Component Functional Description - Door Latch System

Courtesy of CHRYSLER GROUP, LLC

Functional Operation of the Lock Motors:  The Body Control Module (BCM) uses an H-Bridge configuration to drive the lock/unlock motors. To lock the doors, voltage is applied to the Lock pin circuit and ground is applied to both Unlock pin circuits. To unlock the doors, voltage is applied to the Unlock pins and ground is applied to the Lock pin. The passenger doors have a separate Unlock pin so that only the Driver's Door can be unlocked with the first button press on the keyless remote. When the lock motors are activated BCM will drive the motors for approximately 370 ms. The BCM and lock motors operate in the voltage range of 9.0 and 16.0 volts. Typical current draw for the motors are listed in Amps in the table below.

Functional Operation of the Latch Switches:  All of the Door Latches are monitored to determine if they are closed or ajar. All of the ajar switches are wake-up signals to the BCM. The Driver Door Latch must be rationalized in order for some vehicle features to function. Unlike the passenger ajar signals that function like traditional open/closed ajar switches, the Driver Door Latch has redundant switches and signals for diagnostic purposes. The two switches share a common sensor ground. The Door Ajar Switch signals use Battery voltage for a reference. The switches operate in the following manner:

• Driver Door Ajar Switches:  The BCM has an internal pull up 1000 Ohm resistor for each ajar signal. Each switch has an internal 604 Ohm and 1070 Ohm resistor. When the switches are open the resistance through the switches are 1674 Ohms. When the switches are closed the resistance through the switches are 604 Ohms. The switch states are opposing each other, meaning that when the door latch is closed  , switch 1 is in the open state and switch 2 is in the closed state. The opposite is true when the door latch is opened  . For more information refer to the tables below.
• Passenger Ajar Switches:  The ajar switches are open when the latch is closed. When the door latch is ajar the switch closes allowing the signal to connect to ground through the switch. Therefore the signal inputs to the BCM are high when the door is closed and low when the door is ajar.

The BCM monitors the Driver Door Ajar Switch signal circuit and converts the voltage signal into counts to broadcast over the CAN Bus network.

• Electronic Control Modules read an analog signal voltage and converts it to digital counts. Each count is an equal percentage of the signal reference voltage. This can be seen when viewing some sensor or switch voltages and converted values on the scan tool. The values will change in blocks (Ex. 0.08v, 0.12v, 0.16v or 8 Pa, 16 Pa, 24 Pa etc.). The signal voltage could be anywhere from 5.0 volts to a battery voltage reference.
• The resolution of the signal is determined by the required precision of the input by the ECU. Typically, sensor signals are high resolution. This allows the module to read small changes over a wide range in a variable resistance sensor signal. These are typically used on a temperature, pressure, position or MUX type of sensor.
• Switches typically have a much lower resolution than sensors. This is because an ECU is generally looking for a switch to be in only a few positions that use fixed resistors. The lower resolution also helps the ECU to overcome some resistance in a circuit and still read the correct state. This also lowers the sensitivity to setting DTCs.
• The voltage and percentages can vary slightly with calibration changes. The voltage values and percentages in the table are approximate and can be slightly different. Typically, a failure will be noticeably different than the values in the table. The voltage ranges given are based on a nominal reference voltage of 13.5 volts. The voltage on the signal will change with system voltage but the percentage of system voltage should not change.

The Driver Door Ajar Switch signal voltage is divided into 15 counts typically. The table below shows the relation between the Driver Door Ajar Switch states and approximate voltage percentages/ranges. 

DIAGNOSTIC OVERVIEW-DOOR LOCK MOTORS AND SWITCHES: 

• Lock Motors:  The BCM is capable of detecting opens and shorts in the H-Bridge control circuits. The locks are not thermally protected by the BCM. The protection is done by limiting the number of actuations within a calibrated time period.
• Door Latch Switches:  The resistors in the latch switches allows the BCM to perform diagnostics on the circuits and switches. Unlike the passenger door switches the voltage on the two signals should not be high or low. If the BCM determines that either signal falls into the high or low range, a fault is set. If the door latch positions do not match between the two Driver Door Ajar Switches, a correlation fault will set.