System Overview

DIAGNOSTIC FUNCTIONS OVERVIEW

The control module (1) checks itself and all the input and output signals, as well as the function of certain emission-affecting systems via integrated diagnostic functions. The ECM is said to have performed a running cycle each time it goes through all diagnostic functions. Although the time taken to complete the cycle depends entirely on the type of driving, the normal period is about 30 minutes in mixed driving.

If the ECM detects a fault, two parameters (which may differ depending on the DTC) are frozen, and substitute limp-home' values may be adopted to enable the system to continue in operation.

The DTC, type of fault and frozen values are stored in the ECM memory in accordance with a predetermined pattern, which is different for different DTCs. The fault must be present for a predetermined number of intervals before it is recorded permanently and the ECM operates the MIL (in the case of emission-related faults). If the fault is cleared before this interval has expired, the DTC is erased from the memory.


The intervals referred to above are of three main types:
^ Trip (all diagnostic functions have been performed)
^ Running cycle (fuel trim has been initiated)
^ Time (in seconds)

Should a fault disappear for any reason after the DTC has been recorded permanently, the information Will be retained in the ECM memory. Each time the fault recurs, a first counter will advance one step. For each trip completed without recurrence of the fault, a second counter will count down from 3 to 0 in single steps. When the second counter has reached 0. in other words when all of the ECM diagnostic functions have been completed three times and the fault has not recurred, the MIL will be extinguished (if lit) and a third counter will commence to count down.

The third counter records the number of warm-up cycles, starting at 40. A warm-up cycle is one in which the engine temperature is below 40°C (104°F) on starting and exceeds 84°C (183.2°F). If start-up takes place at an engine temperature above 40°C (104°F), the warm-up cycle cannot be implemented. When the third counter reaches 0, the DTC is erased from the ECM memory.

If the fault recurs, the second and third counters revert to their original settings and the first counter advances one step.

Faults which are diagnosed by the TCM may affect emission levels, which is why the TCM may request activation of the MIL lamp via the ECM.