I spent some time repairing the a/c and heater control vacuum lines on my 86 D150. The vacuum control head thats behind the pushbuttons was broken in pieces and the blower switch was broken off at the bracket(like someone punched it). I got some used parts and put it back together. It was stuck in defrost mode no matter what button I pressed. I thought my new(used) control head was bad. After checking further I found mouse damage on the vacuum lines. That took a while to correct. I found if you have one small vacuum leak on any of the vacuum lines that are supposed to have vacuum applied that it defaults to the defrost setting. Since I didn't have a service manual I had to test everything and come up with a diagram for the vacuum lines and the switch functions. I used a hand vacuum pump to pull vacuum on every hose connection to make sure it held vacuum. If it didn't I traced the hose until I found the leak. I thought I would share it with this group as someone may find it helpful. I'm going to leave this in my glove box in case I have any problems in the future.


Vacuum connector at control head

 

I have an 86 W250 and my defrost doesn't seem to work at all. I get front vents only when on the other settings. No cold air from AC, so I'm guessing it needs to be charged or converted to newer style.

Not sure where I should even begin.