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Vacuum Control System

WARNING: This page is about a different car, the 2002 GMC Safari and 2002 Chevrolet Astro. However, it is still accessible from the selected car via links, so may be relevant.
NOTE: Definition: Improper air delivery.
  1. Were you sent here from SYMPTOM DIAGNOSIS  or another diagnostic table? If so, go to next step. If not, see TROUBLE SHOOTING .
  2. Start the engine. Turn the blower motor fan on and select an air outlet mode. Does air flow out of the selected HVAC outlet ducts? If so, go to step  4. If not, go to next step.
  3. Did the customer concern mention that the air discharges out the correct ducts at first, but then changes during higher engine RPM? If so, go to step  8. If not, go to next step.
  4. With the engine running, cycle the HVAC controls through all the modes. Does the air come out of the selected outlet ducts? If so, go to step  19. If not, go to next step.
  5. With the engine running, take a vacuum reading at the manifold vacuum port that supplies vacuum to the HVAC vacuum control assembly. Is there engine vacuum? If so, go to step  7. If not, go to next step.
  6. Repair the no vacuum condition. After repairs, go to step  19.
  7. Take a vacuum reading at the vacuum supply line at the HVAC vacuum control assembly. Is the vacuum reading the same as the engine vacuum? If so, go to step  12. If not, go to step  9.
  8. Disconnect the vacuum supply line from the HVAC vacuum control assembly. With the vacuum gauge connected to the vacuum supply line, restart the engine and turn the engine off. Does the system lose vacuum after turning the engine off? If so, go to next step. If not, go to step  12.
  9. Inspect for leaking or collapsed vacuum lines between the engine manifold vacuum port and the HVAC vacuum control assembly. Are the vacuum lines leaking? If so, go to next step. If not, go to step  11.
  10. Repair or replace the collapsed or leaking vacuum line. After repairs, go to step  19.
  11. Replace the vacuum reservoir tank. See VACUUM STORAGE TANK  under REMOVAL & INSTALLATION. After repairs, go to step  19.
  12. Using an external vacuum source, apply vacuum to the appropriate vacuum line at the connector to the HVAC vacuum control assembly. See Figure. Does the actuator retract? If so, go to step  18. If not, go to next step.
  13. Inspect for leaking or collapsed vacuum lines between the actuator and the connector to the HVAC vacuum control assembly. Are the vacuum lines leaking? If so, go to step  10. If not, go to next step.
  14. Disconnect the vacuum line from the actuator. Apply an external vacuum source to the actuator. Did the actuator retract? If so, go to next step. If not, go to step  17.
  15. With the actuator disconnected, inspect the door for binding. Does the door move freely? If so, go to step  17. If not, go to next step.
  16. Repair the binding door. See appropriate door procedure in REMOVAL & INSTALLATION . After repairs, go to step  19.
  17. Replace the suspect vacuum actuator. See appropriate actuator procedure under REMOVAL & INSTALLATION . After repairs, go to step  19.
  18. Replace the HVAC vacuum control assembly. See HVAC CONTROL ASSEMBLY  under REMOVAL & INSTALLATION
  19. Cycle the HVAC controls through all modes to verify proper operation. Did you find and correct the condition? If so, repairs are complete. If not, go to step  2.
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