GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2001 🢒 Jimmy - 4WD 🢒 HVAC 🢒 HVAC Systems - Automatic 🢒 DTC B0408

Circuit Description

The HVAC control module receives a position signal from the air temperature actuator. The air temperature actuator position sensor is a potentiometer that is tied to a 5-V reference source. The variable output from the potentiometer feeds back a position signal to a pull-up resistor which is located in the HVAC control module. This signal determines the necessary drive signal to the actuator motor which provides the proper air temperature door position. This value is stored in the keep alive memory (KAM) portion of the HVAC control module. When the air temperature door is in a full hot position the scan tool will read above 190 counts. When the air temperature door is in full cold position the scan tool will read below 48 counts. Normal operating range is 46 counts to 194 counts.

Conditions for Running the DTC

The ignition is in the ON position.

Conditions for Setting the DTC

The HVAC control module detects this condition when the ignition switch is in the ON position and one or more of the following conditions exists:

    • Ignition 3 voltage is open.
    • The air temperature actuator signal circuit is shorted to voltage, ground, or open.
    • The 5-V air temperature door control circuit is shorted to voltage, ground, or open.
    • The 5-V reference is shorted to voltage, ground, or open.
    • Low reference is open.

The HVAC control module does not sense the expected change in the temperature door position when the air temperature valve sensor and motor is commanded to move the temperature valve.

Action Taken When the DTC Sets

The HVAC control module drives the temperature valve to the full COLD position when the system is in one of the following modes:

    • PANEL
    • BI-LEVEL
    • AUTO PANEL/AUTO BI-LEVEL

The HVAC control module drives the temperature valve to full HEAT position when the system is in one of the following modes:

    • LOWER
    • DEFOG
    • DEFROST
    • AUTO LOWER

Conditions for Clearing the DTC
    • The DTC becomes history when the HVAC control module no longer detects a failure.
    • The history DTC clears after 50 fault-free ignition cycles.
    • The DTC can be cleared with a scan tool.

Diagnostic Aids
    • Normal operating range is between 40-200 counts. If out of this range check for binding, misalignment, or broken air temperature door.
    • If condition is not present, refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. Test for other DTC that are current DTC that could also set this DTC but would need to be diagnosed first.

    2.

  2. Tests for the proper operation of the circuit to insure that commanded and actual positions are within specifications.

    3.

  3. Tests for the proper operation of the circuit when door is commanded to move.

Step

Action

Values

Yes

No

Schematic Reference: HVAC Schematics

1

Did you perform the HVAC Diagnostic System Check?

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Go to Step 2

Go to Diagnostic System Check - HVAC Systems - Automatic

2

  1. Install the scan tool.
  2. Turn ON the ignition, with the engine OFF.
  3. With the scan tool, check DTC.

Does the scan tool indicate that DTC B0415 or B0416 set along with B0408?

--

Go to Diagnostic Trouble Code (DTC) List

Go to Step 3

3

  1. Install the scan tool.
  2. Turn ON the ignition, with the engine OFF.
  3. With the scan tool, observe the Air Mix Door Commanded data parameter and the Air Mix Door Actual data parameter in the Heater and Air Conditioning Data Display.

Does the scan tool indicate that the Air Mix Door Commanded data parameter is within the specified value of the Air Mix Door Actual data parameter?

0-3 counts

Go to Diagnostic Aids

Go to Step 4

4

  1. With the scan tool, command the air temperature actuator from HOT (Off) to COLD (On).
  2. With the scan tool, observe the Air Mix Door Actual parameter in the Heater and Air Conditioning Data Display.

Does the Air Mix Door Actual data parameter change at all?

--

Go to Step 10

Go to Step 5

5

Test the ignition 3 voltage circuit of the air temperature actuator for an open or a high resistance. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 15

Go to Step 6

6

Test the 5 volt reference circuit of the air temperature actuator for an open, high resistance, short to ground or short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 15

Go to Step 7

7

Test the air temperature door control circuit of the air temperature actuator for an open, high resistance, short to ground or short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 15

Go to Step 8

8

Test the air temperature actuator ground for open, or high resistance. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 15

Go to Step 9

9

Important:: Ensure that the air temperature actuator connector and the HVAC control module connector are connected during this test in order to avoid misdiagnosis.

  1. Turn ON the ignition, with the engine OFF.
  2. With the scan tool, observe the Air Mix Door Actual data parameter in the Heater and Air Conditioning Data Display.
  3. Connect a 3 amp fused jumper wire between the air temperature door control circuit and the 5 volt reference circuit of the air temperature actuator (drives the actuator full COOL).
  4. Remove and reconnect the jumper wire between the air temperature door control circuit and the ground circuit of the air temperature actuator (drives the actuator full HOT).

Does the air temperature actuator drive shaft rotate and do the counts change?

--

Go to Step 12

Go to Step 10

10

Inspect the air temperature door and the air temperature actuator for the following conditions:

    • A misaligned air temperature actuator. Refer to Air Temperature Actuator Replacement .
    • Broken or binding linkages or air temperature door
    • An obstruction that prevents the air temperature door from operating within it's full range of motion
    • Missing seals to the air temperature door
    • Misaligned seals to the air temperature door

Did you find and correct the condition?

--

Go to Step 15

Go to Step 11

11

Inspect for poor connections at the harness connector of the air temperature actuator. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 15

Go to Step 13

12

Inspect for poor connections at the harness connector of the HVAC control module. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 15

Go to Step 14

13

Important:: Perform the recalibration procedure for the air temperature actuator.

Replace the air temperature actuator. Refer to Air Temperature Actuator Replacement .

Did you complete the replacement?

--

Go to Step 15

--

14

Important:: Perform the recalibration procedure for the HVAC control module.

Replace the HVAC control module. Refer to HVAC Control Module Replacement .

Did you complete the replacement?

--

Go to Step 15

--

15
  1. Use the scan tool in order to clear the DTCs.
  2. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC reset?

--

Go to Step 2

System OK