Checking Vacuum System

The vacuum system supplies engine vacuum, by way of the brake vacuum booster vacuum line, to operate the heating and ventilation control servos. The vacuum system is tested by checking for servo operation and vacuum at the servos when the control lever is moved to its different settings. The servos are reached by removing the center console and lower instrument panel tray(s) as described in 4.1 Instrument Cluster and Instrument Panel. Table b gives vacuum system response to control lever setting. The engine must be running for the tests.

No response from all servos at any control lever setting indicates that there is a problem either with vacuum supply, or with the control head. Check vacuum supply by removing the black hose from the rear of the control head with the engine running. No vacuum at the hose indicates a fault in the hose to the vacuum booster, or a faulty check valve. Vacuum at the hose when it is removed indicates a faulty control head.

If only one or two servos fail to respond to lever position, remove the hose from the malfunctioning servo(s) and check for vacuum. No vacuum indicates a faulty control head or vacuum hose. Vacuum indicates a faulty servo.

Table b. Heating and Ventilation Vacuum System Test