I re-soldered a climate control unit for my 83 SD, and magically, the center vent worked. It had never worked before since I got the car. Now my climate control works very well, including variable auto fan speed, center vent etc.

I use a suction gun to de-solder all the joints first, remove as much old solder as I can. When re-soldering the controller board, I also added some wire ends to re-enforce the "vertical" joints between the main board and the two small vertical external connector boards.

You only need to open up the back cover plastic piece to do the re-soldering, no need to disassemble everything.

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Shane83SD