Porsche CAN bus wave form discussion
- 2012 Cayenne S came into the shop because the suspension system was not leveling the vehicle properly as well as exterior lighting was not illuminating properly after the customer changed his own battery
- when connecting our Autel scan tool and attempting an “all system scan” there was some modules the scan tool could communicate with and many more that it could not communicate with
- connected my Snap-on oscilloscope to my DLC breakout box at pins #6 and #14
- CAN high was steady at 2.5V as well as CAN low steady at 2.5V
- with this info, my diagnostic approach was to trouble shoot a “shorted CAN bus”
- to do this, I simply went to each module and checked for 12V and a good ground and then proceeded to unplug the connector and watch my oscilloscope for change.
- after a BUNCH of time, the modules that were causing the grief was the front BCM and suspension module located in the right rear of the vehicle
- now having the suspension working properly as well as the exterior lighting, I did a second “system scan” and was able to talk to all the modules on the bus
- I thought now I would now have a proper CAN bus waveform at the DLC (i.e. 3.5V high and 1.5V low) - I did not. Still 2.5V on CAN high and 2.5V on CAN low
- there was a 2016 Porsche Cayenne Diesel and a 2017 Porsche Macan S at the shop while all of the was happening. I scanned the CAN high and CAN low with my oscilloscope and DLC breakout box - both vehicles had a 2.5V on CAN high and 2.5V on CAN low
QUICKLY, HERE IS WHAT I UNDERSTAND ABOUT CAN bus communication:
- CAN A - transfers data at a rate of 10 kBit per second
- CAN B - transfers data at a rate of 10-250 kBit per second
- CAN C - transfers data at a rate of 500 kBit per second - 1 megabit per second
- CAN D - a diagnostic CAN bus whereby data is transferred at a rate of over 1 megabit per second
HERE ARE MY QUESTIONS BASED ON THE ABOVE EXPERIENCE
- are all CAN bus wave forms (i.e. CAN A, B, C, and D) all the same (i.e. 3.5V high and 1.5V low)?
- Speed of data transfer sets CAN A, B, C, and D apart - true?
- Is it possible to some how use an oscilloscope to determine the rate of data transfer = you can determine if you are on CAN A, B, C, or D
- How the heck to you get a proper CAN bus wave form (i.e. 3.5V on high and 1.5V on low) from these Porsche products?
Thoughts appreciated and thanks in advance.
2.5v/2.6v is average when checked with slower tool ( multimeter, power probe). Can bus can‘t be 2.5v steady without issues. There would be no msg sent Or received with tons of faults. I don’t think vehicle would be operational in this scenario (no crank/no start/no drive) Perhaps your scop settings…???
Not sure what you mean by CAN A,B,C,D. When connected to pins 6 and 14, you are connected to the (one, only) legislated diagnostic CAN bus. The specified communication rate for this bus is 500 kBit/s = 2us/bit. Tomasz is correct, you most likely need to check your scope settings - usually 1V/div and something like 100us/div or less. Good luck. Cheers, Bob
Hi Robert. You are connected to the diagnostic CAN at the DLC. This is only active when your scan tool is communicating with the diagnostic gateway. You will see CAN activity if you connect your scan tool and carry out some operations while monitoring with the scope. Hth. Sean
Ah, yep, good catch Sean! You will only see bus activity on a scope during the request/response period… Cheers, Bob