalents. Mostly available for $50-70 in the wild.
 

Forum Statistics

Threads
23,236
Posts
637,132
Members
18,404
Latest member
Zoie

Members Online

No members online now.