Vacuum Check Valve HVAC: Testing and Inspection

Fig.19 Adjust Vacuum Test Bleed Valve - Typical:

Heater A/C Vacuum System Diagnosis:

Use an adjustable vacuum test set (Special Tool C-3707-B) and a suitable vacuum pump to test the heater-A/C vacuum control system. With a finger placed over the end of the vacuum test hose probe, adjust the bleed valve on the test set gauge to obtain a vacuum of exactly 27 kPa (8 in. Hg.). Release and block the end of the probe several times to very that the vacuum reading returns to the exact 27 kPa (8 in. Hg.) setting. Otherwise, a false reading will be obtained during testing.

One-way Check Valve
1. Disconnect the heater-A/C vacuum supply (Black) tube in the engine compartment. This tube passes through an opening in the dash panel.
2. Remove the one-way vacuum check valve. The valve is located on the (Black) vacuum supply hose at the brake power booster.
3. Connect the test set vacuum supply hose to the heater side of the valve. When connected to this side of the check valve, no vacuum should pass and the test set gauge should return to the 27 kPa (8 in. Hg.) setting. If OK, go to step Step 4. If not OK, replace the faulty valve.
4. Connect the test set vacuum supply hose to the engine vacuum side of the valve. When connected to this side of the check valve, vacuum should flow through the valve without restriction. If not OK, replace the faulty valve.

Heater-A/C Controls
The operation of the Circulation door can be viewed by removing the blower motor and looking up into the unit inlet.
1. Connect the test set vacuum probe to the heater-A/C vacuum supply (Black) hose in the engine compartment. Position the test set gauge so that it can be viewed from the passenger compartment.
2. Start with the Mode control in the Panel position and the Circulation control in the Outside-air position.
3. Move the Circulation control to the Recirculation position (the Circulation door should move into the Recirculation position). After a short pause move the Mode control to the Defrost position (the Circulation door should move to the Outside-air position). The test gauge should return to the calibrated setting of 27 kPa (8 in. Hg.) after each selection is made. If the gauge cannot achieve the calibrated setting, the vacuum circuit or a component has a leak.
4. If the gauge achieves the calibrated setting but the door does not move, there is either a pinched vacuum line or a failed actuator.