Electronic Control Modules (Service Information): Communication: Operation

The primary communication network between electronic control modules on this vehicle is the Controller Area Network (CAN) data bus system. The Controller Area Network (CAN) data bus allows all electronic modules connected to the bus to share information with each other. Regardless of whether a message originates from a module on the higher speed CAN C (500K) Bus or on the lower speed CAN Interior High Speed (IHS) (125K) Bus the message structure and layout is similar, which allows the Body Control Module (BCM) to be a Central Gateway to process and transfer messages between the CAN C and CAN IHS buses. The BCM also stores Diagnostic Trouble Codes (DTCs) for certain bus network faults.

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.

The vehicle communication systems may be diagnosed with the Mopar Scope. Refer to DIAGNOSIS AND TESTING .

CAN BUS FAULTS 

TYPES OF CAN BUS FAULTS
LOSS OF COMMUNICATION	will set by an active receiving/reporting ECU on a CAN Bus network that detects no communication from another ECU on the same CAN Bus network. Insufficient power, ground, bus voltage, or inaccurate vehicle configuration will cause a loss of communication.
IMPLAUSIBLE MESSAGE	will set by an active receiving/reporting ECU, when it determines the data sent from the active transmitting/offending ECU is missing part of the message, or the message is an irrational value over the CAN Bus.
MISSING MESSAGE	will set by an active receiving/reporting ECU, when it determines a data message to be missing partial information when sent from the active transmitting/offending ECU over the CAN Bus network.
BUS OFF	set by an ECU that has experienced approximately 32 transmit errors, this can be caused by ECU internal faults as well as external bus faults like shorts or plugging and unplugging test tools to the diagnostic connector.
PHYSICAL	is only detectable by an ECU that has a transceiver that is able to detect shorts on the bus. If the ECU does not, it generally will set bus off faults due to shorted bus lines.

CAN Bus Voltages (Normal Operation)
CAN-C Bus Circuits	Sleep	Recessive (Bus Idle)	Dominant (Bus Active)	CAN-L Short to Ground	CAN-H Short to Ground	CAN-L Short to Battery	CAN-H Short to Battery	CAN-H Short to CAN-L
CAN-L (-)	0 V	2.4 - 2.5 V	1.3 - 2.3 V	0 V	0.3 - 0.5V	Battery Voltage	Battery Voltage Less 0.75 V	2.45 V
CAN-H (+)	0 V	2.4 - 2.5 V	2.6 - 3.5 V	0.02 V	0 V	Battery Voltage Less 0.75 V	Battery Voltage	2.45 V
CAN-IHS Bus Circuits	Key-Off (Bus Asleep)		Key-On (Bus Active)	CAN-L Short to Ground	CAN-H Short to Ground	CAN-L Short to Battery	CAN-H Short to Battery	CAN-H Short to CAN-L
CAN-L (-)	0.0V		1.3 - 2.3 V	0 V	0.3 - 0.5 V	Battery Voltage	Battery Voltage Less 0.75 V	2.45 V
CAN-H (+)	0.0 V		2.6 - 3.5 V	0.02 V	0 V	Battery Voltage Less 0.75 V	Battery Voltage	2.45 V
Notes
All measurements taken between node ground and CAN terminal with a standard DVOM.
DVOM will display average network voltage.
Total resistance of CAN networks can be measured with the battery disconnected. The average resistance is approximately 60 Ohms. The termination resistors are integral to the Star Connectors.

The CAN-IHS bus network remains active until all nodes on that network are ready for sleep. This is determined by the network using tokens in a manner similar to polling. When the last node that is active on the network is ready for sleep, and it has already received a token indicating that all other nodes on the bus are ready for sleep, it broadcasts a bus sleep acknowledgment message that causes the network to sleep. Once the CAN-IHS bus network is asleep, any node on the bus can awaken it by transmitting a message on the network.

