Mechanics

99 - BLOWER MOTOR NOT WORKING

1999 GMC Yukon

Heater problem
1999 GMC Yukon V8 Four Wheel Drive Automatic 177000 miles

Hi. Having issues with blower fan on 99 GMC Yukon. Hopefully you can help.

99 GMC Yukon SLT 4WD - Front and Rear AC

Issue: Drove the truck to destination and blower worked fine when truck was shut off. Got in it to leave and turned on heat on but no blower. Blower does not run in any of the four positions of the Control Switch. I have checked fuses but all are good. I originally suspected it was the blower motor but now am stumped!

Took the fan motor out and tested voltage. Here's what I saw which, to me, is strange.

There are four " knotches" on the rotary switch corresponding to fan speed. Right? Defined in the schematic as Lo, M1, M2 & Hi. Below is the voltage I see at each setting using a digital meter.

Off - Zero Volts
Low - 8v but climbs slowly to about 9v
M1 - 11.7v
M2 - 11.45v
Hi - Zero Volts
Note: When I turned to a lower setting from any higher setting the voltage drops dramatically and then slowly climbs back to the values from above.

I have now replaced both the Fan Motor, the High Blower Relay with no avail or change in voltages.

I removed the Blower Resistor and check for continuity across all of the various coils and the one resistor. All are good.

The only thing I have not replaced/tested is the HVAC Control Module which includes the rotary switch.

Other notes:

I have checked wiring harness around the heater housing for signs of overheating. No visible signs.
Change other rotary switch for air deflection and the appropriate doors open or close. I can hear them.
The Recirculation button works fine as I can also hear/see the door operate.
Rear AC fan works fine.

Questions:

1. I understand a common problem is that if the resistor goes bad, the fan may only operate on Hi. I am seeing the opposite. Should I be seeing continuity across all poles on the resistor?
2. Couldn't I hook the fan to a 12v power source and it should run? Haven't done it yet, afraid of frying the brushes but seemed like a basic test.
3. Should the HVAC Control Module be the next focus? Is it a pain in the butt to remove?
4. Am I delving too deep here and missing something obvious?

So I'm somewhat stumped. Wondered if you had any specific recommendations based on the above info.

Thank you in advance. Hope you had a great Thanksgiving.

RG
Avatar
Rgreen477
November 30, 2008.




Hi: If you are checking for continutity on the resister, it may appear fine, but under a load fail. I would replace the resister and see if that fixex it.



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