Metering the data cable
You can check a CAN network with a regular meter. Healthy readings are simple, and a bad one points straight at the problem.
The short version
- Power on: each data line to ground reads about 2.5 V. CAN-H sits a little higher, CAN-L a little lower.
- Power off: across the two data lines reads about 60 ohms. That's two 120 ohm terminators in parallel.
- About 120 ohms means one terminator is missing or the cable is open between you and it.
- Near 0 ohms, or a line stuck near 0 V or 12 V, is a short. Unplug modules one at a time until the reading goes normal.
Numbers
- CAN-H to CAN-L, power off: about 60 Ω (two 120 Ω terminators)
- Each line to ground, idle: about 2.5 V
Try it on a service call
Put the lesson to work on a hands-on troubleshooting case.