If the above problem occurs, check the following:
1. The VCT2000 cable. This cable has given problems before. Try with a different cable and a different VCT2000
2. Continuity and signal level on the communication cable
3. Continuity and signal level in the CAN cables between the connector and the central electrical module (CEM).
If points one to three do not produce a result, the fault is probably a permanent cable fault in the CAN network or in a control module. Continue
fault-tracing according to "CAN network hardware and measurement methods" below.
CAN network hardware and measurement methods
The CAN network is connected using resistors (120 ohms each) between the two CAN cables (green (GN) for CAN-L, white (W) for CAN-H) in some
control modules. The two control modules in the low speed network (in principle the passenger compartment) that have resistors are the rear electronic
module (REM) and the upper electronic module (UEM). In the high speed network (in principle the engine compartment), the position of resistors
depends on the engine type and model year. For model years 99-01, the resistors are positioned in the anti-lock brake system module/brake control
module (ABS/BCM) and the electronic throttle module (ETM). In cars from model year 02 with Bosch EMS, the electronic throttle module (ETM) is no
longer part of the CAN network. The resistor in this case is located in the engine control module (ECM).
To localize a permanent fault in the CAN network, the resistance can be measured in the network. When taking resistance readings, the battery negative
terminal must be disconnected and the resistance must be close to 60 ohms. An open-circuit in one or both CAN cables will produce a resistance of 120
ohms. The recommended approach is to measure the resistance using a breakout box connected to the central electronic module (CEM). This has the
1. The reading is taken with the central electronic module (CEM) connected. Note that open-circuits in the anti-lock brake system module/brake
control module (ABS/BCM), steering wheel angle sensor module (SAS), power seat module (PSM), passenger door module (PDM), driver's door
module (DDM) audio module (AUM) will not affect the reading as these control modules are not connected in series. The reading only checks the
"main artery in the CAN network". However, a short-circuit between the two cables will be detected
2. There is easy access to the low speed network, the high speed network and the cables between the central electronic module (CEM) and the
3. If a multimeter is connected, this can be positioned in front of the driver's seat and the cable harness/connectors can be twisted above the central
electronic module (CEM). It is then easy to see any changes in resistance
4. As in point 3, twist and turn the cables/junction behind the radio where the cables lie against the rear wall of the engine compartment
5. As in point 3, move the cables/junction connected to the cable harness in the headlining by the A-post
6. As in point 3, ask someone to twist and turn the cables under the cooling box for the central electronic module (CEM) and the transmission control
If for some reason it is not possible to use the breakout box for the central electronic module (CEM), the low speed network can be measured using the
breakout box for the power seat module (PSM) or in its connector when the power seat module (PSM) is disconnected.
These measurements are particularly effective when diagnostic trouble codes (DTCs) CEM-1A51 to 1A66 and E003 or E001 are stored without
diagnostic trouble codes (DTCs) CEM-DF03 to DF16 being stored. When taking resistance readings, also check that none of the cables are
short-circuited to ground or voltage, which ought to have stored CEM-DF03 to DF16. The resistance should be several kOhms.
For intermittent faults, the oscilloscope is very useful. The reading should be taken with all control modules connected and powered up using the
breakout box. Check that the voltage on CAN-H never exceeds 4.5 V and that CAN-L is never below than 0.5 V. Note that the oscilloscope in VIDA is
not sufficiently accurate (measurement speed too low) for detailed measurement of the traffic on the CAN network, but can give an indication of the
voltage levels that exist and whether communication exists or not.
Note! Never take readings directly in the connector using the test probes of the multimeter. This can result in damage to the connector and
entail insufficient contact when the connector is reassembled. Use a terminal pin instead.
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