Dodge CAN Messages

Objective

This article describes some observations about the data on a newer Dodge Ram pickup. The data are from a simple vehicle dynamics tests from a drive around the block. The work here assumes no prior knowledge of the meaning or interpretation of CAN messages. The following signals are desired in this work:

Some of these signals need more study to determine attribution (i.e. we do not know which of the wheel speeds correspond to each wheel). Also, only the raw data are presented, so calibration is needed to analyze CAN data using engineering units.

Physical Access to the CAN Bus

Dodge is nice enough to publish the electrical wiring diagrams for their work vehicles from 2004 and newer online at http://www.dodge.com/bodybuilder/year.pdf. Select the year, select the vehicle, and select the body style. The year of interest may be incomplete if there were no major redesigns for that model year. For example, to get the wiring diagram for a 2010 Ram, you may need to examine the diagrams for the 2009.

Once you have the option to access the wiring diagrams, it is relatively straight forward to explore the different systems. To read the diagrams, a guide is provided labeled wiring diagram information. The wiring diagram has many codes used for labeling circuits. These codes are described in a document called Wiring Code Identification Information. Finally, you will be able to navigate to the diagram explaining the Bus Communications.

In the Bus Communications diagram, the overall picture is shown below with three distinct CAN bus circuits: Diagnostic CAN, the high-speed CAN C, and the low-speed CAN IHS or "Comfort CAN".

Image of CAN topology

The high speed CAN runs at 500kbps and the comfort CAN runs at 125kpbs. Both use 11-bit identifiers.

The first attempt to access the CAN bus was through the diagnostic port. However, no live data was present. Clearly from the picture above, the CAN C network contains the live data that is shared among the electronic control modules. The following diagram shows part of the CAN C network.

Section of the CAN C network

This diagram shows the CAN C bus is made from two twisted pair wires that are 22 gauge in diameter. The CAN C Bus (+) wire is white with a light green stripe and the CAN C Bus (-) is white with a light blue stripe. Interestingly, the diagnostic port uses a twisted pair of white with dark stripes. So if two sets of white striped twisted pairs are in the same bundle, the twisted pair with the lighter stripes will be the CAN C bus that has the live data. In summary:

CAN C (+): white with a light green stripe
CAN C (-): white with a light blue stripe

The following photo is taken of the wires that are available near the driver's side firewall in the engine compartment. The red material is liquid electrical tape that was applied after the CAN communication wires have been tapped.

CAN C Wires

With physical access to the network, the network traffic can be monitored and recorded.

Example of a Drive Around the Block

With the network monitored, external measurements are needed to correlate signals. For this a Racelogic VBox 3i was used to record the vehicle speed. The following speed record was obtained.

Speed Record Around the Block

The test sequence started with a pickup poised to drive away from a normal residential driveway. After pulling out of the driveway, a series of right hand turns were made to complete the route. Then the pickup was backed into the parking space. The map of the path of travel with the X marking the starting position on the time graph is shown below.

Travel Map

The CAN data from the 500k CAN bus was captured using a Dearborn Group Gryphon S3. The 14Mb raw data file is in a comma separated value format as exported by the DG Technologies Hercules software. Since this data file is large and contains all message traffic, decoding it seems like a daunting task. However, the messages are broadcast using an ID and the ID usually determines what purpose the message has. Therefore, if we can filter by ID, then we can examine the data more efficiently. By efficiently, we mean using plotting tools to generate patterns that can match the dynamic signals measured with external devices. To this end, we created a Python program that takes advantage of the matplotlib tools that parses the CAN data, filters by ID, and plots many combinations of the data. The Python program to decode the data record is described in the next section.

Python CANDecoder Script Download Link

Since it is difficult to evaluate data in a time series by looking at hexadecimal numbers, a method to visualize the data was needed. To this end, a python program was written to parse a CAN data file from the Gryphon S3. The first section of the program evaluates all the CAN IDs, data lengths, and their nominal frequency. For this example, the script produced three pdf files with graphic representations of various byte combinations.

