Theory Of Operation Can-C
- CHARACTERISTICS OF CAN-C
- CAN-C is a multiplexed network that allow all modules to share information using high speed data signals. These networks use a twisted pair of insulated copper wires for the data bus. One wire is designated as CAN (+), the other is CAN (-). Connectors house the bus network circuit terminals in adjacent cavities and connect them to a module. The connectors also connect certain branches of wiring in-line along the wiring harness. A gateway module is used to link CAN-C to other communication networks.
The number of modules placed on a CAN-C bus is limited. In these instances, the modules are organized by interior and exterior location on the vehicle, and placed on different bus networks.
- CAN-C is a multiplexed network that allow all modules to share information using high speed data signals. These networks use a twisted pair of insulated copper wires for the data bus. One wire is designated as CAN (+), the other is CAN (-). Connectors house the bus network circuit terminals in adjacent cavities and connect them to a module. The connectors also connect certain branches of wiring in-line along the wiring harness. A gateway module is used to link CAN-C to other communication networks.
- CAN MESSAGE CONTENT, ARBITRATION, AND TERMINATION
- The CAN network is the most common vehicle communication network and is quickly becoming the industry standard. While other communication network protocols do exist, we will focus on the details of the CAN vehicle communication protocol. Knowledge of the CAN protocol is crucial because every late model FCA vehicle currently uses some form of CAN-based communication network.
A characteristic with CAN communication networks is that each module can bias the network. This means each module can alter the voltage level of the network, both on CAN (+) and CAN (-). Modules transmit information by biasing the network, and receive information by measuring the changing bias.
Information is shared over a CAN network using a sequence of controlled voltage pulses sent out on the bus wires. At the proper time, a module biases the bus voltage to transmit a data signal. It does so with a sequence of voltage pulses that vary in frequency. The combination of these voltage pulses forms a data message and is called a data frame. By design, CAN modules will recognize certain frequency voltage pulses at specific times. The sequence of voltage pulses that form a data frame can be broken down into specific parts.
Start of frame (SOF) - The transmission of data begins with the sending module biasing the bus dominant from a recessive state. Arbitration Field - The sending module identifies itself, and is prioritized; modules that require data from the sending module are alerted. Control Field - Identifies the format and bit construction of a message. Data Field - The actual data or information is contained within this field. The DLC (Data Length Code) in Classic CAN uses 0-8 bytes. Cyclical redundancy check (CRC) - A redundant check that verifies the message was received correctly. Acknowledgment (ACK) - Receiving modules verify if they successfully received the intended information. End of frame (EOF) - The bus returns to a recessive state, signaling the end of the transfer of data. There are two frame formats used in classic CAN: - CAN base format - 11 bit identifier and fixed bit rate.
- CAN extended format - 29 bit identifier and fixed bit rate.
- The CAN network is the most common vehicle communication network and is quickly becoming the industry standard. While other communication network protocols do exist, we will focus on the details of the CAN vehicle communication protocol. Knowledge of the CAN protocol is crucial because every late model FCA vehicle currently uses some form of CAN-based communication network.
- ARBITRATION
- When multiple modules are connected to a single communication network, there is a potential that two modules could attempt to transmit data on to the network at the same time. To prevent this from occurring, modules that are considered more important will have priority when attempting to transmit data. The less important modules are signaled to wait until all other data transfers are complete. This process is called arbitration. When normal operation occurs, arbitration prevents modules from disrupting the network. If a module fails and ceases to adhere to this arbitration, faults could occur on the communication network.
- THEORY AND OPERATION OF CAN-C
- Each CAN-C module can bias the network. This means the module will determine the voltage, and change in voltage, transmitted on the bus wires. Keep in mind there are many modules wired into the CAN-C or FD networks. Arbitration allows for all modules to share information in an orderly manner. Modules with messages deemed highly important take priority.
- TERMINATION RESISTANCE
- The CAN-C network has terminating resistors wired parallel in the circuit. Termination resistance allows for CAN (+) and CAN (-) circuit voltages to pull in different directions as well as absorb any stray voltage spikes throughout the network, preventing network signal interference. Optimal CAN-C bus termination resistance is 60 ohms. In some CAN architectures (Stub, Hub, or Daisy-Chain), this is achieved using two 120-ohm resistors located inside dominant network modules or Star Connectors if equipped. Certain vehicles may contain non-dominant modules with internal resistance, typically around 2.8- 3.0 kilo-ohms, but this resistance is part of the modules' data transmit/receive processor, and is not used to stabilize the network. A non-dominant module with this resistance will have an effect on the total network resistance when measured with a Multi Meter.
Understanding the fault characteristics of the CAN-C bus is key to identifying where to begin diagnosis. Diagnostic strategies are different between concerns of one module experiencing a communication fault versus a fault that affects the entire network. Using techniques like unplugging in-line connectors to break the CAN-C circuit in half and taking voltage and resistance measurements will quickly isolate a communication network fault.
- The CAN-C network has terminating resistors wired parallel in the circuit. Termination resistance allows for CAN (+) and CAN (-) circuit voltages to pull in different directions as well as absorb any stray voltage spikes throughout the network, preventing network signal interference. Optimal CAN-C bus termination resistance is 60 ohms. In some CAN architectures (Stub, Hub, or Daisy-Chain), this is achieved using two 120-ohm resistors located inside dominant network modules or Star Connectors if equipped. Certain vehicles may contain non-dominant modules with internal resistance, typically around 2.8- 3.0 kilo-ohms, but this resistance is part of the modules' data transmit/receive processor, and is not used to stabilize the network. A non-dominant module with this resistance will have an effect on the total network resistance when measured with a Multi Meter.
