Hello. I'm wondering if someone can guide me on how to correct the blower not wanting to switch to defrost. It blows out very strong at the floor vents in front of the 4WD gear shift and at the same time very lightly blows out the dash vents. No matter what position I put the selector switch in, it will not divert to defrost or even fully go to the dash vents. It feels like the slider is engaging when I move it back and forth but nothing changes. Any ideas on how to fix this?

 

sounds like a vacuum issue. find the vacuum line entering into the cab of the truck. it should be by the engine somewhere and make sure its not cracked or unplugged . i believe the k5s defaulted to floor if theres no vacuum going to it

 

Another thing to check is to turn on the heater without the engine running then switch it to defrost and see if any air comes out of the defroster. If it blows out the defroster than you have a vacuum leak. Mine would blow out the defroster when it was not running but would stop as soon as the engine started. It was a bad vacuum connector.

 

Do you have a/c?

Get your head up under the dash and see if you can see what moves when you move the selector lever on the dash.

 

The truck has a\c but I don't have all of the brackets to reconnect the pump so it's just tied up over to the side for now. I was able to find the line coming out of the cab and got it to finally divert. It still isn't completely changing over but is much better than it was.

 

There is a little white plastic clip that breaks on the vent control door in front of the heater core. GM still manufacturers and sells the clip, but you may have to buy a bag of 5 depending on your dealer. Price was reasonable ($3/ea??). I have 2 spares (got a bag of 3 last month- dec2011).

Just unscrew the ducting and push it out of the way underneath your glove box, you'll see the broken clip once you move the black ducts. There is a nail/pin that is a pain to get out, but it will come out with channel locks or big pliers. 10 minute fix. Easy.

GM P/N 301-8715 (03018715), but you can just have the parts guy pull up the GM diagram of the heat system from a pre-87 chevy truck if that number doesn't work out. The pack I have has manufacturer date of Jan07, but mine came from a dealership instead of a GM warehouse.

 

broken vacuum hose