CAN J1939

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CAN J1939

Postby Andrus » Thu Feb 25, 2016 6:43 pm

Hello!

Measured CAN+ and CAN- voltage with multimeter and it was between 2,3 and 2,7 V at engine J1939 connector.( No engine connected. Good.
Then hooked up PICO and captured CAN H and CAN L . Graph is not nice and solid as in PICO guidelines, but it least look as mirror and voltages are in range. I do not like at all voltage rising in graph. Attach CANline capture.

Can anybody give comment regarding CAN capture ?
Attachments
CAN capture.JPG
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Re: CAN J1939

Postby andrewbishop66 » Sat Feb 27, 2016 8:15 pm

Hi Andrus
with the vehicle powered down what's the resistance between can + and can - should be 60 ohms approx with the modules connected that contain the termination resistors if you lose one of the resistors you can get some weird bounce back signals

Andrew
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Re: CAN J1939

Postby Andrus » Sat Feb 27, 2016 9:58 pm

Resistance is 60 ohms, it was first checking.

Got comment that even if CAN capture is not looking perfect, CAN nodes must accept it.. it is still good enough.
I start troubleshooting again from scratch. :oops:
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Re: CAN J1939

Postby FioranoCars » Sun Feb 28, 2016 9:28 am

I don't know your vehicle, but most CAN-BUS termination is 120ohms (at least the cars we work on, niche maybe, but can is can)

Often the resistor is easy to remove too, so we can test with and confirm 120 ohm, remove it and confirm no continuity.

If your bus has two resistors (one at each end) then parralel law would provide that 2 x 120ohm give a result of 60ohms... That might explain it, but I would confirm that the 60ohms is correct, just to save chasing your tail.

HTH

Richard
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Re: CAN J1939

Postby Fat Freddy » Sun Feb 28, 2016 11:06 am

Has the original post been heavily edited? I seem to remember something about 18V's or am I getting confused?

Do you have a specific problem or are you concerned about rising voltage?

Was that capture with or without engine ECU connected?

FF
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Re: CAN J1939

Postby Andrus » Mon Feb 29, 2016 9:06 pm

It is hard to tell what is going on in CAN bus, but I got machine running. There were broken and almost broken wires. Also, CAN wiring itself not perfect but working. Confusing is, that everywhere are CAN pictures with PERFECT signals and You think that that is the way it MUST be.

Yes, there was 18 volt in CAN wiring when +24 volts applied to wiring, No controllers attached.... very confusing.
Anyway, engine running as I can say today.

Attach also CAN capture.
Attachments
CAN capture.JPG
Actually working CAN bus
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