Explained! CAN BUS Diagnosis – How to Troubleshoot Faults.

Summary

This video guides viewers through diagnosing faults in a high-speed CAN bus network using a multimeter. It explains the role of the CAN protocol in automotive communication and outlines a step-by-step process to identify issues with communication between multiple ECUs. The video covers diagnostic techniques including checking battery voltage, performing resistance tests, identifying wiring faults, and detecting short circuits or open circuits. Viewers also learn how to test specific modules and their wiring, helping to pinpoint the source of the issue, whether in the module or the wiring loom.

Takeaways

• 😀 The CAN bus is the most common communication protocol in vehicles, allowing ECUs to communicate without a central computer.
• 😀 CAN High and CAN Low wires are twisted together to reduce electromagnetic interference in the network.
• 😀 A 60-ohm resistance reading between the CAN lines at the OBD2 port is ideal for a healthy CAN bus network.
• 😀 If the resistance is higher than 60 ohms but lower than 120 ohms, it typically indicates high resistance caused by bad terminal contacts.
• 😀 A reading of 120 ohms indicates an open circuit, typically caused by a break in the wiring or a fault in a module with a terminating resistor.
• 😀 If the resistance is below 60 ohms, it suggests a short between CAN High and CAN Low.
• 😀 A bad battery can cause CAN bus communication faults with multiple modules, so always check battery voltage first.
• 😀 If you get a 60-ohm resistance reading but still face communication faults, the issue might be with the module itself or its wiring.
• 😀 Testing each affected module's connector and checking for bent pins, corrosion, or poor terminal tension can help identify faults.
• 😀 Short circuits to ground or voltage on the CAN bus can cause communication faults across the entire network, which should be tested at the OBD2 port.
• 😀 To isolate wiring issues, use a vehicle's wiring diagram to split the circuit and identify which side of the circuit contains the fault.

Q & A

What is the CAN bus in automotive diagnostics?

The CAN bus (Controller Area Network) is a data-sharing system used in vehicles, allowing multiple electronic control units (ECUs) to communicate with each other without needing a central computer or complex wiring.

Why is it important to diagnose CAN bus faults?

Diagnosing CAN bus faults is important because issues in the network can lead to severe symptoms like warning lights, vehicle non-start, and loss of communication between control modules, which can impact the overall functioning of the vehicle.

What tools are necessary for diagnosing CAN bus faults in high-speed networks?

The primary tool for diagnosing CAN bus faults in high-speed networks is a multimeter. Additionally, having access to a full wiring diagram for the vehicle is helpful in identifying modules and fault locations.

What should you check first when diagnosing CAN bus faults?

Start by checking the battery voltage, as a weak battery can cause communication issues with multiple modules.

What does it mean if the CAN network is asleep, and how can you check it?

If the CAN network is asleep, it means the communication lines are inactive. You can check this by measuring the voltage between the CAN high and CAN low pins (6 and 14 on the OBD2 port) when the ignition is off. A zero-volt reading indicates the network is asleep.

What is the significance of resistance tests at the OBD2 port during CAN bus diagnosis?

Resistance tests help identify the health of the CAN network. A reading of 60 ohms between the CAN high and CAN low lines suggests a healthy network. Higher or lower readings indicate potential wiring issues, shorts, or open circuits in the network.

What should you do if the resistance at the OBD2 port reads 120 ohms?

If the resistance reads 120 ohms, it indicates an open circuit between the two terminating resistors, typically caused by a fault in one of the modules or the wiring between them. Use the wiring diagram to isolate and locate the fault.

How do you isolate faults in specific modules when diagnosing CAN bus issues?

To isolate faults in specific modules, disconnect each module and perform the resistance test at its connector. If a module’s wiring is good but resistance shows 0 ohms, it suggests an open circuit, likely due to faulty wiring or module issues.

What should you check if the multimeter reading is 0 ohms when testing a module's connector?

A 0-ohm reading indicates an open circuit in the wiring to the module. Check for physical issues like damaged wires, poor connectors, or corrosion.

How do you test for shorts to ground or voltage in the CAN bus network?

To test for shorts to ground or voltage, set your multimeter to DC volts and check the voltage between chassis ground (pin 4) and both CAN high (pin 6) and CAN low (pin 14). If voltage is present when the network should be asleep, there’s likely a short to ground or voltage affecting the bus.

How can you determine whether the fault lies in the wiring or the modules during diagnosis?

By disconnecting modules and performing resistance tests, you can determine whether the fault is in the wiring or a specific module. If the issue persists after disconnecting a module, the fault likely lies within the wiring loom or network.

What should you do if the fault is found in the module?

If the fault is traced to a module, check its power supply and ground connections. If these are fine, the module may need to be repaired or replaced.