no vent

[ksjms2000]

I have a 98 1500, when I have the knob for the defrost or floor on, it works fine, but when I put it to the vent position, it just blows to the floor, I didn't find any vacum lines, and nothing wrong with the control panel visually, any suggestions??

[Shortbus]

Yes the problem is the Mode valve actuator, it is an electric actuator that moves the doors in the HVAC box for air distribution. Very common problem on these I have sold many of them in the eight years I have done GM parts, depending on what dealer you call "yes you can only get them from the dealer" they will most likely ask you for the stamped number off of the defective actuator so make sure you find the bad actuator and take it out so you don't get the wrong one. I think they list a "temp valve actuator" a "mode valve actuator" and a "blend door actuator" This always confuses the parts guys. Give them the number you can test the actuators by removing them and feeding them direct power to see if the operate or not. Also sometimes when one goes bad you will hear a popping/grinding noise from the HVAC box over on the passenger side.

BTW there are no vacume hoses in these trucks so forget trying to track any of those down.

Good luck with it.

John

[ksjms2000]

Originally Posted by ksjms2000

I have a 98 1500, when I have the knob for the defrost or floor on, it works fine, but when I put it to the vent position, it just blows to the floor, I didn't find any vacum lines, and nothing wrong with the control panel visually, any suggestions??

john thanks for the info, but where is this valve located at??

[Shortbus]

Originally Posted by ksjms2000

john thanks for the info, but where is this valve located at??

Oh man these are every mechanics nightmare...

They are in the center of the vehicle behind the radio and Hvac control. You may have to remove the lower air distribution ducting to see it. I hate to tell you for sure as I am just a parts guy at a GM dealer but I do know the actuator operates the big door on the Drivers side center of the central Vertical ductwork between the floor and the dash panel vents. For a beginner I would say this is a day project even with a repair manual and the right tools. You can also count on breaking some trimming, and clips to boot.