GM/Detroit 6.5L ENGINE CONTROLS – DTC P0251 Injection Pump Cam Sensor Circuit

Circuit Description
The optical sensor provides a pump cam signal to the PCM by counting pulses on the sensor disk located in the injection pump. The pump cam is one of the most important inputs by the PCM for fuel control and timing. This test monitors the number of crankshaft position pulses that have occurred since the last cam pulse. The physical one to one correspondence between the pump cam and the crankshaft implies if more crank pulses are detected than cam pulses, cam pulses have been missed.

Conditions for Running the DTC
• The PCM performs this DTC diagnostic continuously.
• The engine is operating.

Conditions for Setting the DTC
• The number of consecutive missing cam pulses is more than or equal to 8.
• All diagnostic set conditions met for 2 seconds.

Action Taken When the DTC Sets
• The PCM illuminates the malfunction indicator lamp (MIL) on the first consecutive drive trip that the diagnostic runs and fails.
• The Freeze Frame records the operating conditions at the time of failure and updates the Failure Records.
• The PCM will activate Back Up fuel.

Conditions for Clearing the MIL/DTC
• The PCM will turn the MIL off after three consecutive trips without a fault condition.
• A History DTC clears after forty consecutive warm-up cycles, if this or any other emission related diagnostic does not report any failures
• The use of a scan tool.

Diagnostic Aids
Intermittent DTCs (P0251, P0370 and P1216) may be caused by air entering the fuel system when fuel levels get below 1/8 of a tank whiie performing hard acceleration or turning maneuvers. It’s also possible that a P0251, P0370 and P1216 will set the if vehicle has run out of fuel. Customer driving habits should be checked to determine if the vehicle has been performing in these manners. If the vehicle has been performing in these conditions, bleed the fuel system of all air and test drive the vehicle. When the PCM is in backup fuel, fast idle and poor performance problems will exist.

An intermittent condition can be caused by the following:
• Poor connections.
• Rubbed through wire insulation.
• Broken wire inside the insulation.

Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.
3. This step will determine if this is a hard or intermittent DTC.
5. This step will determine if there is a 5 volt reference.
7. This step will check to see if the sensor is sending a signal back to the PCM.
11. The PCM supplies 5 volts on the signal circuit. This step determines if that voltage is present, not present, or too much voltage is present.
16. This step determines if the signal circuit is shorted to 5V. A normal CAM signal circuit will have 3-5mA. Any reading over 50mA indicates a short to 5V.


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