Honda Accord suddenly dropped the engine tachometer and water temperature display on the dashboard to 0

Q: A 2003 Honda Accord CM4, the vehicle condition has been relatively stable, and the mileage is 170,000km. Recently, after driving on a mountain road for about 2 hours, the engine tachometer and water temperature display on the dashboard suddenly dropped to near 0, and the vehicle began to swing violently. Then the air conditioner did not cool, and only hot air blew out. At that time, the driver reduced the speed to about 40km/h, and the vehicle fault phenomenon did not improve. After parking for about half an hour, the vehicle was restarted and the instrument display was normal, and the air conditioning system also resumed normal operation. After the vehicle continued to drive for about 100km, the above-mentioned air conditioning did not cool and the instrument was abnormal. During the maintenance, the instrument, air conditioning related control system and grounding circuit were checked, and no abnormalities were found. The cooling pressure of the air conditioning system was also normal. The Honda HDS tester was used for testing, and three fault codes “U0155”, “B1178” and “B1168” were called out. Among them, U0155 means that the fast communication network from the instrument control module to the engine computer is abnormal; B1178 means that the F-CAN communication line is wrong; B1168 means that the instrument control module has lost communication with the engine ECM/PCM. What is the specific connection between the fault code called out and the fault of this vehicle? Please guide how to repair the fault of this vehicle.

Answer: The meanings represented by the above three fault codes are all related to information communication, that is, related to the structure of the multi-channel communication network system of this vehicle. The network system of this vehicle is divided into two parts: B-CAN body control LAN and F-CAN power fast control LAN. The B-CAN communication network transmits at a lower rate of 33.33kb/s, which is suitable for air conditioning, door locks, audio and anti-theft devices. The data transmission rate of F-CAN is higher, reaching 500kb/s, which is suitable for the transmission of power data such as engine ignition, fuel and emission systems, and transmissions. The network structure of low-speed B-CAN and high-speed F-CAN is shown in Figure 1. Since the transmission rates of the B-CAN and F-CAN buses are different, in order to achieve data sharing between the two networks, the instrument control module also acts as a gateway, responsible for the data conversion between the B-CAN and F-CAN networks.

The Accord CM4 sedan uses a digital instrument. The various sensor signals of the engine are first sent to the engine ECM computer, and then through the communication data of the F-CAN, the information is sent to the dashboard network controller. After being processed by the instrument control module, it is transmitted to the instrument display device.
The remote guidance of the vehicle maintenance process is as follows. Check the communication line, the prompt content is “ERROR1”, indicating that the communication between the instrument control module and F-CAN is faulty. Press the air conditioning A/C switch, the air conditioning panel shows that it is running normally, the indicator light is also normal, but the air conditioning compressor does not run. Use the HDS tester to check the dynamic data flow of the air conditioning system, showing that the air conditioning switch signal is normal. Then test the dynamic data flow of the engine, showing that the air conditioning is still in the off state, indicating that the engine computer does not receive the air conditioning signal. Why do the two have contradictory results?
Check that the communication line from the air conditioning control module to B-CAN is normally connected. Remove the lower trim panel of the right instrument panel, then disassemble the engine computer, and provide a grounding signal to the E18 terminal of the engine computer. At this time, the air conditioning compressor can operate normally and resume cooling (Figure 2).

Remove the instrument control module, unplug the 30-pin connector on the module, and check whether the plug and terminal seat have any bending, loosening or rusting of the terminals. No abnormality was found in the inspection. No fault point was found in the power supply, grounding and transmission line of the connector. From the analysis of the fault phenomenon: the engine tachometer and water temperature display numbers are around 0, and the air conditioning system does not cool. It is suspected that it is caused by intermittent failures in network information transmission or reception. Preliminary analysis shows that the transceiver inside the instrument control module has intermittent failures. Try to replace the instrument assembly. After clearing all fault codes, the vehicle fault is completely eliminated.
The vehicle does not have an independent gateway, but uses the instrument control module as a gateway. If its communication transceiver head has intermittent failures, the high and low speed transmission network transceivers will also have failures that cannot communicate normally. The intermittent failure of the air conditioner in this vehicle is due to the fact that the instrument module receives the air conditioning cooling signal from the air conditioning control module of the B-CAN, but fails to correctly send the command to the engine computer through the user CAN network. The engine computer cannot provide the grounding signal to the air conditioning compressor relay, so the compressor cannot operate. At the same time, the instrument control module cannot receive the signals of various sensors transmitted by the engine computer through the F-CAN, so the instrument display is abnormal.
Here is a reminder for everyone, when replacing the instrument assembly, the mileage of the old instrument must be read with the HDS tester and written into the mileage of the new instrument.