Motronic fuel injection and ignition control system (VW Lt 2d)

The Motronic injection and ignition control unit is equipped with a fault memory, which records faults in sensors or systems, indicating the type of fault. It is installed in the engine compartment above the battery. The control unit detects and differentiates the following faults:

• malfunctions that always occur;
• faults that are detected in no more than three seconds;
• malfunctions that occur more than five times per trip.

If the fault does not reoccur after the 19th engine start, it is automatically cleared from the memory. Faults can be read using the VAG 1551 or VAG 1552 fault code reader. Once the fault has been corrected, it must be cleared from the fault memory. If the ECM connector or power wires are disconnected from the battery, all data and contents of the fault memory are deleted. Self-diagnostic values are automatically updated after 10 trips. The mechanical limitation of the throttle potentiometer is determined by the control unit after a few seconds with the ignition on. The transmission version is determined when the engine is started and the speed exceeds 10 km/h. Self-diagnosis is not possible when the engine control unit checks the immobilizer signal. If the control unit does not receive a signal from the immobilizer, it is blocked. The engine does not start and the immobilizer warning light flashes for approximately 3 seconds after the ignition is turned on. Before you begin troubleshooting when the immobilizer warning light is flashing, interrogate the fault memory of the immobilizer control unit.



Data exchange between the engine control unit and the VAG 1551 fault code reader is carried out in the "LT Diagnostics" mode (LT Diagnosis). The engine control unit version is displayed when connecting a VAG 1551 fault code reader and selecting the "Engine Electronics" function. The layout of the Motronic injection and ignition control system components is shown in the figure 2.66. The available self-diagnostic functions are shown in Table 2.5.

Table 2.5. Available self-diagnostic functions

	Functions	The engine is not running, the ignition is on	The engine is idling
02	Troubleshooting	Yes	Yes
03	Diagnostics of actuators	Yes	No
04	Basic installations	Yes1)	No
05	Clearing the fault memory	Yes	Yes
06	Completion of diagnostics	Yes	Yes
07	Control unit coding	No	No
08	Reading a block of measured values	Yes	Yes
09	Reading individual measured values	No	No
10	Setting up	No	No

¹⁾Should be carried out in the following cases: there is no voltage from the battery; when replacing the control unit, throttle control unit or engine.