J1939 is based on CAN. Both signals start at 2.5V, data (+) toggles to 3.5 while data (-) mirrors that going down to 1.5V. So depending on the amount of traffic on the network, data(+) will be just over 2.5 to 2.8V, while data (-) will be just under 2.5 to 2.2... Login to read more.