In the CAN system, available options are configured into the BCM at the assembly plant, but additional options can be added in the field using the diagnostic scan tool. The configuration settings are stored in non-volatile memory. The BCM also has two 64-bit registers, which track each of the as-built  and currently responding  nodes on the CAN-B and CAN-C buses. The BCM stores a Diagnostic Trouble Code (DTC) in one of two caches for any detected active or stored faults in the order in which they occur. One cache stores powertrain (P-Code), chassis (C-Code) and body (B-Code) DTCs, while the second cache is dedicated to storing network (U-Code) DTCs.

LIN BUS 

The Master module is central to the LIN bus system. All LIN modules connect to a Master module through the LIN bus circuit. The Master module is also wired to a CAN bus for information sharing and to allow for scan tool diagnosis. The Master module is responsible for LIN diagnostics and is capable of setting fault codes for any LIN module or circuit faults. Since a Slave module cannot communicate with any other module directly, the Master module relays LIN Slave module data over the CAN bus to the scan tool, similar to a Gateway Module. Controlled module inputs and commands must be given only to the Master module, which then sends the information to another Slave module or out onto the CAN bus.

The LIN bus is biased through the Master and Slave modules. When at rest, the voltage on the LIN network is close to battery voltage. When LIN bus communication occurs, the voltage is pulled low to nearly 0V, creating a digital signal. If a Slave module is disconnected or removed from the network, that module no longer pulls voltage low on the LIN bus, and the average network voltage level will be lower. LIN voltage reads 12V when in sleep mode. The Master module communicates with Slave modules by pulling the voltage from high to low, creating a digital signal. Controlled modules communicate with the Master by biasing the network voltage in the same manner. Even though the LIN Slave modules are wired to each other, they only recognize the voltage pulses sent by the Master module, making communication possible only between the Slave and the Master. However, LIN communication can occur between Slave modules by using the Master module to relay information. The power feed to the LIN Slave is determined by its function. Controlled modules with a battery feed voltage supply operate and communicate even if the ignition is in the OFF position. For example, to support the hazard lamp function, the instrument panel switch bank is battery fed. If the LIN module only needs to function with the ignition ON, such as the compass module, the power feed is ignition only. The lighting and wiper multifunction switch require both feeds. This allows the lighting functions to be independent of ignition status, while the operation of the wipers requires the ignition feed. LIN Slave modules that receive their power feed from an ignition circuit enter sleep mode when the ignition power is removed. LIN Slave modules that receive their power from a direct battery feed receive a sleep message when the vehicle data bus enters sleep mode. For circuits where the LIN modules are battery fed, either the Master or the Slave module can wake the other. If the Slave module wakes up the Master, the vehicle bus also wakes.

As with any single-wire communication bus, a LIN bus has low fault tolerance. If a short to ground or power occurs on the LIN bus, the entire LIN bus loses the ability to communicate with the Master module. If an open occurs within the network, modules on the other side of the break (that are no longer connected to the Master module) lose communication. If an open occurs within the network, modules still connected to the Master module continue to communicate. Loss of power or ground to a Slave module results in that particular Slave module losing communication as well, whereas loss of power or ground to a Master module results in no LIN bus communication. Generally, one Slave module is triggered for information at a time - meaning zero collision risk and no need for arbitration as is used in CAN (Controller Area Network). If multiple Slave modules respond, a collision occurs and the Master ECU defaults to unconditional frames.

When the LIN bus is shorted, no modules can communicate on the network and a loss of all LIN module functionality occurs. Loss of communication codes (U-codes) will set in the Master module. A short to ground or to power on the LIN bus does not affect Master module communication over the CAN bus. The voltage on the LIN bus should be checked to see if voltage is present. Modules may be isolated by disconnecting Slave modules one at a time and then checking for LIN communication to return when a particular Slave is disconnected. When all Slave and Master modules are disconnected, test the LIN circuit for a short to ground or to power. If an open occurs, modules downstream of the open lose functionality and communication codes will set. If a single LIN Slave module is not communicating but all other LIN modules are functioning normally, always verify power, ground, and LIN voltage at the suspect module before replacing any parts. As long as at least one module on the LIN network is connected, then the circuit is able to be biased.