Page updated on 04-24-2018


02-08-2010, 10:53 AM
I just purchased a 1999 Silverado and the heater and air conditioning control will not make the air blow through the main vents facing you and the passenger when you put the knob to that position.It only seems to blow through the defrost vents no matter what position you put it in.Do these complete heater and ac control units(with the 3 dials and the ac push button in it)go out often or does this sound like the problem?or I have read that sometimes the heater blend door actuator goes out?Is there a way to test either one of these so I dont buy something I dont need?Thanks for your help!

j cAT
02-08-2010, 01:38 PM
these come in two flavors automatic and manual...the auto push button temp blower type , its usually the hvac computer controller..I think around 120.oo for this part...normally when this computer controller fails the hvac will run with the engine / key off...

locate the controller and check all connections first...

02-08-2010, 07:06 PM
I am going to assume that your '99 is an Old Body Style (OBS). If it's a NBS (New Body Style) then there may be a Body Control Module involved. My '98 manual is for the OBS.

The issue you're having is with the mode door (vent, floor, defrost, & anything in between).

Four things in this system and they are not listed in the order of failure!!

1. Heater-A/C dash control panel.
2. Mode door motor. (Located on the drivers side end of the plenum above the tranny hump).
3. Mode door (there may be debris in the plenum that will not allow it to move).
4. Wiring.

02-08-2010, 08:05 PM
i would look at the mode door actuator personally, at the shop i work at i have replaced one to many of these. if a tooth gets stripped inside the motor it won't switch at all.

02-09-2010, 07:58 AM
i would look at the mode door actuator personally, at the shop i work at i have replaced one to many of these. if a tooth gets stripped inside the motor it won't switch at all.

When you say mode door actuator is that the same as the blend door actuator.I have the newer bodystyle 99 with the console.Where is this actuator located and do I have to remove the console to get at it.Thanks for the help guys!
Is this actuator a GM part only or do parts stores carry these now?

02-09-2010, 10:46 AM
I've had a gear tooth break in the fresh/recirc door motor on my Vibe. But because the door only swung 90, all I had to do was remove the gear (there's a big one that connects to the door) and reinsert it 180. Because the bad tooth was now in a spot that the mating gear would never contact, no more stuck door....and a free fix!

It will depend on where the mode door is but it is typically located behind the radio and A/C stack. The door motor will be on one side or the other, likely near the blower under the passenger footwell and behind the glovebox. You should be able to move the selector switch and observe the motor move (or attempt to move) the door. Good luck!

02-09-2010, 09:50 PM
no your blend door actuator controls what the temperature gets put out. your mode door actuator is completely separate. As far as changing the module, the program at my work shows that you have to take the complete dash out and then the heater core casing just to get to it. so yes you would have to remove your console, and all kinds of other stuff lol. i would copy and paste it but it would make this post extremely long lol

05-17-2012, 07:28 AM
I have a 2000 silverado z71 and am having the same problem. Is there any wiring diagram to where i can check and see if the correct voltage is at the mode motor. I only have constant voltage at 1 of the 5 pins on the motor. I took this apart and connected the motor directly to a battery and it worked both directions bypassing the circuit board.

Any input!!!!

06-30-2012, 02:08 PM
if you door works bypassing the board. i would see if you can find any cold solder joints on the board. if you can not find any then replace the board.

Add your comment to this topic!