1.10.3.1 General description

The engine off time is calculated using a relative time counter obtained from the instrument cluster via CAN message. The difference between the relative time counter value at both last engine stop and at the current engine start is calculated in the ECU as the engine off time.

Two methods are used to test the plausibility of the engine-off:

The first method tests the external timer signal from the instrument cluster and the timer in the ECU by monitoring difference of both time signals. This is represented in P2610.

The second method compares the engine off time with the decrease in engine coolant temperature. This is represented by P15E8 and P15E9.

Description of P2610:

This engine off time plausibility test consists of a comparison between the time of the instrument cluster and time of the ECU. Two error detection methods are used in this test.

An error is detected when the difference between the time of the instrument cluster and the time of the ECU exceeds a threshold during the active ECU phase (synchronized running check).

Fig 1: T_DME And T_Instrument Cluster Chart

Courtesy of BMW OF NORTH AMERICA, INC.

Further, an error is detected when the difference between the time of the instrument cluster and the time of the ECU exceeds a threshold when the ECU is waked up from the active PWL phase.

Fig 2: T_DME And T_Instrument Cluster Chart

Courtesy of BMW OF NORTH AMERICA, INC.

Description of P15E8 and P15E9:

This plausibility test compares the engine-off time with the decrease in the engine coolant temperature, taking the ambient temperature into account.

In this diagnosis, an error is detected when either the engine-off time is too low, compared to the decrease in engine coolant temperature (P15E8), or when the engine-off time is too high, compared to the decrease in the engine coolant temperature (P15E9).

When the engine-off time is too slow, the instrument cluster increments too slowly during the engine off phase. When the engine off-time is too fast, the instrument cluster increments too fast while the engine is not running.

Fig 3: Engine Coolant Temperature Versus Time Graph

Courtesy of BMW OF NORTH AMERICA, INC.

In this example, a normal engine coolant temperature cooling down is shown (black curve) at an ambient temperature of 20°C. The green area represents a plausible engine-off time. The orange areas represent an implausible engine-off time, either too slow or too fast incrementing instrument cluster, compared to the decrease in the engine coolant temperature.

Example of an error detection: Engine off time too slow:

When engine coolant temperature decrease is lower than 60°C

(temperature at engine stop - temperature at engine start) and engine off time is lower than 60 minutes, an error is detected.

Example of error detection: Engine off time too fast:

When engine coolant temperature decrease is 5°C

(temperature at engine stop - temperature at engine start) and engine off time is more than 120 minutes, an error is detected.