With the recent warm weather I have noticed that while driving the A/C will suddenly quit blowing out of the dash and start blowing from the defroster. THis only happens for a about a minute and then switches back. I can expect this once every 4-5 minutes. Can I just pop up a access panel on the dash or do I have to tear everything apart to find the actuator and vaccum lines?

Is this a common problem and is the an easy fix?

 

Here are the testing sequences for this malady:

PINPOINT TEST A: IMPROPER/ERRATIC DIRECTION OF AIRFLOW FROM OUTLET
² A1: CHECK THE SYSTEM AIRFLOW
² A2: CHECK THE VACUUM SUPPLY HOSE
² A3: LEAK CHECK THE VACUUM SUPPLY HOSE
² A4: CHECK THE VACUUM CHECK VALVE
² A5: INSPECT THE VACUUM CHECK VALVE
² A6: LEAK TEST THE VACUUM CHECK VALVE
² A7: CHECK THE VACUUM RESERVOIR
² A8: CHECK THE SUPPLY HOSE
² A9: CHECK THE CONTROL ASSEMBLY
² A10: LEAK TEST THE CONTROL ASSEMBLY
² A11: CHECK THE SUPPLY HOSE FOR OBSTRUCTIONS
² A12: LEAK TEST THE JUMPER VACUUM HARNESS
² A13: REVIEW THE VEHICLE'S HISTORY
² A14: CHECK THE VACUUM HOSES TO DAMPER DOORS
² A15: CHECK THE VACUUM HARNESS
² A16: CHECK THE A/C VACUUM CIRCUIT
² A17: CHECK THE VACUUM CIRCUIT CONNECTIONS
² A18: CHECK THE VACUUM HOSE
² A19: CHECK THE VACUUM CONTROL MOTOR
² A20: CHECK THE VACUUM CONTROL MOTOR INSTALLATION

You can tell it can be quite involved as each of these 20 tests have multiple subtests and links depending upon the results. I copied these from the 2000 manual CD.... you should try to get one on Ebay or from Helm, because a vacuum problem can be a pain to trace.

 

I just returned from a trip which was interstate highways with hills. The truck would be fine on a/c and air would be out of panel until reaching a hill. At the hill the truck would pull the hill in overdrive and the panel air would go to defrost. When the truck shifted down out of overdrive on steep hills and th engine revved up the panel air would still be coming out of the panel. This was a constant over the 2 day trip. The truck does not seem to have enough vacuum in overdrive when pulling a hill. I have not talked to the dealership yet, but intend to. I have looked for a line off or something out of order, but haven't found anything.
Anyone else have this problem or has found the reason for this problem?

 

I have noticed that the big panel on the top of the passenger side of the dash will come off to reveal the lines. This is as far as I have gotten on this project.

I do know that I had a vacuum line break about a year ago running my front pumpkin causing loss of my 4wd. It appears to be the same type of vacuum line. I'm gonna check for breaks or cracks in the line running the actuator for the vent / defrost / floor and then look for a bad line that runs to the switch.

Good Luck