Information Bus: Description and Operation
Supplementary Information About Faults In the CAN Network
Supplementary information about faults in the CAN network
Information about "babbling control modules" and "disruptive control modules" in the CAN network
Babbling control module: A control module that sends correctly formed messages on the CAN cables when it should actually be in sleep mode and
as a result keeps the CAN network active. An obvious symptom is a discharged battery but no diagnostic trouble codes (DTCs) stored. A babbling
control module will not produce interference on the CAN network while the car is being driven. A control module with incorrect software can
occasionally produce symptoms similar to those from a babbling control module. This is detected using the fault-tracing method described below.
The usual solution is to load new software
Disruptive control module: A control module that sends incorrectly formed messages on the CAN network, resulting in disruptions in the traffic
between different control modules in the CAN network. This can result in the entire CAN network being shut down. Communication-related
diagnostic trouble codes (DTCs) are normally stored.
Note! When taking readings on discharged batteries, remember that:
The car can draw power from the battery for up to 20 minutes after the key has been removed from the ignition
The Volvo ON Call Plus, Phone module (PHM), will draw power from the battery for approximately 1.5 minutes every 15 minutes to safeguard
the remote unlocking function. This does not cause a major current drain from the battery.
When there are many CAN network-related faults, the fault is usually in the wiring. Diagnostic trouble codes (DTCs) E001 and E000 are usually stored
as a result of a short-circuit between the green and white CAN wiring.
E003 and CEM-1A51-1A66 are usually stored as a result of an open-circuit. CEM-DFxx codes are usually stored as a result of a short-circuit to ground
or supply voltage. Even if the above relate to the majority of the CAN-related faults, there have been cases where the control module itself has been the
cause of the problems.
Each control module on the CAN network has an in-built CAN communication circuit. This is connected to the CAN network and is the component that
regulates the voltage level at start up on the "high" CAN cable (CAN-H) and the "low" CAN cable (CAN-L) so that it is set to 2.5 V. When a control
module starts transmitting on the CAN network, the voltage on CAN-H increases to approximately 3.5 V and the voltage on the CAN-L drops to 1.5 V.
The traffic in the CAN network is redundant, which means that the messages can be interpreted by the receiving control module, even if the message
comes on one of the two CAN cables.
A correctly communicating CAN-bus should have an average value of approximately 2.8 V between CAN H and ground, just below 2.3 V between CAN
L and ground and approximately 0.5 V between both CAN-cables.
It is easiest to take the voltage readings using a multimeter. A VIDA oscilloscope can also be used to check the activity on the CAN network. However,
it is not possible to see individual pulses or packages on the CAN network if the resolution of the oscilloscope is too low.
An internal fault in a control module can result in the control module not communicating correctly on the CAN network. This will be detected by the
other control modules in the same section of the CAN network, which then start sending fault messages (known as "error frames").
In some special cases, the other control modules store diagnostic trouble code (DTC) E000 or E001 and then stop communicating on the CAN network.
An example of this is where a control module intermittently short-circuits CAN H to 3.5 V and CAN L to 1.5 V. The other control modules will detect
this and start sending "error frames", but do not necessarily store diagnostic trouble codes (DTCs). "Error frames" can be detected from dramatic
increases in traffic on the CAN network as described in Case 2 below.
"Babbling control modules" can discharge the battery because the CAN network is kept active. In individual cases, a "clicking sound" may be heard from
the passenger door module (PDM) and the driver's door module (DDM) every 30 seconds after the key has been removed from the ignition switch.
This clicking sound is typical when there is a babbling control module somewhere in the low speed network, although it is not necessarily the case that
anything will be heard. Babbling control modules are detected through increased activity on the CAN network and can be detected as follows:
Measure the average voltage value between the white and green CAN cables. This value is normally approximately 0.5 V
Close all the doors, including the hood and the tailgate, remove the key from the ignition switch and wait for at least one minute.
Note! Do not lock the doors.
Connect a voltmeter between the green and white CAN cables.
Note! The VIDA cart cannot be connected when taking this reading. If VIDA is connected, the CAN network will never go into sleep
mode as VIDA will ensure that there is still traffic. Use a free-standing measuring instrument instead and take the reading directly on the
The voltage between the green and white CAN cables must drop to 0 V. No activity (pulses) should be measurable on the network. If there is activity, cut
the power to the control modules that are constantly powered (Steering wheel module (SWM), Upper electronic module (UEM), Rear electronic module
(REM), Driver's door module (DDM), Passenger door module (PDM), "VOLVO On-call Plus" Phone module (PHM), Accessory electronic module
(AEM) and Central electronic module (CEM)) one by one in that order until the voltage drops to 0 V and no activity can be seen on the CAN network.
Use the relevant fuses for the control modules to cut their power. Do not disconnect them physically as this can affect other control modules on the CAN