ASE A6 Electrical Electronics Practice Test
9. The resistance between pins 6 and 14 of the OBD II DLC connector above measures 60 ohms. Technician A says this is correct. Technician B says the modules must be powered down or the battery disconnected to test resistance in the CAN bus. Who is correct?
- A. Technician A
- B. Technician B
- C. Both A and B
- D. Neither A or B
Answer A is wrong. CAN buses use twisted pair wires that reduce noise and interference. Twisted pair wire has an impedance of around 120 ohms.
Answer B is wrong. All modules must be powered down or the vehicle's battery disconnected to measure resistance in the CAN bus circuit. Testing a powered circuit can damage the ohmmeter.
Answer C is correct. To prevent reflection, the CAN bus has two 120 Ohm resistors placed in parallel. They are often inside a control module like the ECM. This configuration results in 60 Ohms of resistance on the bus.
Answer D is wrong. It is 60 ohms because two 120 ohm resistors installed parallel between CAN HI and CAN LO have an equivalent resistance of half the value of either resistor or 60 ohms.