Defrost not blowing air to top vents

ID Status Date Year Make Model Transmission Type A/C Controls Public/Private
#15664 In Progress 1999 GMC SLT public

I have a 1999 GMC Sierra SLT truck and noticed ac is cold but not freezing cold and heat works but if you turn to defrost it doesn’t work. Also last year had a complete new ac system installed compressor,dryer,lines etc. What do you think it could be and what could fix this ? Thanks

Sparky

Is it the 1999  old or new body style?

I believe it’s a new body style

Sparky

There are several things that could be causing the problem of the mode actuator not moving to the defrost position. The most common would be a faulty mode door actuator. Others would be poor wiring connections and a faulty control panel. There are some tests that can be performed with test lights and multi meters but it is fairly difficult to do due to lack of room to access the harness connector at the actuator.

Where would it be and how to replace it? Is there a video ?
Thanks

Sparky

This article would be similar to your vehicle as long as it is a new body style 1999. That part number for the replacement part may be different.

https://sparkys-answers.com/2011/02/2003-chevrolet-silverado-changing-mode.html

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