Computers/Controls - CANbus Fault Diagnosis
NO: 37-25DATE: 11-08-2002
MODEL/YEAR: MY99- S80; MY01- S60, V70, and V70XC
SUBJECT: Battery Drain and CANbus Disturbances
CHASSIS:
REFERENCE: TNN 37-22
DESCRIPTION:
This Tech Net Note is intended to provide additional information to assist in the diagnosis of CANbus related faults.
Definition:
Babbling node:
A node that sends correct messages (frames), when it should be in sleep mode, thus keeping the CANbus active. The symptom is typically a battery drain, but no DTCs. A babbling node will not cause CAN disturbances while driving. A node with wrong SW may sometimes have symptoms similar to a babbling node(1). This will be detected by the same method described below, but the remedy is to download correct SW.
Disturbing node:
This node will send incorrect frames and will cause error frames on the CANbus. A disturbing node may cause the CANbus to collapse. Communication related DTCs are typically set.
NOTE:
When measuring battery drain, be aware that:
^ The final parasitic draw may not stabilize until approximately 20 minutes after the key is taken out of the ignition.
^ The "Volvo On-Call Plus" (PHM) will have a current draw for 1.5 minutes every 15 minutes related to the remote unlock feature. This feature will not cause excessive drain on the battery.
SERVICE:
Normally when there is a number of CANbus related faults, the root cause is a problem in the cable harness. DTC E001 or E000 codes are normally related to a short between green and white wire, E003 and CEM 1A51-1A64 to open circuits (e.g. a loose terminal), and CEM DFxx codes are shorts to ground or battery. See TNN 37-22 for information about CANbus fault-tracing in general, this will resolve the majority of CANbus related faults. Although the majority of CANbus related faults can be diagnosed as above, a few cases have been identified in which a specific node was the root cause.
(1) - Wrong software can be due to an unsuccessful "Reload" after hardware replacement (See TNN 34-05 for more information).
Every node on the CANbus has a CANbus controller on the circuit board. This is the interface to the CANbus and it is the component that sets the level of 2.5V as the base (start) voltage for both high side and low side. As a node start sending a message, the high side will go up to approximately 3.5V and the low side will simultaneously go down to 1.5V. The signals are redundant, i.e. a message will be interpreted by the receiving node even if it only arrives on the CAN high side.
A properly communicating CANbus should have >2.5V average between white wire to ground, <2.5V average between green wire and ground and >0V average between green and white wire. The average voltage can be measured with a voltmeter. Although the VADIS oscilloscope can be used to see activity on the CANbus, it can not be used to see single pulses or even frames (CANbus data packages).
With an internal fault in a node, it is possible that the node does not properly communicate on the CANbus. This will be detected by the other nodes and they will start sending error frames. In some cases, the other nodes may store DTC E000/E001 and stop communicating on the CANbus. An example would be if a node intermittently shorts the high side to 3.5 V and low side to 1.5 V. The other nodes will detect it, start sending the error frames, but will not necessarily set a DTC. The error frames can be detected by the dramatic increase in CANbus activity, which will be described in case 2 below.
1. "Babbling nodes" may cause battery drain by keeping the network alive. In some cases, an audible "clicking" from PDM and DDM may be heard every 30 seconds after the key is taken out of ignition. The clicking is a clear indication of a babbling node somewhere in the low speed CANbus, but may not always occur. Babbling nodes can be detected by an increase in activity on the CANbus, and can be identified as described below.
^ Measure the average voltage between green and white wire in the low speed CANbus. During normal operation the voltage should be approximately 0.5V.
^ Close all doors, including hood and trunk, take the key out of the ignition and wait at least one minute. Do NOT lock the door.
^ Using the VADIS oscilloscope set to the smallest time base and 1 volt per div., the voltage between the green and white wires should go down to 0 volts. There should be no activity (pulses) measured. If there is activity, depower the battery fed nodes (SWM, UEM, REM, DDM, PDM, "Volvo On-Call Plus" (PHM), AEM and last GEM) one by one, in that order, until the voltage goes to 0 volts with no activity on the oscilloscope. Make sure not to depower the nodes by disconnecting them since that may cut off other nodes further down the CANbus. Use the fuse, but pay attention that PDM and DDM are fed through the same fuse.
^ The node that was last depowered when the CANbus voltage went to 0 and activity stopped on the oscilloscope is most likely the culprit. In case the PDM/DDM are considered as faulty, they can be disconnected from the CANbus since they are not in series. That will show which one is faulty, by noting which disconnection causes the CANbus to go to 0 volts. Both the DDM and PDM should not be replaced at the same repair.
^ Make sure to verify that the node was babbling by repeating the test, with and without the node powered up.
If the suspected node is only electrically fed when the key is in the ignition, this method can not be used since these nodes would not cause a battery drain.
2. "Disturbing nodes". The advantage with these cases is that there is often a DTC set, typically DTC EOO1 and GEM 1A51-1A64. The below method should be used in conjunction with TNN 37-22 to distinguish between a wiring related fault, which is most common, and a "disturbing node". The customer symptom is normally that the DIM gauges went down (which is the common "customer states -" symptom for all CANbus faults).
Both "babbling nodes" and "disturbing nodes" may be very intermittent and when the car is in the workshop the fault may not be reproducible.
In certain cases, a "disturbing nodes" may not set DTCs. In such a case, use the following method:
There is a threshold in which a "disturbing node" will cause error frames on the CANbus, but will not collapse the CANbus. The following method can be used to identify a "disturbing node" whether or not it collapses the CANbus.
^ Measure the voltage between the green and white wire in the relevant CANbus. In a normal case, key in position 2, the average voltage would be approximately 0.5V.
^ If a node is disturbing, the voltage is likely to exceed 0.7V.
^ If so, depower the nodes in the same way as described above to find the root cause.
^ Make sure to verify that the node was disturbing by repeating the test, with and without the node powered up.
Note! After a "babbling node" or "disturbing node" failure, some nodes may stay in a limp home mode also after the key is cycled. To make sure that you recover the node from the limp home mode, disconnect and reconnect the battery.