- TOLERANCE
- CAN-C has limited fault tolerance. The majority of faults result in a complete loss of communication with the CAN-C network. One of the first steps in proper diagnosis is identifying whether a CAN-C fault is present. The symptoms of a CAN-C communication network fault are not unique. Lost time and improper repairs may result if the concern is not properly verified and identified. Whenever a CAN-C communication network fault is suspected, the first step is to connect the scan tool and check for network communication and U-codes.
- FAULT SYMPTOMS AND DIAGNOSIS
- When communication hard faults occur on the CAN-C bus circuits, such as a short to ground or short to power, typically the vehicle will experience a no crank/no start with the MIL illuminated. U-codes may be found in any of the modules on the bus. Check power, ground, the connector, and both bus circuits to the suspect module before replacing the module. If communication on an entire network or networks is not possible, ensure the scan tool is functioning properly and operating with the latest software level.
There are several faults that can interrupt normal operation of the CAN-C communication network. The network wire may chafe, resulting in one or both wires shorting to chassis ground, shorting to power, or shorting to each other. Damage to a connector can result in the same faults. If either CAN (+) or CAN (-) is shorted to ground, voltage, or together at any place in the circuit, communication is not possible. When an open occurs in the circuit, any module downstream of the open will not communicate. It is possible for a module to fail, resulting in loss of communication with that particular module. If internally shorted, a module fault may bring the whole network down, resulting in no communication on the entire CAN-C network. If this type of concern occurs, typically the network will begin to communicate normally if the faulty module is disconnected. Remember to verify power and ground, as well as connector faults, before replacing any module.
- When communication hard faults occur on the CAN-C bus circuits, such as a short to ground or short to power, typically the vehicle will experience a no crank/no start with the MIL illuminated. U-codes may be found in any of the modules on the bus. Check power, ground, the connector, and both bus circuits to the suspect module before replacing the module. If communication on an entire network or networks is not possible, ensure the scan tool is functioning properly and operating with the latest software level.
- CAN BUS FAULTS
- There are five types of CAN Bus messaging faults, they are as follows:
TYPE DESCRIPTION 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/sending 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/sending 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.
- There are five types of CAN Bus messaging faults, they are as follows:
- CORRUPTION
- A certain type of fault called corruption exhibits unique fault symptoms. One type of corruption occurs when an open on CAN (+), CAN (-), or both, separates a dominant module or Star Connector if equipped from the network. Removing the dominant module (or Star Connector if equipped) greatly affects the optimal termination resistance. Failed modules may produce network corruption. When a module that has termination fails, it may attempt to communicate (bias) on only CAN bus (+) or (-), causing corruption. Another type of corruption is caused by EMI due to stray, high-voltage magnetic fields or untwisted communication wires. When corruption occurs, communication may still be possible, but erratic. When viewed with a diagnostic scan tool, corruption causes the network modules to flash between yellow and red, indicating they are communicating one moment but not the next. If one particular module stays red (not communicating) and does not flash, suspect a concern with either this module, its connectors, or the bus circuits leading up to that module.
Electronic Control Units require the following four components to properly communicate on a Controller Area Network (CAN) data bus: Power (Battery/Fused B+/Ignition), Ground, CAN bus voltage and correct vehicle configuration.
If the message detected conflicts with the CRC the ECU receiving it will determine the message to be an error and consider that communication has not been possible. Diagnosis of this condition using the Mopar Scope may reveal activity that appears to be bus data messaging, even if no actual communication is possible. Communication problems that affect the whole bus, as a result of opens and terminal push outs are more likely to occur on data busses that operate at a high speed than a data bus that operates at a lower speed.
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 Notes 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).CAN BUS TERMINATION RESISTANCE TABLE NON-DOMINANT 0 1 2 3 4 5 6 7 8 9 10 ONE DOMINANT 120.00 115.38 111.11 107.14 103.44 100.00 96.77 93.75 90.90 88.23 85.71 TWO DOMINANT 60.00 58.82 57.69 57.60 55.55 54.54 53.57 52.63 51.73 50.84 50.00 NON-DOMINANT 11 12 13 14 15 16 17 18 19 20 ONE DOMINANT 83.33 81.08 78.94 76.87 75.00 73.17 71.42 69.76 69.18 66.66 TWO DOMINANT 49.18 48.38 47.61 46.87 46.15 45.45 44.77 44.11 43.47 42.85
- A certain type of fault called corruption exhibits unique fault symptoms. One type of corruption occurs when an open on CAN (+), CAN (-), or both, separates a dominant module or Star Connector if equipped from the network. Removing the dominant module (or Star Connector if equipped) greatly affects the optimal termination resistance. Failed modules may produce network corruption. When a module that has termination fails, it may attempt to communicate (bias) on only CAN bus (+) or (-), causing corruption. Another type of corruption is caused by EMI due to stray, high-voltage magnetic fields or untwisted communication wires. When corruption occurs, communication may still be possible, but erratic. When viewed with a diagnostic scan tool, corruption causes the network modules to flash between yellow and red, indicating they are communicating one moment but not the next. If one particular module stays red (not communicating) and does not flash, suspect a concern with either this module, its connectors, or the bus circuits leading up to that module.