Identifying a Control Module In the CAN Network

Fig 1: Checking Traffic Load On CAN Cables

Courtesy of VOLVO CARS OF NORTH AMERICA.

HINT: Use the wiring diagram to provide support when taking readings.

For general information about CAN-troubleshooting, see:

See General Information About Fault-Tracing In the CAN Network

HINT: DiCE can be used as fault detection tool if fault is suspected on the CAN-net that cannot be detected with normal troubleshooting. Interference, intermittent faults, faulty messages on the CAN-net (chattering control modules) etc., are examples of such faults. The tool registers fault messages used on the CAN-net. Fault messages may be caused by short intermittent malfunctions on the CAN-net or by a control module sending fault messages. See tools: 951 3010 (memory card) as well as associated TJ 20402.

If the load on the CAN network is too high, this may be due to a control module sending faulty messages on the CAN network (known as a disruptive control module). The fault may also be due to an intermittent short-circuit to supply voltage or ground in one of the CAN cables. This results in the control modules on the CAN network starting to send faulty messages. The faults can occur without a diagnostic trouble code (DTC) being stored by the control modules. The voltage can be measured between the CAN cables in order to check the load on the CAN network.

NOTE: It is not necessarily the control module(s) that stored the diagnostic trouble code (DTC) that is communicating faultily. Faulty messages can come from any control module in the same section of the CAN network. However, faulty communication in one network cannot affect the communication on the other network. In other words, faulty communication on the LS-CAN cannot disrupt communication on the HS-CAN and vice versa.

Ignition on.
Connect OBDII-Box (951 3015) to the diagnostics socket.

Check communication/load on CAN-net by measuring voltage between CAN-L and CAN-H. Measure voltage between the following connections on the OBDII-box/diagnostics socket.

#6 and #14. The average value of the voltage must be approximately 0.6-1.4 V. The voltage shall normally be between 0.55 - 0.90 V. A higher voltage may indicate increased traffic on the CAN-net. In case of fault in communication/too high traffic on the CAN-net, the voltage is considerably higher than the above stated values.

In order to identify which control module has faulty communication, cut the power supply to one control module at a time. Check the voltage in the CAN cables again. The easiest way to cut the power supply is to remove the fuse(s) for the relevant control module. Repeat until the voltage between the CAN wiring drops to the normal level.

NOTE: When connecting and disconnecting individual control modules, other diagnostic trouble codes (DTCs) may be stored in certain control modules.

HINT: When the suspected defective control module has been identified, connect and disconnect the supply voltage to the control module a number of times to verify that the malfunction appears and disappears. Switch off the ignition between each check.

NOTE: Do not cut the power to the central electronic module (CEM) unless it is the last control module left to check.

A control module that is communicating incorrectly must be replaced. See Repairing and installing/Removing, replacing and installing.

NOTE: After fault with control modules that interfere with communication, other control modules may remain in Limp-Home (that is, they work with reduced functionality) even when the ignition has been turned off and on a few times. For certain faults in the CAN-net where control modules have stopped communicating, it may help to disconnect/reconnect the battery to "restart" the control module/CAN-net.

Fig 2: Checking Communication/Load On CAN-Net Between CAN-L And CAN-H

Courtesy of VOLVO CARS OF NORTH AMERICA.

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Refer to Verification