GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2004 🢒 GMC Sierra Denali 🢒 Engine 🢒 Engine Cooling 🢒 DTC P1AC1 or P1AC2

Circuit Description

The following DTCs are for the starter/generator control module (SGCM) coolant fan relay:

    • P1AC1 is for the SGCM coolant fan relay control circuit low voltage condition.
    • P1AC2 is for the SGCM coolant fan relay control circuit high voltage condition.

The SGCM controls the coolant fan operation by applying voltage to the coolant fan relay control circuit with an internal solid state device called a driver. When the SGCM is commanding the relay ON, the voltage of the control circuit should be low, near 0 volts. When the SGCM is commanding the relay OFF, the voltage potential of the control circuit should be high, near 12 volts.

The SGCM monitors the relay control circuits for the following conditions:

    • Short to ground
    • Short to voltage
    • An open circuit

If the SGCM detects an improper voltage level on the coolant fan relay control circuit , then DTC P1AC1 or P1AC2 will set and the driver will be disabled.

Conditions for Running the DTC
    • The engine has been running for 30 seconds.
    • The SGCM has received voltage on the ignition 0 voltage circuit.
    • The SGCM has turned ON and no configuration DTC are present.
    • For DTC P1AC1 the SGCM coolant pump is commanded OFF.
    • For DTC P1AC2 the SGCM coolant pump is commanded ON.

Conditions for Setting the DTC
    • For DTC P1AC1 the SGCM detects a low voltage condition on the SGCM coolant fan relay control circuit.
    • For DTC P1AC2 the SGCM detects a high voltage condition on the SGCM coolant fan relay control circuit.

Action Taken When the DTC Sets
    • The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
    • The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

Conditions for Clearing the MIL/DTC
    • The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
    • A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
    • A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
    • Clear the MIL and the DTC with a scan tool.

Diagnostic Aids

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. During the SGCM cooling fan relay operation, listen for an audible click. Command the ON and the OFF states. Repeat the commands as necessary.

    2.

  2. Verifies that the SGCM is providing voltage to the SGCM cooling fan relay.

    3.

  3. Tests for an open in the ground circuit to the SGCM cooling fan relay.

    4.

  4. Tests for constant voltage to the SGCM cooling fan relay.

Step

Action

Yes

No

Schematic Reference: Engine Cooling Schematics

Connector End View Reference: Cooling System Connector End Views

1

Did you perform the Engine Cooling Diagnostic System Check?

Go to Step 2

Go to Diagnostic System Check - Engine Cooling

2

  1. Install a scan tool.
  2. Turn ON the ignition, with the engine OFF.
  3. With a scan tool, command the SGCM Fan Speed ON and OFF.

Does the SGCM cooling fan relay turn ON and OFF with each command?

Go to Diagnostic Aids

Go to Step 3

3

  1. Turn OFF the ignition.
  2. Disconnect the starter/generator control module (SGCM) cooling fan relay.
  3. Turn ON the ignition, with the engine OFF.
  4. Connect a test lamp to ground.

    5. Probe the control circuit of the SGCM cooling fan relay with a test lamp that is connected to battery positive voltage.

  5. With a scan tool, command the SGCM Fan Speed ON and OFF.

Does the test lamp turn ON and OFF with each command?

Go to Step 4

Go to Step 5

4

  1. Connect a test lamp between the control circuit of the SGCM cooling fan relay and the ground circuit of the SGCM cooling fan relay.
  2. With a scan tool, command the SGCM Fan Speed ON and OFF.

Does the test lamp turn ON and OFF with each command?

Go to Step 8

Go to Step 10

5

Does the test lamp remain illuminated with each command?

Go to Step 7

Go to Step 6

6

Test the control circuit of the SGCM cooling fan relay for a short to ground and an open. Refer to Circuit Testing and to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 13

Go to Step 9

7

Test the control circuit of the SGCM cooling fan relay for a short to voltage. Refer to Circuit Testing and to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 13

Go to Step 9

8

Inspect for poor connections at the SGCM cooling fan relay. Refer to Testing for Intermittent Conditions and Poor Connections and to Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 13

Go to Step 11

9

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

Did you find and correct the condition?

Go to Step 13

Go to Step 12

10

Repair the ground circuit of the SGCM cooling fan relay. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

Go to Step 13

--

11

Replace the SGCM cooling fan relay. Refer to Relay Replacement in Wiring Systems.

Did you complete the replacement?

Go to Step 13

--

12

Important: Perform the programming procedure for the SGCM.

Replace the SGCM. Refer to Generator Control Module Replacement in Engine Electrical.

Did you complete the replacement?

Go to Step 13

--

13
  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