Technician Handbook ! 673 Electronic & Computer Controlled Systems Self-Diagnosis A significant reason ECUs have become so common in automobile systems is their ability to perform self-diagnosis . ECUs can identify faults in circuits, components, and even within the ECU itself. When a fault is detected, the ECU can: • Illuminate a warning light • Set a diagnostic trouble code • Begin operating in a fail-safe mode by:
– Disabling a system that is working incorrectly – Using sensor data from alternate sources – Applying alternate rules for operating the vehicle or subsystem to maintain maximum safety Differences in Self- An ECU’s self-diagnosis capabilities can range from very simple to highly sophisticated. Each ECU has its own features and limitations, and very few work in exactly the same way. Diagnosis The example above is a throttle position sensor circuit. The electronics inside the engine control module (ECM) are designed so that an open or a short to ground on VTA or VTA2 can be detected and a DTC set. The circuit arrangement inside the ECM is not able to distinguish a short from an open, however.
In either case, the voltage the ECM is monitoring goes to 0V. Technical Training 17