Checking the resistance of the control area network (CAN) cables

Fig 1: Identifying Ignition In Off Position And Disconnecting Negative Cable From Battery

Courtesy of VOLVO CARS OF NORTH AMERICA.

• Disconnect the battery negative lead. The network must not be powered when taking resistance readings. Otherwise an incorrect value may be obtained.
• Connect the adapter wiring to the central electronic module (CEM).

  HINT: Where a breakout box cannot be used, take readings from the rear of the affected connectors if possible. This prevents terminal pins and socket housings being damaged. 

  When taking readings on the low speed network 

  • When taking readings on the low speed network the adapter wiring should be connected to the Central electronic module (CEM) and readings taken from there. Readings can even be taken in the connector for the Power seat module (PSM). The advantage of the adapter wiring to the Central electronic module (CEM) is that all control module can be connected in the network when taking readings and access can be gained to both the high speed and low speed networks.

    The resistance reading only measures the main arteries in the low speed network, not the junctions that exist for different control modules.

  When taking readings on the high speed network 

  • When taking readings on the high speed network the adapter wiring should be connected to the Engine control module (ECM) and readings taken from there. The Engine control module (ECM) must be connected. This resistance reading only measures the main arteries in the high speed network.

  Connect an ohmmeter to the terminals for CAN-L and CAN-H on the relevant network. Turn the contacts and turn the cable harness. Resistance must be approximately 60 Ω.

  NOTE:

  Resistance will only be 60 Ω if the main artery of the CAN network is whole and the network is connected. If the main artery of the CAN network is split somewhere and a reading is taken between the CAN cables, resistance will be 120 Ω.

  Other information 

  • See Battery, disconnecting
  • See Connecting the breakout box
  • See Checking wiring and terminals
  Fig 2: Identifying Open-Circuit

  Courtesy of VOLVO CARS OF NORTH AMERICA.

Is the value OK? 

• YES 

  Refer to Checking a short-circuit in CAN cables 

• NO 

  Refer to Identifying fault causes in the CAN network