Byte Plots For Drive Around The Block.pdf

Even Byte Plots For Drive Around The Block.pdf

Odd Byte Plots For Drive Around The Block.pdf

There are 55 unique IDs in the log file that are summarized in the following table. The timing data in the table below is approximate. Byte counts start at 0 and go to 7 for an 8 byte message.

CAN ID (Hex) CAN ID (Dec) Data Length Frequency (Hz) Period (sec.) Notes
0A0 160 1 10.0173 0.0998271  
101 257 4 7.49224 0.133472  
111 273 6 0.00488412 204.745  
1C0 448 6 5.02087 0.199169  
1C6 454 7 0.99636 1.00365  
1D0 464 6 0.781459 1.27966  
1E6 486 7 0.99636 1.00365  
200 512 8 37.3586 0.0267676 Speed like signals on Bytes 2-3, 4-5 and 6-7
208 520 8 37.3586 0.0267676 Speed like signals on Bytes 4-5 and 6-7
210 528 8 37.4514 0.0267013 Byte 2 has brake pressure like signals
212 530 8 37.4514 0.0267013  
215 533 7 37.3635 0.0267641

Speed like signal on bytes 0-1
Distance like on bytes 2-3 and 4-5

218 536 8 37.4514 0.0267013  
236 566 8 74.6781 0.0133908 Steering data on 0-1
248 584 8 49.9889 0.0200044  
300 768 8 37.3635 0.0267641 Signed signal for 6-7
308 776 8 74.9028 0.0133506  
312 786 8 74.9028 0.0133506 Engine like data
315 789 6 49.9889 0.0200044  
325 805 7 49.9889 0.0200044  
328 808 8 37.3635 0.0267641 Byte 2
330 816 8 37.3635 0.0267641  
338 824 8 37.4514 0.0267013 Speed on 0-1, 6 and 7 (maybe steering?)
358 856 8 37.3488 0.0267746  
392 914 8 9.99779 0.100022  
3D0 976 4 7.49224 0.133472 Bytes 0 and 1 have different dynamic data
3E4 996 3 0.99636 1.00365 Bytes 1 and 2 are distance like signals
3E6 998 1 1.9976 0.5006  
3FB 1019 5 1.00613 0.993909  
3FD 1021 8 0.49818 2.00731  
3FF 1023 6 19.9956 0.0500111 Dynamic on Byte 0
408 1032 8 9.99779 0.100022  
410 1040 8 49.9889 0.0200044 Low resolution dynamic on byte 1
415 1045 7 7.49224 0.133472 Dynamic signal on byte 1
418 1048 8 37.4514 0.0267013  
428 1064 7 37.3488 0.0267746  
445 1093 6 16.6646 0.0600074 Dynamic level sensor like data on 0
4A0 1184 8 7.49224 0.133472 All dynamic signals
4D0 1232 4 0.99636 1.00365  
520 1312 4 7.49224 0.133472  
5B0 1456 8 2.00737 0.498164  
5FE 1534 8 0.99636 1.00365  
608 1544 8 7.48735 0.133559 Dynamic signals on bytes 0, 5, and 6
67F 1663 8 0.49818 2.00731  
6F9 1785 6 0.49818 2.00731  
6FA 1786 8 7.48735 0.133559  
6FD 1789 8 0.49818 2.00731  
6FE 1790 8 7.48735 0.133559  
6FF 1791 8 1.9976 0.5006  
700 1792 8 3.57029 0.280089  
702 1794 8 3.57517 0.279707  
706 1798 8 3.57029 0.280089  

The following figures contain dynamic signals extracted from the CAN data. These are contained within the pdf documents generated by the Python script. There were 6 different speed signals that match the pattern from the VBox record.

 

208-6-7

The following is likely an accelerator pedal signal.

Steering like signals:

RPM like signals:

Distance signals:

The following signals are unknown:

In summary, there are many signals on the CAN that contain dynamic signals that can be useful for studying vehicle dynamics. The data presented so far has not been calibrated. However, identifying messages of interest is the first step.