Diagnostics
This article is a true description of an AECS technical help desk problem and how it was solved.
PROBLEM PRESENTED TO THE TECHNICAL SUPPORT TEAM:
“I would like to measure the vehicle’s CAN bus low and would like to run the data through a CAN analyser to check for communication issues. Can you please assist me in carrying out this set up on my four-channel wireless scope?”
The workshop had an ATS WiFi 6004XM scope, a diagnostician who had completed the AECS CAN data bus training and subscribed to AECS technical support.
The diagnostician suspected there could be an issue on the CAN communication line between several ECUs and wanted to identify if this was the case.
MEASURE
We asked the technician to go into the scopes software and use one of the pre-set measuring templates that are connected to the wiring diagrams in the automotive test and information software provided with these scopes. The wiring diagrams make location of the appropriate wires easy as pin out information is provided. From the lists on right side of the diagram, an object tree on the screen pre-set measuring templates can be loaded into the scope by one click of a mouse.
The pre-set on the right sets up the scope ready to carry out measurements after running a test to check lead connections.
HOOK UP
Even though the pictures in ATIS are clearly showing, with photographs, how to hook up to the vehicle, we advised differently.
He had to back probe the channel one scope lead to the engine ECU connected to the CAN low and channel two to the transmission ECU’s CAN low wire, so the measurements of both channels could be compared for analysis of CAN low wiring.
The recorded pattern was posted to our forum for analysis.
DO THE MATH
The ATS scopes offers the unique ability to analyse the two recorded signals automatically.
The Math functions of the scope allow many analytical tricks, but also a simple option like Ch1 minus Ch2.
Please think about it; if you measure with two volt meters on both ends of the same wire, you should have on both meters the same voltage. Be it 12V, 2.5V, or 1.5V on both sides, does not matter. 12V – 12V = 0V, 2.5V – 2.5V = 0V, etc.
So if the wiring and connections are in good order you shall always have 0V as outcome on the math channel. In the recording the math channel (pink) is not looking like a 0Volt line! Let’s zoom in.
Zoomed in, it is visible that the math channel only shows very thin spikes when the CAN bus voltages switch. That is nothing to worry about, and has everything to do with wiring impedance, which we shall not cover in this article.
DECIPHER
What we did wanted to quickly see is if all CAN messages where transmitted and received by all ECUs connected to the CAN bus.
The difficulty is that only the CAN low was measured, and that for ‘normal analyses’ the CAN high is used.
MATH TO THE RESCUE (AGAIN).
The measurement software in the scope for CAN analysis needs to see rising signals, were the CAN low switches from 2.5 volts in a recessive state down to 1.5 volts when transmitting a bit.
With the mathematical functions within the scope known as I/O (input/ output) you can even modify signals to make for example the CAN low diagnostics still possible.
We selected gain/offset function to invert the CAN low for our CAN analyser to function. This is as simple as dragging in the gain/offset I/O into the analyser.
The function of the CAN analyser table is to decode all the signals recorded on the screen and create an alphanumeric representation so further analysis can be made of data transmitted.
Clearly visible in the Gain recording are for example the Acknowledge pulses. Those pulses are where other ECU’s confirm the data reception of a message.
In the CAN analyser the Acknowledges are represented by 1s.
Also the data pack ID is clearly visible, the ID is send by the ECU which send’s the data pack.
Nothing untoward was seen in this recording, we just wanted to share what we do here at the AECS technical help desk when a simple recording is sent in for evaluation.
CONCLUSION
Very quickly with the help of a few mathematical functions we could tell that the wiring on the CAN bus was intact and operating correctly. We could see that the data was being acknowledged on the recorded pattern and also on the CAN table. We could look at the CAN table and see that the same message data was not being repeated by the same sender. Much the same as when we have to repeat ourselves, it means someone is not listening. When computers have to repeat themselves the receiver is not listening (faulty). Something is wrong. No selective hearing issues here! Very quickly the technician could confidently eliminate CAN low as an issue. How long would this take you to determine?
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