Theory Of Operation

WARNING: This page is about a different car, the 2017 RAM 2500 HD. However, it is still accessible from the selected car via links, so may be relevant.

For Model Years 2016, 2017, and 2018 vehicles equipped with the 6.7L Cummins Turbo Diesel, misfire detection uses two different algorithms depending on vehicle configuration. Vehicles equipped with the 68RFE automatic transmission use the Exhaust Manifold Pressure (EMP) sensor to detect misfires, while vehicles equipped with the G56 manual or the AISIN automatic transmission use the crankshaft position (CKP) sensor to detect misfires. Descriptions of the two different algorithms are below.

Vehicles Equipped with G56 Manual or AISIN Automatic Transmissions:

The Crankshaft Position (CKP) Sensor is a hall-effect type sensor. The Powertrain Control Module (PCM) provides a 5-Volt supply to the position sensor and return circuit. A notched tone wheel integrated into the damper is used to determine the position of the engine by the PCM. As the notches on the tone wheel moves past the position sensor, a signal is generated on the position sensor circuit. The change in engine speed when different calibrated fuel quantities are added by the injector to a cylinder is a measure of how well the cylinder is firing. If more than one cylinder is failed, a multiple cylinder misfire fault is set in addition to the individual faults of the failed cylinders. This diagnostic runs while the engine is at idle and at operating temperature and not in active regeneration mode. This is a two trip fault with active regeneration being disabled when the MIL lamp is on. The PCM will turn off the MIL lamp after the monitor runs and passes three consecutive times.

Vehicles Equipped with the 68RFE Automatic Transmission Only:

This algorithm detects misfires by monitoring the Exhaust Manifold Pressure (EMP) sensor pulses. The EMP sensor pulses are monitored during a 1000 revolution, 3000 firing event window. During this window, normal EMP sensor pulses cause a rise in pressure due to a combustion event. When there is a misfire, there is a lower EMP sensor pulse due to the lack of combustion. The system sees this dip in pressure, calculates a value, and then compares it to a threshold. If the calculated value is less than or equal to the threshold, one misfire event is counted. Once the counter reaches a threshold, the algorithm determines which cylinder is misfiring and sets the individual fault accordingly. If more than one cylinder contributed to the misfire counts, the multi-cylinder fault is set. After the 3000 firing event window is complete, the counter is reset and another 3000 firing event window is started. This diagnostic runs when the ambient temperature is above -6°C (20°F), the coolant temperature is above 15°C (60°F), the fuel level is above 15%, and the engine is operated anywhere between 0 to 75% maximum engine speed and 0 to 75% maximum net engine torque. This diagnostic is a two trip fault, and active regeneration will be disabled when the MIL lamp is on. The PCM will turn off the MIL lamp after the monitor runs and passes three consecutive times.