89 Dakota 3.9 V6. All of a sudden when I accelerate, the AC goes from the dash vents to the floor vents and then back to the dash vents. I replaced a section of that thin plastic line coming from the fire wall to the heater valve again. It always worked in the past, but now the AC is stuck on the floor vents. ??

I think Dodge added, or installed a vacuum canister under the hood to solve this problem. You may want to look for that canister and be sure everything is OK with it. Its under the hood on the passenger side, tucked way down low, it looks like an old coffee can painted black and has a few hose connections on it. Gene

Pretty good odds are you have a vacuum leak or bad vacuum actuator

No coffee can on this truck. I do have one in my Charger so I knew exactly what to look for. Anyway, I reexamined the white plastic line and it was fine, but the black line right next to it was hanging by a thread. AC working fine now. When it gets cooler here in Florida, I'll trace those 2 lines back behind the fire wall and replace them both.

Check all the actuators and hoses under the dash. Any leakage is going to cause problems.