P0251

Wiring Diagram:

CIRCUIT DESCRIPTION
The optical sensor provides a pump cam signal to the Powertrain Control Module (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. This is a type A DTC.

CONDITIONS FOR SETTING THE DTC
- RPM less than 300.
- 8 consecutive cam pulses missing for 8 #1 cylinder events.
or
- RPM greater than or equal to 300.
- 8 consecutive cam pulses missing for 32 #1 cylinder events.

ACTION TAKEN WHEN THE DTC SETS
Backup 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 will clear when forty consecutive warm-up cycles that the diagnostic does not fail (coolant temperature has risen 5°C (40°F) from start up coolant temperature and engine coolant temperature exceeds 71°C (160°F) that same ignition cycle.

DIAGNOSTIC AIDS
When PCM is in backup fuel, fast idle and poor performance problems will exist. If P0251 is also stored there is a possible problem with signal circuit. P0251 and P0370 will set if vehicle has run out of fuel.

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.
12. 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.
17. This step determines if the signal circuit is shorted to 5V. A normal CAM signal circuit will have 3-5 mA. Any reading over 50 mA indicates a short to 5V.