1.10.3.1 General Description

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

Two methods are utilized to test the plausibility of the calculated engine-off time:

The first method, indicated by P2610, tests the external timer signal from the instrument cluster and the timer in the ECU by monitoring the difference between both time signals. The second method, indicated by P15E8 and P15E9, compares the engine off-time to the decrease in the ECT.

Description of P2610:

This engine off-time plausibility test is executed by comparing the instrument cluster time to the ECU. Two error detection methods are utilized 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 certain threshold value during the active ECU phase (synchronized running test).

Fig 1: Identifying T_DME And T_Instrument Cluster Chart

Courtesy of BMW OF NORTH AMERICA, INC.

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

Fig 2: Identifying 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 to the ECT decrease, taking the AAT into account.

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

The instrument cluster increments too slowly during the engine-off phase when the engine-off time is too slow. While the engine is not running, the instrument cluster increments too fast when the engine off-time is too fast.

Fig 3: Engine Coolant Temperature Versus Time Graph

Courtesy of BMW OF NORTH AMERICA, INC.

In the example above, a normal ECT cooling down is shown (black curve) at an AAT of 20°C. The green area represents a plausible engine-off time and the orange areas represent an implausible engine-off time. Compared to the ECT decrease, the instrument cluster is incrementing either too slowly or too fast in the orange areas.

Error detection example: Engine off-time too slow:

When ECT decrease < 60°C (temperature at engine stop - temperature at engine start) and engine off-time < 60 minutes, an error is detected.

Error detection example: Engine off-time too fast:

When ECT decrease = 5°C (temperature at engine stop - temperature at engine start) and engine off time > 120 minutes, an error is detected.