no defrost

my sons 98 s-10 will not blow out the defrost ducts. it blows out the other vents just fine, but when you flip the selector over to defrost nothing changes. any ideas on what to do?

Under the dash... find the red and the blue vacuum lines coming from the back of the mode switch. They both go to the heat/defrost valve actuator. If I remember right, it's on the driver side, close to the transmission tunnel, and above the gas pedal. With the engine at idle, the blue vacuum line should have vacuum when the mode switch is in the MAX AC, NORMAL, BI LEVEL and VENT positions. There should not be vacuum in any other mode switch position. If vacuum is correct, the door might be stuck. Try to move the actuator shaft in and out by hand to move the valve, (door). Post your results.

thanks i will give it a try and report back!!