GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2001 🢒 Catera 🢒 HVAC 🢒 HVAC Systems - Automatic 🢒 DTC 018

Circuit Description

The LH temperature valve actuator is a DC stepper motor by design. The stepper motor has four field windings that are controlled by the heater and A/C control. When the heater and A/C control needs to move the stepper motor, it energizes the proper field windings which rotates the armature a fixed amount in the proper direction. This fixed amount is called a step. Battery voltage for the stepper motor is supplied by the heater and A/C control when the ignition is in the ON position.

Attached to the stepper motor is a small gear. When the stepper motor is installed on the HVAC module, it meshes with a larger gear which is part of an assembly that drives the linkage of the air mix door.

Conditions for Setting the DTC

When the ignition is in the ON position, the heater and A/C control measures the current of all four coil wires of the stepper motor . The measured current is averaged by the controller and a "fail threshold" is calculated. After the output current is measured it is compared with the threshold value. If the current is too low then the output from the controller to the stepper motor is presumed to be open or shorted to ground. This step is repeated 3 times within a 10 second time frame.

Action Taken When the DTC Sets
    • Stores a DTC 18 in the heater and A/C control memory.
    • The fault detection process is repeated 3 times at a fixed interval of 70 ms to allow for debouncing.
        When the fault is set, the stepper motor can no longer be controlled.

Conditions for Clearing the DTC
    • The conditions for the fault are no longer present.
    • Using a scan tool.
    • A history DTC will clear after 20 consecutive ignition cycles if the conditions for the fault is no present.

Diagnostic Aids

If condition 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. Tests to ensure that the heater and A/C control has the ability to command the LH temperature valve actuator.

    2.

  2. Tests for the proper operation of the circuit in the high voltage range when the actuator is at rest.

    3.

  3. Tests for the proper operation of the circuit in the high voltage range when the actuator is at rest.

    4.

  4. Ensures the heater and A/C control has the ability to control the LH temperature valve actuator.

    5.

  5. Ensures the heater and A/C control has the ability to control the LH temperature valve actuator.

    6.

  6. Ensures the heater and A/C control has the ability to control the LH temperature valve actuator. Test lamp will be illuminated when connection made. Test lamp should increase intensity when commanded.

    7.

  7. Ensures the heater and A/C control has the ability to control the LH temperature valve actuator. Test lamp will be illuminated when connection made. Test lamp should increase intensity when commanded.

    8.

  8. Verifies the heater and A/C control is getting power from fuse CLAMP 15 Fuse 14.

Step

Action

Value(s)

Yes

No

Schematic Reference: HVAC Schematics

1

Did you perform the HVAC Diagnostic System Check?

--

Go to Step 2

Go to Diagnostic System Check - HVAC Systems - Automatic

2

  1. Install a scan tool.
  2. Turn ON the ignition, with the engine OFF.
  3. With a scan tool, command the Left Air Mix Door Mot. ON and OFF.

Does the scan tool indicate that the Left Air Mix Door Mot. parameter goes inactive within 10 seconds when the door is commanded?

--

Go to Step 3

Go to Step 10

3
  1. Turn OFF the ignition.
  2. Disconnect the LH temperature valve actuator.
  3. Turn ON the ignition, with the engine OFF.
  4. Measure the B+ power supply voltage at the LH temperature valve actuator connector to a good ground.

Does the voltage measure near the specified value?

B+

Go to Step 4

Go to Step 12

4

Important: Ensure the connector for the LH temperature valve actuator is connected during this step to avoid misdiagnosis.

  1. Turn OFF the ignition.
  2. Connect LH temperature valve actuator.
  3. Turn ON the ignition, with the engine OFF.
  4. Measure voltage on terminals 3 and 6 at the LH temperature valve actuator to a good ground.

Does the voltage measure near the specified value?

0.5 V

Go to Step 5

Go to Step 13

5

Important: Ensure the connector for the LH temperature valve actuator is connected during this step to avoid misdiagnosis.

Measure voltage on terminals 1 and 4 at the LH temperature valve actuator.

Does the voltage measure near the specified value?

B+

Go to Step 6

Go to Step 14

6

  1. Turn OFF the ignition.
  2. Disconnect the LH temperature valve actuator.
  3. Connect test lamp from terminal 2 to terminal 1 of the LH temperature valve actuator.
  4. Turn ON the ignition, with the engine OFF.
  5. With a scan tool, command the Left Air Mix Door Mot. ON and OFF.

Does the test lamp illuminate with each command?

--

Go to Step 7

Go to Step 15

7

  1. Turn OFF the ignition.
  2. Connect test lamp from terminal 5 to terminal 4 of the LH temperature valve actuator.
  3. Turn ON the ignition, with the engine OFF.
  4. With a scan tool, command the Left Air Mix Door Mot. ON and OFF.

Does the test lamp illuminate with each command?

--

Go to Step 8

Go to Step 16

8

  1. Turn OFF the ignition.
  2. Connect test lamp from terminal 2 to terminal 3 of the LH temperature valve actuator.
  3. Turn ON the ignition, with the engine OFF.
  4. With a scan tool, command the Left Air Mix Door Mot. ON and OFF.

Does the test lamp dim when commanded?

--

Go to Step 9

Go to Step 17

9

  1. Turn OFF the ignition.
  2. Connect test lamp from terminal 5 to terminal 6 of the LH temperature valve actuator.
  3. Turn ON the ignition, with the engine OFF.
  4. With a scan tool, command the Left Air Mix Door Mot. ON and OFF.

Does the test lamp dim when commanded?

--

Go to Step 20

Go to Step 18

10

  1. Turn OFF the ignition.
  2. Disconnect the heater and A/C control connector C1.
  3. Turn ON the ignition, with the engine OFF.
  4. Measure the voltage on the ignition positive voltage circuit at the heater and A/C control connector.

Does the voltage measure near specified value?

B+

Go to Step 11

Go to Step 19

11

Test the ground circuit of the heater and A/C control for a high resistance or an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 24

Go to Step 21

12

Test the B+ power supply circuits of the LH temperature valve actuator for an open, high resistance or short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 24

Go to Step 21

13

Test the circuits in terminals 3 and 6 of the LH temperature valve actuator for an open, high resistance or short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 24

Go to Step 21

14

Test the circuits in terminals 1 and 4 of the LH temperature valve actuator for an open, high resistance or short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 24

Go to Step 21

15

Test the circuit in terminal 1 of the LH temperature valve actuator for an open, high resistance or short to power. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 24

Go to Step 21

16

Test the circuit in terminal 4 of the LH temperature valve actuator for an open, high resistance or short to power. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 24

Go to Step 21

17

Test the circuit in terminal 3 of the LH temperature valve actuator for an open, high resistance or short to power. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

--

Go to Step 24

Go to Step 21

18

Test the circuit in terminal 6 of the LH temperature valve actuator for an open, high resistance or short to power. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 24

Go to Step 21

19

Repair the ignition positive voltage circuit to the heater and A/C control. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 24

Go to Step 21

20

Inspect for poor connections at the harness connector of the LH temperature valve 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 24

Go to Step 22

21

Inspect for poor connections at the harness connector of the heater and A/C control. 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 24

Go to Step 23

22

Replace the LH temperature valve actuator. Refer to Air Temperature Actuator Replacement - Left Side .

Did you complete the replacement?

--

Go to Step 24

--

23

Important: Perform the recalibration procedure for the heater and A/C control.

Replace the heater and A/C control. Refer to HVAC Control Module Replacement .

Did you complete the replacement?

--

Go to Step 24

--

24
  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