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I have a M.Bz clk55amg getting a no com error Checked break out box got power,grounds are all good no activity on can bus I check the resistance on can bus and I got 72.8 and got like what does that mean maybe a partial short somwhere any suggetion of what direction to go from here I got power and ignition power and grounds are good to ecu. I did have to resolder ground connection after that the windows started working right and the seats are now moving bu still a no com
I don't love that number but it may be acceptable, if someone else wants to chime in.
Not sure if my thinking is right here, but do you have any activity on any can lines at any modules? Just wondering if you can see signals going out, but maybe not transmitting on the main line if that resistance is too high?
I also don't love 72.8 ohms. :lol: That wouldn't be a short, but excessive resistance at some point.
KOEO, what are the CAN high and low voltages? I know you said there's no activity, and that's fine. I'm interested to know if the bias voltage is there or not.
The purpose of the termination resistors for impedance matching of the source and the load. The transmission lines have resistance, capacitance, and inductance. This only matters because this is a high-speed signal.
Low-speed GMLAN (which is single wire CAN) is fast, but slow enough that termination occurs in the modules.
Another way of putting it is that the termination resistors eliminate signal reflection which corrupts the signals. This doesn't mean however that you will get no communication with the wrong resistance.
If you have NO communication with 72Ω of resistance, I would NOT suspect the resistance value as being an issue.
What I would want to do is scope the signals. You probably have a module putting out trash on the network or distorting the signals.
I would start looking at the Modules/Topology and see what all is on the network(s) gateway that could potentially be taking the CAN communication down. Someone mentioned already about the bias voltage the question is what are the readings for the bias voltage?
