For 1990-2009 cars

### Circuit Description

When the Power Take-Off (PTO) switch is turned to the ON position and all PTO engage conditions are met, the engine RPM should increase to the standby speed. This symptom diagnostic will diagnose Powertrain Control Module (PCM) and Throttle Actuator Control (TAC) Module initial speed increase failure. When the PCM receives the signal from the PTO switch the PCM will send a signal to the TAC module. The TAC module controls the TAC motor which changes the position of the throttle. Communications between the two modules is monitored and checked for accuracy by the PCM. PCM or TAC DTCs may be recorded in the event of the failure.

### 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. This step tests the PTO Enable signal circuit.

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

4. This step tests the PTO Switch Output Enable circuit for an open, high resistance or short to ground.

5. 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 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 in Wiring Systems | Go to Step 3
3 | Did you perform a Diagnostic System Check? | Go to Step 4 | Go to Diagnostic System Check - Power Take-Off (PTO)
4 | Does the PTO solenoid engage? | Go to Step 5 | Go to Power Take-Off (PTO) Does Not Engage
5 |

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 switch to ON.
4. Using a scan tool read the PTO Enable Status on the Engine Controls Data List.

Does the scan tool indicate that the PTO is enabled?

Go to Step 6

Go to Step 7

6 | 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 in Cruise Control | Go to Step 8
7 | Test the PTO Switch Output Enable circuit for an open, high resistance or short to ground. Did you find and correct the condition? | Go to Step 11 | Go to Step 10
8 | Test the Serial Data circuits for an open, high resistance or short to ground. Did you find and correct the condition? | Go to Step 11 | Go to Step 9
9 | Replace the TAC module. Refer to Electronic Throttle Actuator Control Module Replacement in Engine Controls 8.1L. Did you complete the replacement? | Go to Step 11 | --
10 | Replace the PCM. Refer to Powertrain Control Module Replacement in Engine Controls 8.1L. Did you complete the replacement? | Go to Step 11 | --
11 | 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