1990 Silverado Defroster Puzzle
 

I have searched this forum and others and have not found a problem like the one I am experiencing. I can't get the defroster to actuate.

The actuator itself works well. I even tried replacing it with a brand new spare, and still the problem remains.

The actuator, when looking at it installed, has three positions; left (heater), center (vent), and right (defroster). My system is acting as if the linkage on the right is too long. It maxes out its travel before the actuator can fully rotate to the right.

The linkages themselves are designed to extend from their resting states, not compress. In the attached photo, I have marked the linkage with a red arrow.

This is a tough problem to explain in writing. I am hoping that a reader will have experienced it, had the good fortune to repair it, and can understand what it is I am trying to explain.

Thanks in advance for taking the time to work on this challenge......

Defroster Puzzle Fix
 

I fixed the problem by drilling a hole in the actuator linkage. The photo attached shows the approximate placement of the hole (the little red dot).

To determine the proper placement, I removed the top of the linkage from the damper arm, switched to Defrost mode, made sure the damper was in its closed state, and marked the linkage at the point where the damper arm was at its apex.

The system works like a charm. I have no idea why this had to be done, though. Truly, it breaks all the rules of good engineering. But it's kept me from having to disassemble the dash in order to fix it.

Good old Yankee ingenuity.....

Thanks to all who read my post and wanted to help but couldn't figure out what I was talking about. :-)