Brakes - ABS Module CAN Fault Diagnostics
No: LA100-004Issue: 1
Date: 01 Dec 2006
SECTION: 100-00 - GENERAL INFORMATION
CAN Faults Relating to the ABS Module Diagnosis
AFFECTED VEHICLE RANGE:
LR3 (LA) VIN: 5A000360 onwards
CONDITION SUMMARY:
DIAGNOSTIC GUIDANCE FOR CAN FAULTS RELATING TO ABS MODULE DIAGNOSIS
Situation: This bulletin has been issued to provide diagnostic guidance for Control Area Network (CAN) faults relating to the Anti-lock Braking System (ABS) module.
Action: Should a CAN fault arise, refer to the Repair Procedure detailed in this bulletin to resolve the concern.
PARTS:
No parts required for this bulletin. Diagnostic information only.
WARRANTY:
No warranty provided for this bulletin. Diagnostic information only. Normal warranty policy and procedures apply.
REPAIR PROCEDURE
CONTROL AREA NETWORK (CAN) FAULTS AND PRINCIPLES OF OPERATION
--CAN FAULT CATEGORIES--
NOTE: CAN faults may be categorized as either 'CAN Data indicating a fault' or 'Missing CAN Data' and will indicate the potential problem as follows:
^ 'CAN Data Indicating a Fault' is where a CAN signal is being received by a control module and contains data that is either indicating a fault condition or is invalid because it is out of the specified range for the signal. This type of data will occur because of a specific fault with the system from where the signal has been sent.
^ 'Missing CAN Data' is where CAN signals are not being transmitted. Signals may be identified as missing from a particular system or a general CAN bus failure may be identified. The DTC description will identify the system from which specific signals may be missing. A general CAN bus failure can be caused by a problem in a control module, but is typically an indication of a problem with the CAN bus wiring.
--PLAUSIBILITY FAULTS RELATED TO THE CAN--
NOTE: A plausibility fault is not a CAN fault, though a CAN issue could have contributed to the fault being raised. To trace a plausibility fault it will be necessary to check all sensors relating to the plausibility and the CAN communication between the relevant control modules.
Principles: Land Rover vehicles use a variety of sensors located around the vehicle. The systems relying on these sensors use plausibility checks to monitor their 'health'. For example, the health of a brake pedal switch may be monitored by checking if the brakes master cylinder pressure increases when the brake pedal switch operates. Often plausibility checks will be carried out by comparing a sensor signal to another signal that is being sent via the CAN bus. A plausibility fault is therefore not a CAN fault but in order to trace a plausibility fault it may be necessary to be aware that CAN issues could have contributed to the fault being raised. If a system shows a plausibility fault it is necessary to check all sensors relating to the plausibility and the CAN communication between the relevant control modules.
--TRACING A CAN FAULT CHAIN TO ROOT CAUSE--
NOTE: The root cause of a CAN fault can only be determined after the fault chain is traced back to the first control module. The first control module should contain DTCs indicating a hard fault or that the module may have shut down completely. A module has shut down completely if diagnostic communication with that module can not be established.
Principles: Complex vehicle systems such as Anti-lock Braking System (ABS) or Dynamic Stability Control (DSC) use input signals from sensors that are wired directly into the control module and from the CAN bus. These same systems also publish signals on the CAN bus so that other systems such as the Engine or Instrument Pack can access them. Many control modules on the vehicle are therefore dependent on other control modules if they are to provide fault free functionality. This dependency may cause a hard fault in one control module to shut down functionality in a second control module. The loss of functionality in the second control module may in turn knock out a third control module. The DTCs in the third control module may point only to faulty or missing CAN data in the second control module. The root cause of the problem can therefore not be solved until the chain is followed to the first control module. The first control module should contain DTCs indicating a hard fault or it may have shut down completely.
