GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 2003 🢒 Century 🢒 HVAC 🢒 HVAC Systems - Automatic 🢒 Vacuum Control System Diagnostic

Step

Action

Yes

No

DEFINITION: Improper air delivery.

    • Vacuum applied to the wrong actuator
    • Vacuum leak

1

Were you sent here from Symptoms or another diagnostic table?

Go to Step 2

Go to Symptoms - HVAC Systems - Automatic

2
  1. Start the engine.
  2. Turn the blower motor fan ON and select an air outlet mode.

Does air flow out of the selected HVAC outlet ducts?

Go to Step 4

Go to Step 3

3

Did the customer concern mention that the air discharges out the correct ducts at first, but then changes during higher engine RPM?

Go to Step 8

Go to Step 4

4

With the engine running, cycle the HVAC controls through all the modes.

Does the air come out of the selected outlet ducts?

Go to Step 19

Go to Step 5

5

With the engine running, take a vacuum reading at the manifold vacuum port that supplies vacuum to the vacuum control assembly.

Is there engine vacuum?

Go to Step 7

Go to Step 6

6

Repair the no vacuum condition.

Is the repair complete?

Go to Step 20

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7

Take a vacuum reading at the vacuum supply line at the vacuum control assembly.

Is the vacuum reading the same as the engine vacuum?

Go to Step 12

Go to Step 9

8
  1. Disconnect the vacuum supply line from the vacuum control assembly.
  2. With the vacuum gauge connected to the vacuum supply line, re-start the engine and turn the engine OFF.

Does the system lose vacuum after turning OFF the engine?

Go to Step 9

Go to Step 12

9

Inspect for leaking or collapsed vacuum lines between the engine manifold vacuum port and the vacuum control assembly.

Are the vacuum lines leaking?

Go to Step 10

Go to Step 11

10

Repair or replace the collapsed or leaking vacuum line.

Is the repair complete?

Go to Step 20

--

11

Replace the vacuum reservoir tank. Refer to Vacuum Tank Replacement .

Is the repair complete?

Go to Step 20

--

12

Using an external vacuum source, apply vacuum to the appropriate vacuum line at the vacuum control assembly connector. Refer to HVAC Vacuum Schematics .

Does the actuator retract?

Go to Step 18

Go to Step 13

13

Inspect for leaking or collapsed vacuum lines between the actuator and the vacuum control assembly connector.

Are the vacuum lines leaking?

Go to Step 10

Go to Step 14

14
  1. Disconnect the vacuum line from the actuator.
  2. Apply an external vacuum source to the actuator.

Did the actuator retract?

Go to Step 15

Go to Step 17

15

With the actuator disconnected, inspect the door for binding.

Does the door move freely?

Go to Step 17

Go to Step 16

16

Repair the binding door. Refer to the appropriate repair.

    •  Mode Door Replacement
    •  Vent Door Replacement

Is the repair complete?

Go to Step 20

--

17

Replace the vacuum actuator. Refer to the appropriate repair.

Is the repair complete?

Go to Step 20

--

18

Test the operation of the HVAC control module. Refer to Air Delivery Improper .

Is the HVAC control module operating properly?

Go to Step 20

Go to Step 19

19

Replace the vacuum control assembly. Refer to Vacuum Control Assembly Replacement .

Is the repair complete?

Go to Step 20

--

20

Cycle the HVAC controls through all modes to verify proper operation.

Did you find and correct the condition?

System OK

Go to Step 2