P251D

DTC P251A-P251F

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle (Diagnostic System Check - Vehicle) prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis (Strategy Based Diagnosis) for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions (Diagnostic Procedure Instructions)provides an overview of each diagnostic category.

DTC Descriptor
DTC P251A
- Power Take-Off (PTO) Enable Switch Circuit

DTC P251E
- Power Take-Off (PTO) Engine Shutdown Control Circuit Low Voltage

DTC P251F
- Power Take-Off (PTO) Engine Shutdown Control Circuit High Voltage

Diagnostic Fault Information





Circuit/System Description

The power take-off (PTO) is an upfitter integrated system that allows the user to create an auxiliary power source. The PTO system specifically controls engine speed to values higher than normal base idle, PTO load relay engagement, and remote starting and shutdown of the engine. When the operator requests PTO using either the PTO switch or PTO remote, the engine RPM will increase to a default standby speed. The engine control module (ECM) converts the data from the engine speed sensor to a 2 pulses/engine revolution signal. When PTO is engaged, the power take-off module (PTOM) sends a GMLAN serial data message to the ECM requesting engine speeds, in accordance with the depressed switches from the PTO switch or remote.

Conditions for Running the DTC

The PTO module becomes active.

Conditions for Setting the DTC

P251A
* The PTOM detects a high voltage signal on one or more of the in-cab PTO switch circuits: SET -, RES +, 1 or 0.
* The above condition is present for greater than a predetermined time threshold.
* This code moves to history after 90 seconds.

P251E or P251F
* The PTOM detects that the PTO engine shutdown signal circuit is above or below a predetermined voltage threshold.
* The above condition is present for greater than a predetermined time threshold.

Action Taken When the DTC Sets

* The PTOM stores the DTC information into memory when this diagnostic runs and fails.
* The malfunction indicator lamp (MIL) will not illuminate.
* PTO becomes disabled.

Conditions for Clearing the 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 failure is no longer present.

Reference Information
Schematic Reference

Power Take-Off (PTO) Schematics ([1][2]Electrical Diagrams)
Connector End View Reference

Component Connector End Views (Connector End Views By Name)
Description and Operation

Power Take-Off (PTO) Description and Operation (Description and Operation)
Electrical Information Reference

* Circuit Testing (Circuit Testing)
* Connector Repairs (Connector Repairs)
* Testing for Intermittent Conditions and Poor Connections (Testing for Intermittent Conditions and Poor Connections)
* Wiring Repairs (Wiring Repairs)

Scan Tool Reference

Control Module References (Control Module References)for scan tool information

Circuit/System Verification

P251A
1. Ignition ON, verify that P0641 is not set as current in the PTO module.

• If the specified DTC is set as current, refer to DTC P0641 (P0641).

2. Engine ON, press 1 on the PTO switch to engage PTO. The engine RPM should increase to the default engine speed within a few seconds. Press RES + to increase the engine speed and SET - to decrease the engine speed accordingly. Press 0 to disengage PTO and to return the engine speed back to normal idle.

P251E or P251F
1. Engine ON, with PTO enabled using the in-cab PTO switch, momentarily press the 0 switch. The engine RPM should return to normal idle speed.
2. With PTO enabled, momentarily press the remote engine shutdown switch. The engine RPM should return to normal idle speed and the engine will shutdown.

Circuit/System Testing

P251A
1. Ignition OFF, disconnect the harness connector at the PTO module.
2. Ignition OFF, disconnect the harness connector at the in-cab PTO switch.
3. Ignition OFF, test for infinite resistance between the harness connector at the PTO switch mode select A signal circuit terminal 2 and ground.

• If less than the specified value, test the mode select A signal circuit for a short to ground.

4. Ignition OFF, test for less than 1.0 ohm of resistance in the mode select A signal circuit between the PTO switch connector terminal 2 and the PTO module connector terminal 26.

• If greater than the specified range, test the mode select A signal circuit for an open/high resistance.

5. Ignition OFF, test for infinite resistance between the harness connector at the PTO switch mode select B signal circuit terminal 9 and ground.

• If less than the specified value, test the mode select B signal circuit for a short to ground.

6. Ignition OFF, test for less than 1.0 ohm of resistance in the mode select B signal circuit between the PTO switch connector terminal 9 and the PTO module connector terminal 11.

• If greater than the specified range, test the mode select B signal circuit for an open/high resistance.

7. Ignition OFF, test for infinite resistance between the harness connector at the PTO switch mode select C signal circuit terminal 4 and ground.

• If less than the specified value, test the mode select C signal circuit for a short to ground.

8. Ignition OFF, test for less than 1.0 ohm of resistance in the mode select C signal circuit between the PTO switch connector terminal 4 and the PTO module connector terminal 12.

• If greater than the specified range, test the mode select C signal circuit for an open/high resistance.

9. If all circuits test normal, replace the PTO switch.

P251E or P251F
1. Ignition OFF, disconnect the harness connector at the PTOM.
2. Ignition ON, test for 0 volts between the PTO engine shutdown signal circuit terminal 33 and ground.

• If greater than the specified value, test the PTO engine shutdown signal circuit for a short to voltage. If the circuit test normal, replace the ECM.

3. Ignition OFF, disconnect the harness connector X1 at the ECM.
4. Ignition OFF, test for infinite resistance between the PTO engine shutdown signal circuit X1 terminal 84 and ground.

• If less than the specified range, test the PTO engine shutdown signal circuit for a short to ground.

5. Ignition OFF, test for less than 1.0 ohm on the PTO engine shutdown signal circuit between the PTO connector terminal 33 and the ECM connector X1 terminal 84.

• If greater than the specified range, test the PTO engine shutdown signal circuit for an open/high resistance.

6. Ignition OFF, test for less than 1.0 ohm on the PTO engine shutdown signal circuit between the PTO connector terminals 33 and 31.
7. If greater than the specified range, test the PTO engine shutdown signal circuit for an open/high resistance. If the circuit tests normal, test or replace the upfitter remote switch assembly.
8. If all circuits test normal, replace the PTOM.

Repair Instructions

Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.

* Power Take-Off Switch Replacement (Power Take-Off Switch Replacement)
* Control Module References (Control Module References)for PTO module replacement, setup, and programming