CAN-bus must have 120 Ohm resistor termination on BOTH ends of the bus to prevent "reflection noise" ruining the signal (and communications).
This means that DVM measures 60 Ohm
- with module connected to the actual CAN-bus, but power off
- (2 pieces of 120 Ohm resistors in parallel, effectively).
This procedure must always be followed:
In general just measure across CAN+/- with DVM (ignition off)
- if 120ohm, add 120ohm termination (so it will be 60 Ohm). Just a 120 Ohm resistor between CAN+ and CAN-
- if 40 Ohm, there is an extra 120 Ohm termination somewhere (3 in total, there should be only 2)
- 60 Ohm: good, only 2 terminators are there. Hopefully at the end of the bus (long CAN-bus will not work if terminators are not at the very end, both ends). Shorter CAN-buses are less sensitve.
If termination is OK (60 Ohm), watch aim2can moduls LED;
- if 1Hz blink all is well
- if 3Hz blink there is some problem (eg. termination BAD, or no acknowledge)