Page 452 - Foton Workshop Manual - Auman EST-M

DI - 398 Diagnosis - Diagnosis-fault diagnosis of electronic and electrical parts

CAN communication failure

Overview

1. CAN (controller area network) is a real-time application and serial communication network. It is a on-board
multi-thread communication network capable for high-speed data transmission and fault detection.
2. There are many electric control units on vehicle and these units are connected mutually via CAN
communication system to realize information share.
3. In CAN communication system, those control units are connected via two communication lines (CAN-H
line, CAN-L line), which can utilize the less conductor to realize the high-speed information transmission.
Each control unit can transmit/receive data information but read only the required data information.
4. The sensor data and system control signal can share with ECU to further reduce the use of line.
5. Two data communication modes are adopted on this vehicle: CAN conductor and K conductor.
6. Description of CAN bus:
a. CAN is the abbreviation of Control Area Network and its full name is controller area network bus to control
the mutual connection of equipment for data exchange.
b. The communication medium of CAN bus is twisted pair. At the end of twisted pair, there are two 120Ω
resistors. One end of high-speed CAN bus is located in engine ECM and the other end is located in
combination instrument.
c. In case of communication signal loss, the program will set the fault diagnosis code of communication loss
for each control module. This fault diagnosis code can be read by fault diagnostic apparatus.
Caution
● The fault diagnosis code of serial data loss does not mean the fault of the module setting this fault
diagnosis code.
● At two ends of CAN bus, 120Ω terminal resistor which can inhibit reflex will be connected. They play
an important role in matching with bus impedance. If this resistor is ignored, it will largely reduce the
anti-interference performance and reliability of data communication and even result in failure to
communication.
7. Description of K bus
a. K bus is used for diagnostic communication between external testing device and on-board ECU (such as
ABSECU). The transmission rate is 10.47kbps.
b. In case of signal transmission, the voltage will switch between 0V and 12V: 12V, logic "1"; 0V, logic "0".

Precautions
1. It is necessary to use a high-impedance digital multimeter to measure the resistance or voltage of
CAN bus.
2. Before inspection of circuit, turn off the ignition switch and disconnect the battery negative cable.
3. After CAN bus is repaired, use PVC tape to wrap the required wire harness.
Warning
● CAN-H conductor and CAN-L conductor must always be installed together. During installation, these
two conductors shall be twined mutually.
● If the bus fails to be twined, CAN bus signal will be easy to suffer noise interference. In case of
twisted-pair, two signals can present neutral to shield from external interference.
● When the bus is repaired, the peeling length of twisted-pair shall be less than 100mm.
4. When the broken circuit is repaired, do not replace the broken wire harness with a branch conductor.
Warning
If a branch conductor is adopted, the twisted pair will not protect the signal and the system signal
transmitted via CAN will not be correct.

DI-398