Vacuum System Diagnosis

If the airflow through the vents changes modes while driving, or remains in any mode and is unable to change, diagnose the system using Vacuum Harness Diagnosis. Vacuum Harness Diagnosis

Start the engine and allow it to idle. Move the selector knob to each position and refer to the vacuum diagrams and operational charts for airflow, air door functioning, and vacuum circuits. If airflow is not coming out of the proper outlet at each selector knob position do the following:

Inspect
1. The hose connections at the vacuum actuators, control head valve, and vacuum tank.
2. The vacuum source circuit:
^ Install a vacuum tee and gage (with restrictor) at the vacuum tank outlet. Idle the engine and read the vacuum. At all selector knob positions, normal vacuum is equivalent to manifold vacuum.
^ If the vacuum is less than normal at all positions, remove the tee and connect the vacuum gage line to the tank and read the vacuum. If the vacuum is still low, the problem lies in the feed circuit to the tank or the tank itself. If the vacuum is now normal, the problem lies downstream.
^ Vacuum may be less than normal at some positions. If vacuum was low at one or several of the selector knob positions, a leak is indicated in these circuit(s).
^ Vacuum may be normal at all positions. If vacuum was normal and even at all positions, then the malfunction may be caused by improperly connected or plugged lines or malfunctioning vacuum valve(s).
3. Specific vacuum circuit check:
^ Place the selector knob in the malfunctioning position and check for vacuum at the pertinent vacuum actuators.
^ If vacuum exists at the actuator but the door does not move, then the actuator is defective or the door is mechanically bound.
^ If low or no vacuum exists at the actuator, determine whether the cause is the vacuum harness or the vacuum valve.
^ Check the vacuum harness first.
4. Vacuum harness circuit check:
^ Disconnect the vacuum harness at the control head.
^ The green line should show engine vacuum. If not, trace back through the connector to the vacuum tank.
^ To check any individual circuit, place the selector knob at the involved circuit position and check for vacuum.