GND to meassure CAN

I have a question about CANbus.

Do i need a GND to measure the differential value of canH and canL?

For example, i have an socket where i can plug in to read can, it only has CanH and CanL on it.

Is it possible to sent and receive on the bus like this with a diagnostic tool? Or is it possible this gives bus errors while doing so? Or dont i get any signals?

CAN is a differential bus. The signaling is on the two wires relative to each other. Either the CANH > CANL or CANL > CANH. There may be multiple devices on the bus simultaneously (so it's OK to plug in a CAN sniffer to examine the signals), or another device that both receives and transmits on the bus.

All the devices on the bus must operate at the same bit speed (baud rate). So attaching a diagnostic device that transmits at the wrong rate can cause bus errors. A common mistake is to get reverse CANH and CANL, so if you are hooking something up for the first time and it sees errors, try swapping the signal wires.

Hopefully that is helpful -- let us know what exact devices you are trying to use and the environment and perhaps we can provide some more guidance.

You may find that this wikipedia article will fill in some details:

https://en.wikipedia.org/wiki/CAN_bus

I am aware of how that all works, let me just refraise that, english isnt my main launguage so forgive me

If i use for example an can controller, which controls a ac motor. it always has gnd and a (12v) + signal. So it always had a gnd to compare its can h and can l. For example:

Gnd - can h is 2,5V

Gnd - can L is 1,5V

Difference is 1V.

It is possible to mearuse this without gnd, but how accurate is it? Could it cause problems during high bus load?

I am working as a machine builder for custom made logistical electronic vehicles, only for a half year now and never done this type of electircal work before. And i can build them, i just dont engineer them. We have 2 engineers who design the electrical systems, all scematics and such.

But we have a lot of problems with faults during flashing of can controlled hydraulic valve units. This happens over can causing high bus load, and in this case while flashing the hydraulic valve systems arent connected to gnd. So after looking over the electrical scematics a few times i began to notice that the socket on wich we plug in to flash isn't connected to ground either. And the unit is connected to an external power supply while doing this procedure.

Now i know that for example an serial interface with rx/tx wires need gnd in order to see the signals. So i was wondering if having no gnd could cause problems during high busload. And the engineers at work couldn't give me an answer actually, who studied automotive enginering... so i thought, maybe HPA can help me out with this one

A simple voltmeter will not work to measure the differential voltage on an active CAN bus. You need to use an oscilloscope with CAN low for the ground, and CAN high for the measured signal. You could also connect two channels, and reference to a ground at any device on the bus. High bus load just means that the there are many messages and the bus isn't in the idle state as much.

It does sound like your equipment could have ground loops, with different devices at different ground references. Again an oscilloscope it useful to measure this type of noise, and you can also attach a current probe to see if there is current flowing where you don't expect it.

I have experienced problems using laptop computers to connected to grid mains grounds as well as through USB to another device. Running the laptop on batteries allowed more error-free communication.

Good luck with your fault finding...

Thanks for the tips so far! Fault finding will continue but it is a difficult one. Since it isnt always, just occurs random that flashing isn't working properly and it gives the strangest errors.