Can/BYTEFLIGHT Bus Diagnostics (61 03 03)

1. Identification of engine type

Bus diagnosis identifies the type of engine in a vehicle. The test module has to evaluate the fault memory entries differently based on the engine type. Make sure that the vehicle is identified automatically.

2. Read off all fault memories of all control modules

• Perform a "Short Test" on the entire vehicle using DIS CD 44 or higher.
• Ensure that all the control modules can be identified.
• If more than 2 CAN faults or more than 2 Byteflight communication faults are stored, the "CAN/Byteflight" virtual module will be marked with an "X". An "x" will appear next to the "virtual" control module "CAN/Byteflight" or "CAN". Virtual means this is not a real control module but a wild card for all control modules on the CAN bus or Byteflight.
  Fig 1: Displaying Fault Memories Of All Control Modules

  
• Page to the right after a "Short Test" is complete.

3. DIS Test Plan.

• Highlight "System analysis B6131_xxxxx" test plan from the DIS test plan section and continue.
  Fig 2: Displaying DIS Test Plan

  

4. Evaluation of fault memory entries

• Once the test plan has been started, the following messages will be displayed:

"Evaluating fault codes stored in the system.

Time: approx. 30 seconds

Please wait.

Evaluation of quality of results:

***** Very High Quality

* Low Quality"

Fig 3: Displaying Evaluation Of Fault Memory Entries

5. The determined causes of the fault

• Once the evaluation process is complete, the screen will display the following: (as an example)

"The determined causes of fault are as follows:

1. "[1] **** DSC
2. [2] *** PT-CAN
3. [3] ** ARS
4. [4] End test module

The most probable cause of the fault is listed under [1]. Select test step"

Fig 4: Displaying Causes Of Fault

• Select [1] and continue.

6. Continue test plan

• The following messages are displayed (as an example) for this test plan.

"DSC

Dynamic stability control

Analysis of the bus faults stored in the vehicle showed that the corresponding control module did not participate at least temporarily in bus communication."

• Continue with test plan.
  Fig 5: Displaying Test Plan

  
• The following messages are displayed (as an example) for this test plan.

"Possible causes of faults:

• Voltage supply
• Bus connection
• Dynamic stability control

Check voltage supply and bus connection (branch lines, plug connections) for loose contacts. Replace control module if fault reoccurs."

Fig 6: Displaying Message

• Continue with test plan.
• The BMW diagnosis system proposes a separate test plan for each of the 3 most probable fault causes
• If problem was not found then use selection "[1] Return to selection", and continue in test plan.
• If problem was found and repaired use selection "[2] End test module."
  Fig 7: Selections "[1] Return To Selection", And "[2] End Test Module."

  
• The fault memories can be displayed by selecting "Control Module Functions/CAN Byteflight/Read Info memory". The lists of faults that the system analysis looks at are displayed and can be selected for viewing.
  Fig 8: Displaying Fault Memories

  

7. Inspection procedure for impedance measurement (PT-CAN, F-CAN, Local-CAN)

When directed by the DIS test plan or in cases where communication is lost on an entire bus system it may be necessary to measure the impedance of the complete bus circuit. When measuring impedance, it is necessary to disconnect  the circuit being tested from the power supply beforehand. The vehicle's battery and battery charger should therefore be disconnected.  Wait about three minutes to allow all capacitors in the system to discharge.

Inspection procedure for resistance test:

• The CAN bus must be disconnected from the power supply
• No other testing equipment must be in use (connected in parallel)
• The measurement is taken between the CAN Low and CAN High leads
• The measurements recorded may differ from the specified levels by a few Ohms

Impedance measurement with matching resistor PT-CAN, F-CAN, Local-CAN 

On the Tester, switch to Measurement system -> Multimeter

• Measurement function: Resistance
• Measuring range: automatic

In order to prevent signal reflection, a 120 Ω resistor is fitted to two CAN bus nodes (at the extremities of the PT-CAN network). The two terminal resistors are connected in parallel and form a shunt impedance of 60 Ω. When the power supply is switched off, the shunt impedance can be measured across the communication leads. In addition, the individual resistors can be tested independently of one another. (Tip for 60 Ω measurement: disconnect an easily accessible control module (not fitted with terminal resistor) from the bus and then measure the impedance between CAN Low and CAN High leads on the connector.)

Terminating resistor locations:

8. DC voltage measurement PT-CAN, F-CAN, Local-CAN

Precondition for the measurement: battery connected and ignition on! 

On the Tester, switch to Measurement system -> Multimeter 

• Measurement function: Voltage
• Measurement type: =
• Measuring range: automatic

In order to establish whether the CAN Low or CAN High lead is defective, you can measure the CAN Low (CAN High) voltage to ground.

CAN Low to ground: voltage approx. 2.4 V

CAN High to ground: voltage approx. 2.6 V

These values are approximate values and can vary by a few hundred mV depending on the bus load.

