Vacuum Operated Duct Test

The purpose of the duct system is to provide warm air from the heat shroud to the air intake system. When vehicle is warm it maintains a temperature between 70°F (21°C) and 105°F (40°C) by mixing warm and cool air. Surrounding air temperature should be above 60°F (15°C) when testing system operation.

1. Block wheels and apply parking brake. Inspect heat riser tube, repair or replace as necessary. Remove components required to observe operation of duct door. Door should be open to fresh air at this time. Check for sticking or binding, repair as necessary.
2. Check vacuum source and condition of vacuum hoses at bimetal sensor, Cold Weather Modulator (CWM) and vacuum motor. Start engine. If door closes to fresh air (open to heated air from manifold), go on to step 3). If not, place finger over bleed hole in bimetal sensor. Duct door should quickly close to fresh air (open to manifold heated air). If not, replace vacuum motor and retest.
3. If vehicle is equipped with a delay valve before vacuum motor, replace with straight through vacuum hose for this test. With engine off, cool bimetal sensor and CWM with refrigerant for 20 seconds.
   CAUTION: Do not cool sensor with refrigerant while engine is running. If drawn into intake system, poisonous phosgene gas will be exhausted into work area. Perform test in well ventilated area.
4. Restart engine. Door should fully close to fresh air (open to manifold heated air). If not, replace sensor. Stop engine, cool CWM and sensor. Restart and run engine briefly (under 15 seconds). Door should close to fresh air (open to manifold heated air). See the AIR CONTROL DOOR CLOSING VACUUM table.
5. Shut off engine and observe door. On vehicles without CWM, the door will slowly open to fresh air (closed to manifold heated air) in 10 to 30 seconds. Vehicles with CWM, the door should not open to fresh air (closed to manifold heated air) for at least 2 minutes. If less replace CWM. Cool CWM and sensor, then retest.