Theory Of Operation Can-IHS
WARNING: This page is about a different car, the 2022 Chrysler Voyager and 2022 Chrysler Pacifica. However, it is still accessible from the selected car via links, so may be relevant.
- CHARACTERISTICS OF CAN-IHS
- CAN-IHS is an event-driven multiplexed network that allows modules to share data. CAN-IHS is a medium-speed bus operating at 125 kbps. The voltages, termination resistance, and wiring configuration, are very similar to that of CAN-C. CAN-IHS is primarily used for interior and body systems. FCA vehicles that utilize a CAN bus architecture for module communication use either a CAN-IHS bus or another network called the CAN-B bus for interior module communication. While both CAN-IHS and CAN-B communication networks are utilized for similar purposes, modules on the CAN-IHS network are internally different from modules on the CAN-B bus. The CAN-IHS is wired using stub, or backbone, topology.
- 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-IHS
- Like CAN C voltages can only be seen using an oscilloscope. Each module on the CAN-IHS bus provides its own bias. This means that every module can apply power and ground on the data bus, allowing all modules to send and receive data. The active, idle, and sleep voltages are approximately the same for CAN-IHS as they are for both Diagnostic CAN-C, CAN-C and FD. The CAN (+) and CAN (-) will be biased to 2.5V. When active, or dominant, CAN (+) is pulled high to 3.5V, and CAN (-) is pulled low to 1.5V. The CAN-IHS bus is always awake when the ignition is ON. Multi Meter's will average the readings. When using a Multi Meter, in the active state, normal voltages levels on CAN (+) are approximately 2.5V to 2.7V, and on CAN (-) are approximately 2.5V to 2.3V. CAN-IHS can go into sleep mode when the ignition is in the OFF position. When in sleep mode, voltages on both CAN (+) and CAN (-) are 0V. Modules that are powered by an ignition-OFF draw (IOD) fuse can awaken the network if a switched input, such as a door ajar switch, is activated.
- CAN-IHS SLEEP VOLTAGES
- The CAN-IHS bus is awake whenever the ignition switch is turned to RUN. With the ignition in the OFF position, modules on the CAN-IHS network will time out and go to sleep. When the modules are asleep, voltages on the CAN-IHS bus will read 0V. The CAN-IHS can awaken without the ignition switch turning to the RUN position due to direct inputs, such as a door ajar switch, or bus inputs from another module.
- TERMINATION RESISTANCE
- Just like the CAN-C and FD network, termination resistance is used in the CAN-IHS circuit to separate CAN (+) and CAN (-) voltages and to absorb any stray voltage spikes throughout the communication network, preventing network signal interference. Optimal CAN-IHS bus total circuit 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 CAN-IHS non-dominant modules have some level of termination resistance, typically around 2.8-3.0 kilo-ohms. Remember that with the modules on the CAN-IHS network, the termination resistors are wired in parallel in the network. Because of this, the 2.8-3.0 kilo-ohms resistance in other modules will have some effect on total circuit resistance. Initially, all non-dominant CAN-IHS modules had some internal termination resistance. To permit common modules on multiple platforms, termination resistance is no longer present in many of these modules.
- TOLERANCE
- CAN-IHS has a limited fault tolerance. If either CAN (+) or CAN (-) are shorted to voltage or together, communication is not possible. When an open occurs on both wires in the circuit, any module downstream of the open will not communicate. CAN-IHS can withstand certain short to ground faults on the CAN (-) circuit. Communication can still occur because a voltage difference between CAN (+) and (-) is possible when the CAN (+) voltage is pulled above 0V.
- FAULT SYMPTOMS AND DIAGNOSIS
- When communication hard faults occur on the CAN-IHS communication network, such as a short circuit, open circuit, or an internally shorted module, the vehicle's windshield wipers typically will run continuously and the head lamps will stay on. U-codes will be found in the gateway module and possibly other modules. If a specific module is separated or drops off the network, loss of functions controlled by that module will occur, and U-codes will be set in several modules. When diagnosing a CAN-IHS fault, remember to:
Begin by measuring bus voltages and total circuit termination resistance. Check power, ground, and voltages on both bus circuits to any suspect module before replacement of that 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 Failed modules or an open circuit fault that isolates a dominant module may produce network corruption.
- When communication hard faults occur on the CAN-IHS communication network, such as a short circuit, open circuit, or an internally shorted module, the vehicle's windshield wipers typically will run continuously and the head lamps will stay on. U-codes will be found in the gateway module and possibly other modules. If a specific module is separated or drops off the network, loss of functions controlled by that module will occur, and U-codes will be set in several modules. When diagnosing a CAN-IHS fault, remember to:
- 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-IHS 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-IHS 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.