I Think I CAN – Decoding the Bus

 

The 2014 Chrysler 200 Touring car originally arrived with a DTC U1503 (Implausible Message Data Length Received from TIPM). The shop checked the CAN B and CAN C circuits for open or shorted conditions, that the TIPM was configured correctly, powers and grounds at the PCM, TIPM and ABS modules, and battery voltage. The battery and TIPM were replaced. The ABS light would turn on intermittently.

An increasing number of computers and devices are being added to CAN buses and as more modules are added the available bus time becomes more occupied. When the traffic reaches around 40% of the bus time, errors can start to occur. At this point an oscilloscope may be required to debug the network.

What can cause errors? Voltage spikes and electrical interference from collapsing magnetic fields, inductors and power devices can all affect CAN communication. Data bits are fixed amounts of time. CAN data consists of a Start-Of-Frame (SOF) bit, an Identifier field consisting of 11 or 29 bits (indicating the nature of the data, such as engine PIDs, ABS data, etc.);

The Control field indicates the size of the Data field and contains what is called the Data Length Code (DLC). The Data field is the information transferred (such as engine rpm, water temp, oil temp, etc.). The Cyclical Redundancy Check (CRC) field is an error checking method to ensure the transferred data is not corrupted by any electro-magnetic disturbances. The Acknowledge field (ACK) is a very simple method of indicating to the transmitting computer that all the receiving computers have received the data uncorrupted. Then there is the End-Of-Frame bit.

The particular areas of interest for reading data from the CAN Bus are the Identifier and Data fields. The PICO Scope is capable of serial decoding and I used it to decode the CAN data bus. The IDs and Data are shown in coded hex (Hexadecimal), which is standard in the digital communications industry. I noticed that the bus frame rate and bus load of this CAN Bus system was shown as approximately 28%. Max bus load of a Powertrain type system is usually around 40%. I noticed that at 64.87 seconds after the start of collection, a few Error frames began to occur and then disappeared.

We started moving connectors and when we moved the ABS module connector, multiple Error frames appeared on the PICO Scope. Upon closer inspection of the 47-way connector, Pin #12, which is CAN C (+), had green corrosion, barely discernible, until I inspected it with my 40x, illuminated loupe. These days a quality magnifier is a must!