9. Oscilloscope measurement PT-CAN, F-CAN, Local-CAN

Precondition for the measurement: battery connected and ignition on! 

On the Tester, switch to Measurement system -> Oscilloscope setting 

• Measurement type: =
• Measuring range: +/- 5V
• Frequency range: 1 kHz

In order to obtain a clear idea of whether the CAN bus is functioning properly, it is very useful to be able to observe activity on the bus. What is important here is not to analyze the actual data being transmitted, but simply to be able to see that the CAN bus is operating. The oscilloscope test can state that "the CAN bus is probably operating without faults".

If the oscilloscope is used to measure the voltage differential between the CAN Low and CAN High ground leads, a square wave signal with the voltage limits U (min) = 1.5 V and U (max) - 2.5 V is obtained.

If the oscilloscope is used to measure the voltage differential between the CAN High and CAN Low ground leads, a square wave signal with the voltage limits U (min) - 2.5 V and U (max) - 3.5 V is obtained.

These values are approximate values and can vary by a few hundred mV depending on the bus load.

10. Resistance test K-CAN

No defined resistance test can be carried out at the K-CAN data bus, as the resistance varies depending on the internal switching logic of the control modules!

11. Measuring K-CAN DC Voltage

Precondition for the measurement: battery connected and ignition on! 

On the Tester, switch to Measurement system -> Multimeter 

• Measurement function: Voltage
• Measurement type: =
• Measuring range: +/- 10 V

In order to establish whether the CAN Low or CAN High lead is defective, you can measure the CAN Low (CAN High) voltage to ground.

CAN Low to ground: voltage approx. 4.8 V

CAN High to ground: voltage approx. 0.2 V

These values are approximate values and can vary by a few hundred mV depending on the bus load.

12. Oscilloscope measurement K-CAN

Precondition for the measurement: battery connected and ignition on! 

On the Tester, switch to Measurement system -> Oscilloscope setting 

• Measurement type: =
• Measuring range: automatic
• Frequency range: 1 kHz

In order to obtain a clear idea of whether the CAN bus is functioning properly, it is very useful to be able to observe activity on the bus. What is important here is not to analyze the actual data being transmitted, but simply to be able to see that the CAN bus is operating. The oscilloscope test can state that "the CAN bus is probably operating without faults". If the oscilloscope is used to measure the voltage differential between the CAN Low and CAN High ground leads, a square wave signal with the voltage limits U (min) = 1 V and U (max) = 5 V is obtained. If the oscilloscope is used to measure the voltage differential between the CAN High and CAN Low ground leads, a square wave signal with the voltage limits U (min) = 0 V and U (max) = 4 V is obtained. These values are approximate values and can vary by a few hundred mV depending on the bus load.

13. CAN bus without function

If the K-CAN or PT-CAN data bus indicate no function, it is likely that a short circuit has occurred on the CAN Low and/or CAN High lead, or that a control module is defective. In order to localize the cause of the fault, it makes sense to use the following procedure:

• Disconnect one CAN-bus node after the other until the unit causing the fault (= control module x) is found
• Check the lines to control module x for short circuits
• If possible, check control module x

However, this procedure only leads to success if a tap line from a control module to the CAN bus has a short circuit. If a CAN bus lead itself has a short circuit, the wiring harness must be checked.

14. Diagnosis information

Two different bus faults can be entered in the CAN bus control modules:

• CAN communication fault
• CAN wire fault

The communication fault provides an overview of the control modules that have failed on the CAN bus, i.e. were no longer able to communicate. The "CAN communication fault" can only be read out if the fault is currently not present. If the fault is currently present, it is no longer possible to communicate with the control module. This means that the fault memory cannot be read either! Physical wire fault can be detected by the use of fault-tolerant CAN transceivers. However, at the moment there are only fault-tolerant transceivers for the K-CAN data bus. This means that only control modules that are connected to the K-CAN  data bus can have made the fault memory entry "CAN wire fault  ". The CAN transceiver is also unable to distinguish between the individual fault categories listed below. If the bus fault "CAN wire fault" is entered in a control module, this can mean:

• Break in wiring CAN High
• Break in wiring CAN Low
• Short circuit CAN High to ground or positive
• Short circuit CAN Low to ground or positive
• Short circuit CAN High to CAN Low

Break in wiring (single-wire operation): each control module contains a separate bus termination. This means that, even in the case of a break in wiring, the voltage level can be maintained across the entire K-CAN network. The consequence of this is that a transmitting control module does not detect this fault and continues to work in two-wire operation. However, if a control module transmits a message across the break, the receiving control module only detects activity on the undamaged bus line. The receiving control module thus detects single-wire operation and sets the fault "CAN wire fault". If different control modules receive messages across the break, a number of control modules can have made this fault memory entry in single-wire operation!

Short circuit:  if there is a short circuit in the system, all  K-CAN control modules must have entered the fault "CAN wire fault". In order to localize the short circuit, follow the procedure for "CAN bus without function".