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For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2005 🢒 TopKick C-Series Commercial (Cutaway) C4/C5 🢒 Vehicle Control Systems 🢒 Vehicle DTC Information 🢒 Engine RPM Does Not Increase with Power Take-Off Engaged

Circuit Description

When the power take-off (PTO) enable switch is turned to the ON position and all PTO engage conditions are met, the engine RPM should increase to the standby speed. When the powertrain control module (PCM) receives the signal from the PTO switch the PCM will send a signal to the throttle actuator control (TAC) module. The TAC module controls the TAC motor which changes the position of the throttle. Communications between the 2 modules is monitored and checked for accuracy by the PCM. Diagnostic trouble codes (DTCs) may be recorded in the event of the failure. Diesel engines do not have a TAC system and the engine RPM is strictly controlled by the engine control module (ECM).

Test Description

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

  1. This step checks for other system DTCs that may affect the PTO.

    2.

  2. This step tests the PCM for the PTO Enable signal.

    3.

  3. This step tests the communication between the PCM and the TAC module.

    4.

  4. This step tests the PTO On Switch Signal circuit.

    5.

  5. This step tests the PTO On Switch-Signal circuit for an open, high resistance or short to ground.

    6.

  6. This step tests the Serial Data circuits for an open, high resistance or short to ground.

Step

Action

Yes

No

Schematic Reference: Power Take-Off (PTO) Schematics and Cruise Control Schematics

1

Did you review the Power Take-Off (PTO) Description and Operation and perform the necessary inspections?

Go to Step 2

Go to Power Take-Off (PTO) Description and Operation

2

Verify the fault is present.

Does the system operate normally?

Go to Testing for Intermittent Conditions and Poor Connections

Go to Step 3

3

Did you perform the Diagnostic System Check - Vehicle?

Go to Step 4

Go to Diagnostic System Check - Vehicle

4

Does the PTO solenoid engage?

Go to Step 5

Go to Power Take-Off (PTO) Does Not Engage

5

Is the engine a diesel?

Go to Step 8

Go to Step 6

6

  1. Start the engine.
  2. Ensure that all of the PTO engage conditions are met. Refer to Power Take-Off (PTO) Description and Operation .
  3. Turn the PTO enable switch to ON.
  4. Using a test lamp test for voltage to the PCM on the PTO On Switch Signal circuit.

Does the test lamp illuminate?

Go to Step 7

Go to Step 9

7

Using the scan tool read the TAC/PCM communication signal on the Engine Controls Data List.

Does the scan tool display OK?

Go to Diagnostic Starting Point - Cruise Control

Go to Step 10

8

  1. Depending on the specific diesel engine application, disconnect the appropriate connector at the ECM.
  2. Turn the ignition switch to RUN.
  3. Turn the PTO enable switch to ON.
  4. Using a test lamp test for Voltage to the ECM on the PTO On Switch Signal circuit.

Does the test lamp light?

Go to Diagnostic Starting Point - Engine Controls for the 6.6L engine or Diagnostic Starting Point - Engine Controls for the 7.8L engine

Go to Step 9

9

Test the PTO On Switch-Signal circuit for an open, high resistance or short to ground.

Did you find and correct the condition?

Go to Step 13

Go to Step 12

10

Test the Serial Data circuits for an open, high resistance or short to ground. Refer to Cruise Control Schematics .

Did you find and correct the condition?

Go to Step 13

Go to Step 11

11

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

Did you complete the replacement?

Go to Step 13

--

12

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

Did you complete the replacement?

Go to Step 13

--

13

Test drive the vehicle and operate the PTO system as described in the supporting text.

Does the symptom occur?

Go to Step 3

System OK