IDENTIFY CAN FAULT
NOTE: IDS tools such as 'Network Integrity Tests' and 'Sorted DTC Display Tool' should be used along with diagnostic information detailed in this bulletin to resolve CAN issues. The electrical circuits section of GTR contains 'Module Communications Network' circuit diagrams which will assist in identifying the control modules, connectors and wires in the vehicles CAN bus.
NOTE: After following the guidance in this bulletin, if no definite cause was found for a CAN issue and all DTCs have been cleared and have not returned after a test drive of at least 10 miles (16 km), the vehicle can be returned to the customer.
NOTE: A Midtronics battery charger PSC550 must be connected to the vehicle battery during diagnostic sessions.
1. Connect the diagnostic equipment to the vehicle and begin a WDS or IDS session.
NOTE: DTCs and snap shot data should be noted at all stages during the diagnosis. This information will be needed in the event further assistance is requested from Land Rover Technical Helpline.
2. Note DTCs and collect snap shot data.
3. Determine whether the CAN fault is related to 'CAN Data indicating a fault', 'Missing CAN Data' or a 'Plausibility Fault Related to the CAN' and refer to the appropriate section title that follows.
DIAGNOSE 'CAN DATA INDICATING A FAULT'
NOTE: Diagnosing 'CAN Data indicating a fault' requires locating the control module that may be reporting a hard fault with a sensor or another part of its system. Hard faults that could relate to the faulty CAN data must be fixed before moving on to any further diagnosis. A module should not be replaced until the root cause of the fault has been determined.
Principles: If a control module (like the ABS module) reports 'CAN Data indicating a fault' from another control module, this is not an indication of a fault within the ABS module. It is an indication that the ABS module has detected a fault from another control module or vehicle system. Changing the ABS Module will not fix the fault. The diagnostic investigation should therefore concentrate on the control module from where the faulty data is being transmitted. It is likely that the transmitting control module will contain DTCs that indicate a fault with a sensor or other part of its dependent system. That is most likely to be the root cause of the problem. When diagnosing CAN Data indicating a fault, it may be necessary to trace the 'CAN Fault Chain' to get to the root cause of the problem.
1. If a control module reports 'CAN Data indicating a fault' from another control module, locate the control module from where the faulty data is being transmitted.
2. Check the control module for DTCs to determine if the fault is with a sensor or another part of its dependent system.
3. Where necessary, trace the CAN fault chain back until the root cause of the problem is determined.
DIAGNOSE 'MISSING CAN DATA'
NOTE: Diagnosing 'Missing CAN Data' requires locating the control module that has stopped sending signals.
A module should not be replaced until the root cause of the fault has been determined.
Principles: If a control module (like the ABS module) is reporting missing CAN data' from another control module, this is not an indication of a fault within the ABS module. It is an indication that the ABS module has detected that another control module has stopped sending CAN signals. Changing the ABS/DSC Module will not fix the fault. In this case the diagnosis should concentrate on tracing which control module or modules have stopped sending signals. It may be necessary to trace the 'CAN fault chain' to get to the root cause. It may also help to look for common patterns in the DTCs stored by a number of control modules. For example, if several control modules are reporting missing CAN data from the Transmission, then the Transmission Control Module (TCM) should fall under suspicion as the origin of the fault.
DTCs relating to 'Missing CAN Data' can sometimes be logged at ignition 'ON' or during engine cranking if the vehicle battery condition is poor. Environmental conditions that put extra strain on the battery, such as cold weather, may make these faults more likely to occur. Ignition on/cranking faults of this type occur because low voltage causes a mis-match in timing between control modules powering up and putting signals on the CAN bus. The vehicle battery condition should therefore always be confirmed as good when diagnosing missing CAN data.
If a control module (like the ABS module) is 'missing' from the CAN bus it will not be possible to read any DTCs from it because DTC diagnosis is done over the CAN network. If a control module is missing from the CAN bus it may be due to one of the following:
^ The control module is damaged.
