If there ends up being other pages you see on the index that you need let me know, but I think this will have you mostly covered. Hope it helps!

 

I've been working on my 86 D150 the last few days with similiar issues. The first thing to check is to make sure the diaphram in the water flow control valve isn't blown which will cause a vacuum leak and prevent full vacuum from getting to the rest of the vacuum actuators. It's the one in the heater hose under the hood. If it's good then you're going to need to check to see if you're getting full vacuum at the control head. You'll have to pull the cover off for the gauge cluster to get it out. You might as well pull the radio out too. You'll have more room to access the vacuum lines if you do. You'll need to do a visual inspection first to check for mouse damage, broken vacuum lines or one of the soft rubber hoses has just pulled apart from the hard plastic lines(mine had all of the above). You can also pull the speaker grill and speaker out of the top of the dash then pull the glove box down to look at the actuators on top of the heater box. Lastly, when you put the control head back in the dash, make sure you route the temperature control cable properly so it doesn't bind up. If you don't it will cause the cable to bind up and the wire inside will kink at the control head. Found that out the hard way. You can test the vacuum actuators individually at the control head after you disconnect the vacuum harness. You'll need to make a jumper tube and stick it in the holes in the vacuum plug. I made one out of a straw that comes with an aerosol can of brake parts cleaner. Start the engine and find the line that has vacuum all the time, then bend the tube in a horseshoe shape and jumper between the vacuum source and the other vacuum lines. You should see the vacuum actuators move each time you do.