Unique AC problem in desert

[caughenbaugh]

Truck is a 1998 Tahoe LS 4Dr 5.7L. I have two common problems with my A/C but none of the common fixes I've found have had the least effect on them. First off the blower motor works fine on 1,2 and 4, but not on 3. Second problem is that while I get plenty of cold air on the floor and defroster settings very little comes through the dash vents.

What I've done so far: replaced blower motor relay, replaced blower motor, cleaned out heater core and check the blend door and actuator and both are working fine. There is also a rotten food smell when the A/C is on, but not when the heat is on. I also noticed the blower vibrates pretty good on the highest setting, enough that you can feel it in the floor if the engine is off.

I'm guessing the fan control not working on 3 may be the HVAC control module, but I have absolutely no idea about the dash vents.

 

[Monstar_est87]

http://www.tahoeyukonforum.com/forum/showthread.php?t=29095

As for the 3rd speed await 4 sunlit...

 

[SunlitComet]

if speed three is out the fan switch or an internal fault with the non-high speed power resistor could be present. test for system voltage on the light blue wire on back of fan switch connection on speed medium 2 which is the second highest of the whole setup. if full voltage then go after the resistor. if not then the fan switch. for the dash vent you need to inspect the motor and linkage (output mode)above and to the right of gas pedal it controls the outputs not the blend(temperature) door and motor. the fan switch can be pulled from the hvac control head for replacement. and lastly but important do you get defrost outlet on the windshield going at any speed? if so on some speeds but changes on it own back to floor when changing speed of blower but nothing else or ntothing but floor period then also go after the fan switch. when positioned to off it should kill the fan and default vent outputs to floor but the temp door will function normally. this allows ventilation air to move thru the cab while driving but still allow a temperature change.

 

[caughenbaugh]

Thanks guys. Wish I would have known about that while I had the blower assembly out yesterday. And the assembly by the gas pedal is the one I checked. I just referred to it as the blend door because that's what the post I saw on it called it.

As for the air flow everything works correctly and air comes out of the right vents it's just very weak out of the dash but normal out of the floor and defrost. That and the noise coming from the blower compartment leads me to believe there's debris in the channel described in the first post.

I'll check the voltage at the resistor when I pull the blower to clean the compartment. Thanks for the tip on being able to just replace the switch though as the complete unit was at least $130 everywhere I looked.