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I am trying to connect to the CAN bus of a heavy trucks 24V OBD2 port.

I have already tested my setup with a van, and confirmed it working.

The only differences I can see between the 2 vehicle's OBD ports is that in my truck there is 0v difference between the CANH and CANL lines, and 0.3 on the van that worked.

I can't find anything online around the "issue"(if it even is one) of no voltage difference between can lines.

The truck works fine, and there are no issues reported in the instrument cluster.

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The CAN bus is differentially driven, that means whenever the CAN+ line is high the CAN- line is low and vice-versa. So the average voltage between them is always going to approach 0V. Your 0.3V is close enough to 0V and is within the normal range for what would be expected.

A voltmeter, therefore, is not the correct instrument to test a CAN bus system.

If the CAN bus was down in a modern vehicle, you would have multiple failures going on throughout the vehicle as various pieces of the electronics would be unable to communicate.

If you need to diagnose to some degree and lack a CAN bus analyzer, an oscilloscope would be a good choice.

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  • Thank you for responding. I understand that and unfortunately don't have access to proper tools like an oscilloscope. From reading online, I should see a value of 2.2 for the low and 2.7 for the high line when compared to ground. In my case both of these are 2.45v. Also, the resistance between them with the battery disconnected is 120 ohm. I also tried using the 120ohm resistor on the mcp2515 board but no luck Commented May 2, 2020 at 11:08

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