Principles of Operation

Handles, Locks, Latches and Entry Systems

Principles of Operation

Power Locks

The Body Control Module (BCM) monitors the door lock control switches. When the BCM receives an unlock/lock command from either door lock control switch, the BCM energizes a relay (internal to the BCM) to supply the correct power and grounds for the door lock actuators.

Remote Keyless Entry (RKE)

The Remote Keyless Entry (RKE) feature is controlled by BCM. The BCM receives the RKE inputs from the Tire Pressure Monitor (TPM) module over the Medium Speed Controller Area Network (MS-CAN). When any RKE transmitter button is pressed, the signal is received by the TPM module internal antenna. The TPM module then filters the RKE signal received to make sure it is a programmed transmitter. If the signal received is from a programmed transmitter, the TPM module sends the RKE command to the BCM over the MS-CAN. The BCM then interprets the RKE command and activates the corresponding output.

The BCM requests the illuminated entry feature to turn the interior lamps on when an unlock command is received. If a lock command is received, the illuminated entry feature turns off. The BCM then supplies voltage to the appropriate door lock actuator(s) to lock or unlock the doors. The RKE transmitter can also be used to activate the panic alarm. On vehicles with memory, the RKE transmitter also causes the BCM to send a MS-CAN message to the Driver Seat Module (DSM) to activate all memory features to the positions associated with the RKE transmitter being used.

Remote Start

The factory equipped remote start system is controlled by the BCM. How the BCM receives the remote start command from an Integrated Keyhead Transmitter (IKT) is very similar to the RKE function. For additional information on the factory equipped remote start system, refer to Starting System - Diesel Engine Testing and Inspection (diesel engines).

Keyless Entry Keypad

The keyless entry keypad is hardwired to the BCM. The BCM interprets the inputs from the keyless entry keypad and then controls the associated operation. The keyless entry keypad is illuminated for 5 seconds when any button is pressed. The BCM requests the illuminated entry feature to turn the interior lamps on when a valid entry code is received. If a lock all doors code is entered, the illuminated entry feature turns off.

NOTE: The keyless entry keypad does not lock the doors if the driver door is ajar.

The keyless entry keypad supplies a signal to the BCM when the buttons are pressed. The BCM then supplies voltage to the appropriate door lock actuator(s) to lock or unlock the doors. On vehicles with memory, the keyless entry keypad also causes the BCM to send an MS-CAN message to the DSM to activate all memory features to the positions associated with the personal entry code entered in on the keypad.

Field-Effect Transistor (FET) Protection

Field-Effect Transistor (FET) is a type of transistor that when used with module software can be used to monitor and control current flow on module outputs. The FET protection strategy is used to prevent module damage in the event of excessive current flow.

The BCM utilizes an FET protective circuit strategy for many of its outputs (for example, a headlamp output circuit). Output loads (current level) are monitored for excessive current (typically short circuits) and are shut down (turns off the voltage or ground provided by the module) when a fault event is detected. A short circuit DTC is stored at the fault event and a cumulative counter is started.

When the demand for the output is no longer present, the module resets the FET circuit protection to allow the circuit to function. The next time the driver requests a circuit to activate that has been shut down by a previous short (FET protection) and the circuit is still shorted, the FET protection shuts off the circuit again and the cumulative 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 short circuit tolerance based on the harmful effect of each circuit fault on the FET and the ability of the FET to withstand it. A module lifetime level of fault events is established based upon the durability of the FET. If the total tolerance level is determined to be 600 fault events, the 3 predefined levels would be 200, 400 and 600 fault events.

When each tolerance level is reached, the short circuit 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 code 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), DTC U1000:00 and the associated DTC (the DTC related to the shorted circuit) automatically clears and the circuit function returns.

When each level is reached, the DTC associated with the short circuit sets along with DTC U1000:00. These DTCs can be cleared using the module on-demand self-test, then the Clear DTC operation on the scan tool (if the on-demand test shows the fault corrected). The module never resets the fault event counter to zero and continues to advance the fault event counter as short circuit fault events occur.

If the number of short circuit fault events reach the third level, then DTCs U1000:00 and U3000:49 set along with the associated short circuit DTC. DTC U3000:49 cannot be cleared and the module must be replaced after the repair.

The BCM FET protected output circuit for the handles, locks, and latches system is the keypad illumination output circuit.