Plug that yellow line and see if you get back control of desired airflow? It sounds like your temperate blend door actuator is bad or the shaft on that door is stuck, these can rust and then get hung up. If airflow stays on defrost, or goes to defrost while driving under load(low manifold vac) your reserve tank is cracked, or the supply line/check valve is broken someplace, usually this is a black line, but Ford does use white too! This line comes in from the engine to the control head. Here's the diagnostic test, just in case your had bad info on how to do it.
Run the self diagnostic test on it by turning the key to on, then press off and floor at the same time, then within 2 secs, press auto, If there is a problem a trouble code will appear on the display...To exit self test, press the BLUE, cooler button this also retains error codes, To erase codes and exit, press defrost, choose one of these before you power down the system... Test with engine at 120 degrees or more, and cabin between 40-90 degrees.Post any codes recorded here and I let you know what's defective...
Electronic Blend Door Actuator The electronic blend door (temperature blend door) is operated by a reversible electric motor (actuator) and potentiometer, in response to input signals from EATC module. The actuator is located on top of evaporator core housing. Temperature blend door actuator contains a reversible electric motor and a potentiometer. The potentiometer wiper is connected to the actuator output shaft and moves with the output shaft to indicate temperature blend door position. Vacuum Actuators Three vacuum actuators control movement of air distribution doors. The floor/panel door actuator and panel/defrost door actuator are mounted on the heater air plenum chamber. The fresh/recirculated air door actuator is located in the air duct and controls the fresh/recirculated air door.
Sunday, October 4th, 2009 AT 12:54 PM