A/C Problem

After reading many threads re vacuum lines,canisters,controls,dampers I still have the same problem--fan returns to defrost outlets when engine under load on my 2000 Astro. Have replaced vacuum lines and checked for leaks, have disassembled and greased control,checked damper actuators for leaks,checked vacuum canister?. Anybody have any further suggestions?

There is a check valve in the vacuum line between the source line and the "T" fitting that splits the vacuum between the reservoir (plastic ball) and HVAC controls. If the check valve is not functioning, the symptoms you describe will occur.

Is it possible that someone removed this check valve as when I replaced the 1/8" plastic line it went direct from the left side of the intake to the "T" fitting? If so could that be my problem-missing check valve?

The black and white "T" fitting contains the check valve. the hard nylon tubing connects to the black side, and the reservoir and controls are connected to the white side. The check is built into the "T" to prevent vacuum leakage back to the intake. You should be able to check this by removing either of the rubber lines from this to see if there is vacuum present.

http://72.19.213.157/files/NVanVacSourceID.jpg

Just to be sure I have replaced this "T" with check valve but it did not appear to be stuck. Any further help greatly appreciated.

Connect a vacuum gauge downstream of the check valve. Apply vacuum or run the engine, then shut off the vacuum source (or engine). Vacuum should remain downstream of the check valve almost indefinitely. If not, isolate the vacuum reservoir from the system and test again. Then isolate the control line into the passenger compartment, reconnect the reservoir, and test again. One of them should revela leakage.