Have a 96 f150 302 5 speed manual 50000 mi. When under wot and a/c on max. A/c will go from vents to floor and stay there until I let off. Takes several seconds to shift to the floor and several seconds to return to vents when I let up. Plumbed in a hand vacuum pump in the engine compartment at the source of vacuum for this heater a/c controls. Everything works fine and shifts from vents to defrost and floor as it should until I go to max A/C then i loose vacuum and cannot pump it up. Is there a fresh air/ recirculating actuator that I need to find or is it a bad control on the dash? When I apply vacuum to the actuator under the hood with a white hose, it moves and does hold vacuum. All vacuum lines are in very good condition, Also tested one acuator under the dash, blue line. It holds vacuum also. Am I missing something? Thanks

 

Sounds like the typical failure is somewhere in the white hose going to the actuator under the hood. Not to say you can't have an other than typical failure, such as a vacuum reservoir leak or a check valve not checking.

 

so looking at the vacuum diagram its the white hose, the actuator which I tested ot the control? Can I find the other end of the white hose cap it and vacuum test it?

 

Haven't done it myself, but see no reason why you couldn't. The white hose should go to the HVAC control panel in the dash. I suppose it's possible that there is a problem with the control panel, or the hose connection fell off - causing the symptoms you're experiencing.

Many users have simply ran standard rubber vacuum line as a replacement. I'm not so ambitious - I ty-wrap the actuator to recirculate in the summer months leaving the controls on regular A/C, and undo the ty-wrap for normal airflow in the winter.

 

I would just as soon diagnois and repair it. Truck has a lot of life left. Eliminate the vacuum leak. So i guess I will try to isolate the white line.

 

Tested white line. It did not hold vacuum. Closer inspection revealed a couple of cracks in the tubing right where it bends and passes over the blower motor under the hood. About 10 inches from the actuator. Cut it and spliced in a repair. Problem solved. Very hard to see the one or two cracks. otherwise appeared new. Thanks for the help.