Dakota 4WD V8-4.7L VIN J (2007)
Body Control Systems: Description and Operation
Operation
OPERATION
The Controller Area Network (CAN) data bus allows all electronic modules or nodes connected to the bus to share information with each other.
Regardless of whether a message originates from a module on the low speed CAN-B bus or on the high speed CAN-C or CAN-D bus, the message
structure and layout is similar, which allows the Front Control Module/Central GateWay (FCM or FCMCGW) to process and transfer messages between
the buses. The FCM also stores a Diagnostic Trouble Code (DTC) for certain bus network faults.
All modules (also referred to as nodes) transmit and receive messages over one of these buses. Data exchange between nodes is achieved by serial
transmission of encoded data messages. Each node can both send and receive serial data simultaneously. Bus messages are carried over the data bus in
the form of Variable Pulse Width Modulated (VPWM) signals which, when the high and low voltage pulses are strung together, form a message. Each
node uses arbitration to sort the message priority if two competing messages are attempting to be broadcast at the same time.
The voltage network used to transmit messages requires biasing and termination. Each module on the bus network provides its own biasing and
termination. Each node terminates the bus through a terminating resistor and a terminating capacitor. There are two types of nodes on the bus. The
dominant node terminates the bus through a 1 KW resistor and a 3300 pF capacitor, typically resulting in about a 3300 ohm termination resistance.
However, this resistance value may vary somewhat by application. The FCM (or FCMCGW) is the only dominant node in this network. A non-dominant
(or recessive) node terminates the bus through an 11 KW resistor and a 330 pF capacitor, typically resulting in about a 10800 ohm termination
resistance.
The communication protocol being used for the CAN data bus is a non-proprietary, open standard adopted from the Bosch CAN Specification 2.0b. The
CAN-C is the faster of the two primary buses in the CAN bus system, providing near real-time communication (500 Kbps).
The CAN bus nodes are connected in parallel to the two-wire bus using a twisted pair, where the wires are wrapped around each other to provide
shielding from unwanted electromagnetic induction, thus preventing interference with the relatively low voltage signals being carried through them. The
twisted pairs have between 33 and 50 twists per meter (yard). While the CAN bus is operating (active), one of the bus wires will carry a higher voltage
and is referred to as the CAN High or CAN bus (+) wire, while the other bus wire will carry a lower voltage and is referred to as the CAN Low or CAN
bus (-) wire. Refer to the CAN Bus Voltages table.
All measurements taken between node ground and CAN terminal with a standard DVOM.
DVOM will display average network voltage.
Total resistance of CAN-C network can also be measured (60 ohms). Cannot measure total resistance of CAN-B network.
