GM Service Manual Online
For 1990-2009 cars only

DTC P0568 6.6L and 7.8L

Circuit Description

The cruise control set/coast switch is an input to the engine control module (ECM). The ECM monitors the cruise control set/coast switch signal circuit in order to detect when the driver has requested to set the vehicle speed or to allow the vehicle to coast. The ECM detects a voltage signal on the cruise control set/coast switch signal circuit when the switch is applied. This DTC sets if the ECM detects that voltage is present on the cruise control set/coast switch signal circuit for longer than 90 seconds.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0568 (6.6L and 7.8L) Cruise Control Set Switch Circuit

Conditions for Running the DTC

    • The ignition is ON.
    • The cruise switch is ON.

Conditions for Setting the DTC

The ECM detects the set/coast switch is ON for longer than 90 seconds.

Action Taken When the DTC Sets

    • The ECM stores the DTC information into memory when the diagnostic runs and fails.
    • The malfunction indicator lamp (MIL) will not illuminate.
    • The ECM records the operating conditions at the time the diagnostic fails. The ECM stores this information in the Failure Records.
    • The cruise is disabled.

Conditions for Clearing the MIL/DTC

    • A last test failed, or the current DTC, clears when the diagnostic runs and does not fail.
    • A history DTC clears after 40 consecutive warm-up cycles, if failures are not reported by this or any other emission related diagnostic.
    • Use a scan tool in order to clear the MIL/DTC.

Diagnostic Aids

    • Inspect for a set/coast switch stuck in the engaged position or the signal circuit is shorted to voltage.
    • For an intermittent, refer to Testing for Intermittent Conditions and Poor Connections .

Test Description

The number below refers to the step number on the diagnostic table.

  1. This step determines if a condition is present.

Step

Action

Yes

No

Schematic Reference: Cruise Control Schematics

Connector End View Reference: Engine Control Module Connector End Views for the 6.6L (LLY) engine, or Engine Control Module Connector End Views for the 7.8L engine

1

Did you perform the Diagnostic System Check - Vehicle?

Go to Step 2

Go to Diagnostic System Check - Vehicle

2

  1. Install a scan tool.
  2. Turn ON the ignition, with the engine OFF.
  3. Turn the cruise On/Off control switch ON.
  4. With the scan tool, observe the Cruise Set/Coast Switch parameter in the powertrain control module (PCM) Engine 1 Data, data list.

Does the Cruise Set/Coast Switch parameter display On?

Go to Step 3

Go to Diagnostic Aids

3

  1. Turn OFF the ignition.
  2. Disconnect C1 of the engine control module (ECM).
  3. Turn ON the ignition, with the engine OFF.
  4. Connect a test lamp between the cruise control set/coast switch signal circuit and a good ground.

Does the test lamp illuminate?

Go to Step 4

Go to Step 6

4

Test the cruise control set/coast switch signal circuit for a short to voltage. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Step 9

Go to Step 5

5

Inspect for poor connections at the harness connector of the cruise control switch. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

Go to Step 9

Go to Step 7

6

Inspect for poor connections at the harness connector of the ECM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

Go to Step 9

Go to Step 8

7

Replace the cruise control switch. Refer to Cruise Control Switch Replacement .

Did you complete the replacement?

Go to Step 9

--

8

Replace the ECM. Refer to Control Module References for replacement, setup, and programming.

Did you complete the replacement?

Go to Step 9

--

9

  1. Use the scan tool in order to clear the DTCs.
  2. Operate the vehicle within the Conditions for Running the DTC.

Does the DTC reset?

Go to Step 2

System OK

DTC P0568 8.1L

Circuit Description

The cruise control set/coast switch is an input to the throttle actuator control (TAC) module. The TAC module uses the cruise control set/coast switch signal circuit in order to detect when the driver has requested to set the vehicle speed or to decelerate the vehicle speed. The TAC module detects a voltage signal on the cruise control set/coast switch signal circuit when the switch is applied. The TAC module sends a serial data signal to the powertrain control module (PCM) via the serial data circuit. This DTC sets if the PCM receives a serial data signal from the TAC module indicating that voltage is present on the cruise control set/coast switch signal circuit for longer than 90 seconds.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0568 (8.1L) Cruise Control Set Switch Circuit

Conditions for Running the DTC

    • The ignition is ON.
    • The cruise control switch is ON.

Conditions for Setting the DTC

The PCM receives a serial data signal from the TAC module indicating that the set/coast switch is ON for longer than 90 seconds.

Action Taken When the DTC Sets

    • The PCM stores the DTC information into memory when the diagnostic runs and fails.
    • The malfunction indicator lamp (MIL) will not illuminate.
    • The PCM records the operating conditions at the time the diagnostic fails. The PCM stores this information in the Failure Records.
    • The cruise control system is disabled.

Conditions for Clearing the MIL/DTC

    • A last test failed, or the current DTC, clears when the diagnostic runs and does not fail.
    • A history DTC clears after 40 consecutive warm-up cycles, if failures are not reported by this or any other emission related diagnostic.
    • Use a scan tool in order to clear the MIL/DTC.

Diagnostic Aids

    • Ensure that the set/coast switch is not stuck or sticking in the engaged position.
    • For an intermittent condition, refer to Testing for Intermittent Conditions and Poor Connections .

Test Description

The number below refers to the step number on the diagnostic table.

  1. This step determines if a condition is present.

Step

Action

Yes

No

Schematic Reference : Cruise Control Schematics

Connector End View Reference: Powertrain Control Module Connector End Views for the 8.1L engine

1

Did you perform the Diagnostic System Check - Vehicle?

Go to Step 2

Go to Diagnostic System Check - Vehicle

2

  1. Install a scan tool.
  2. Turn ON the ignition, with the engine OFF.
  3. Turn the cruise On/Off control switch ON.
  4. With the scan tool, observe the Cruise Set/Coast Switch parameter in the powertrain control module (PCM) data list.

Does the Cruise Set/Coast Switch parameter display On?

Go to Step 3

Go to Diagnostic Aids

3

  1. Turn OFF the ignition.
  2. Disconnect the cruise control switch.
  3. Turn ON the ignition, with the engine OFF.
  4. With the scan tool, observe the Cruise Set/Coast Switch parameter in the PCM data list.

Does the Cruise Set/Coast Switch parameter display On?

Go to Step 4

Go to Step 5

4

Test the cruise control set/coast switch signal circuit for a short to voltage. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Step 9

Go to Step 6

5

Inspect for poor connections at the harness connector of the cruise control switch. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

Go to Step 9

Go to Step 8

6

Inspect for poor connections at the harness connector of the throttle actuator control (TAC) module. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

Go to Step 9

Go to Step 7

7

Replace the TAC module. Refer to Control Module References for replacement, setup, and programming.

Did you complete the replacement?

Go to Step 9

--

8

Replace the cruise control switch. Refer to Cruise Control Switch Replacement .

Did you complete the replacement?

Go to Step 9

--

9

  1. Use the scan tool in order to clear the DTCs.
  2. Operate the vehicle within the Conditions for Running the DTC.

Does the DTC reset?

Go to Step 2

System OK