Field-Effect Transistor (FET) Protection

The SJB  utilizes a Field-Effect Transistor (FET) protective circuit strategy for many of its outputs (for example, the headlamp output circuit). Output loads are monitored for excessive current (typically short circuits) and are shut down when a fault event is detected. A continuous DTC is stored at the time of the fault event and a cumulative counter is started.

The module resets the FET protection and after a customer demand of the function (previously disabled), attempts to provide the requested output (voltage or ground). Each time the module detects an excessive load on the protected circuit, a counter advances.

When the excessive circuit load occurs often enough, the module shuts down the output until a repair procedure is carried out. Each FET protected circuit has 3 predefined levels of fault tolerance based upon each circuits' total protective capability. For example, if the total predefined tolerance level is 600 fault events, the tolerance levels are reached at 200, 400 and 600 fault events.

When each tolerance level is reached the continuous DTC that was stored on the first failure cannot be cleared by a command to clear the continuous DTCs. The module does not allow this DTC to be cleared or the circuit restored to normal operation until a successful self-test proves that the fault has been repaired. After the self-test has successfully completed (no on-demand DTCs present), the continuous DTC clears and the circuit function returns.

When the first or second fault tolerance level is reached, the continuous DTC (associated with the short circuit) sets along with DTC B106E. These DTCs can be cleared using the Clear DTC operation on the scan tool as long as the fault itself has been corrected.

If any of the circuits are shorted past the third fault tolerance level, then DTCs B106F and B1342 set along with the associated continuous DTC. These DTCs cannot be cleared and the module must be replaced.