^ There is a problem with the power supply or grounds to the control module.
^ There is a problem with the CAN wiring to the control module.
1. Ensure the vehicle battery condition is good.
2. Check all control modules on the vehicle CAN bus for DTCs.
3. Identify the control module that is at the source of failing to publish data on the CAN bus.
4. Check the powers and grounds on the suspect control module.
5. If any of the following are true, refer to the 'Checking the Integrity of the CAN Bus' section to check the integrity of the CAN bus to the suspect control module:
^ DTC analysis indicates that one or more control modules is missing from the CAN bus.
^ DTC analysis indicates 'CAN bus off' faults.
^ It is not possible to establish diagnostic communication with any control module on the vehicle and the diagnostic tool has been proven to not be faulty.
CHECKING THE INTEGRITY OF THE CAN BUS
NOTE: Some ABS/DSC modules are connected to the vehicle CAN bus in addition to a separate 'private CAN bus' that goes only to the vehicles Yaw/lateral acceleration sensor. The circuit diagram should be checked to ensure the CAN bus has been correctly identified.
1. Check the circuit diagram to ensure you have correctly identified the vehicle CAN bus.
2. Turn the ignition to the 'OFF' position.
3. If the DTC analysis has identified a 'suspect control module', unplug this control module.
4. If the DTC analysis has not identified a 'suspect control module', unplug the ABS/DSC module.
NOTE: The control module connector will have either two or four CAN bus pins, depending on how the module is linked into the CAN bus. A four pin connector will be grouped as two pairs. CAN pins in a control module connector are always grouped as pairs, known as 'CAN_high' and 'CAN_low'. Each pair of CAN pins can normally be identified by the wires to them being twisted together.
5. Identify the CAN pins in the unplugged control module connector.
NOTE: Intermittent faults can exist. If the resistance or voltage measurements are outside of the tolerance range, the cable harnesses should be 'wiggled' where possible while carrying out the tests. Also, connectors to all control modules on the CAN bus should be disconnected and re-connected as this is often enough to clean up light corrosion which is causing intermittent connectivity.
NOTE: The resistance value across each pair of CAN pins must be either 60 Ohms (± 5 Ohms) or 120 Ohms (± 5 Ohms). The value depends on where the control module is located in the CAN bus relative to other control modules, however, for this bulletin test it does not matter which value is recorded as long as the value is either 60 Ohms or 120 Ohms.
6. Measure and record the resistance across each pair of CAN pins.
7. With the connector still unplugged, switch the ignition to the 'ON' position.
NOTE: The voltage between CAN_high and ground and between CAN_low and ground is constantly varying, but should be in the range 2 volts to 4 volts.
8. Measure and record the voltage between CAN_high and ground.
9. Measure and record the voltage between CAN_low and ground.
10. Switch the ignition to the 'OFF' position.
11. Re-connect the connector to its control module.
12. Select another control module on the CAN bus and repeat tests 6 to 12.
13. If any unacceptable measurement is recorded, test each section of the CAN bus to identify the location of the fault as follows:
^ To isolate a faulty control module, carry out steps 6 to 12 from each control module on the CAN bus.
^ To isolate a wiring fault, check the integrity of the CAN bus wiring between control module connectors, looking for open circuits or shorts to power or ground.
DIAGNOSE A CAN RELATED PLAUSIBILITY FAULT
NOTE: Plausibility faults can be logged due to CAN issues where sensor and vehicle system plausibility tests are carried out using CAN signals.
1. Ensure the vehicle battery condition is good.
2. Check that the sensor that relates directly to the plausibility fault, identified by the DTC, is correctly and securely mounted.
3. If there are any DTCs relating to CAN communication between control modules, resolve these issues.
4. If the plausibility fault relates to the DSC Sensor Cluster or the Steering Angle Sensor, check the vehicle geometry and Steering Angle Sensor calibration.
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