Vacuum Control System

Vacuum control is used to operate the mode-air doors within the HVAC housing. Testing the operation of the heating-only and heating-A/C system mode control will determine if the vacuum and mechanical controls are functioning. However, it is possible that a vacuum control system that operates correctly at engine idle (high engine vacuum) may not function properly at high engine speeds or loads (low engine vacuum). This can be caused by leaks within the vacuum system, or a faulty vacuum check valve.

A vacuum system test will help to identify the source of poor vacuum system performance or vacuum system leaks. Before starting this test, stop the engine and make certain that the problem is not a disconnected vacuum supply line at the engine intake manifold vacuum tap or at the vacuum reservoir.

Adjust Vacuum Test Bleed Valve:

Use an adjustable vacuum test set (Special Tool C-3707-B) and a suitable vacuum pump to test the HVAC 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 verify that the vacuum reading returns to the exact 27 kPa (8 in.Hg.) setting. Otherwise, a false reading will be obtained during testing.

VACUUM CHECK VALVE
1. Remove the vacuum check valve. The valve is located in the vacuum supply line (black) at the HVAC system vacuum tee.
2. Connect the test set vacuum supply hose to the A/C-heater control side of the vacuum check 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 3. If not OK, replace the faulty vacuum check valve.
3. Connect the test set vacuum supply hose to the engine vacuum side of the vacuum check valve. When connected to this side of the check valve, vacuum should flow through the check valve without restriction. If not OK, replace the faulty vacuum check valve.

A/C HEATER CONTROLS
1. Connect the test set vacuum probe to the HVAC vacuum supply (black) line at the tee in the engine compartment. Position the test set gauge so that it can be viewed from the passenger compartment.
2. Place the A/C-heater mode control in each mode position, one position at a time, and pause after each selection. The test set gauge should return to the 27 kPa (8 in.Hg.) setting shortly after each selection is made. If not OK, a component or vacuum line in the vacuum circuit of the selected mode has a leak. See the procedure in Locating Vacuum Leaks.

LOCATING VACUUM LEAKS

WARNING: ON VEHICLES EQUIPPED WITH AIRBAGS, DISABLE THE AIRBAG SYSTEM BEFORE ATTEMPTING ANY STEERING WHEEL, STEERING COLUMN, OR INSTRUMENT PANEL COMPONENT DIAGNOSIS OR SERVICE. DISCONNECT AND ISOLATE THE BATTERY NEGATIVE (GROUND) CABLE, THEN WAIT TWO MINUTES FOR THE AIRBAG SYSTEM CAPACITOR TO DISCHARGE BEFORE PERFORMING FURTHER DIAGNOSIS OR SERVICE. THIS IS THE ONLY SURE WAY TO DISABLE THE AIR-BAG SYSTEM. FAILURE TO TAKE THE PROPER PRECAUTIONS COULD RESULT IN AN ACCIDENTAL AIRBAG DEPLOYMENT AND POSSIBLE PERSONAL INJURY OR DEATH.

CAUTION: Do not use lubricant on the switch ports or in the holes of the connector, as lubricant will ruin the vacuum valve in the switch. A drop of clean water in the connector holes will help the connector slide onto the switch ports.

1. Disconnect the vacuum harness connector from the back of the A/C-heater control.
2. Connect the test set vacuum hose probe to each port in the HVAC housing half of the vacuum harness connector, one port at a time, and pause after each connection. The test set gauge should return to the 27 kPa (8 in.Hg.) setting shortly after each connection is made. If OK, replace the faulty A/C-heater control. If not OK, go to Step 3.

Vacuum Circuits Chart:

3. Determine the vacuum line color of the vacuum circuit that is leaking. To determine the vacuum line colors, refer to the Vacuum Circuits Chart.
4. Disconnect and plug the vacuum line from the component (fitting, actuator, valve, switch, or reservoir) on the other end of the leaking circuit. Instrument panel disassembly or removal may be necessary to gain access to some components. See the appropriate service procedures.
5. Connect the test set hose or probe to the open end of the leaking circuit. The test set gauge should return to the 27 kPa (8 in.Hg.) setting shortly after each connection is made. If OK, replace the faulty disconnected component. If not OK, go to Step 6.
6. To locate a leak in a vacuum line, leave one end of the line plugged and connect the test set hose or probe to the other end of the line. Run your fingers slowly along the line while watching the test set gauge. The vacuum reading will fluctuate when your fingers contact the source of the leak. To repair the vacuum line, cut out the leaking of the line. Then, insert the loose ends of the line into a suitable length of 3 millimeter (0.125 inch) inside diameter rubber hose.