How To Test CAN BUS With A Multimeter [CAN Bus Resistance Check] Mechanic Mindset

Summary

In this video, the presenter demonstrates how to test a CAN Bus network using a multimeter, focusing on the resistance check. By measuring between the CAN high and CAN low lines, a reading of around 60 ohms indicates the network's health. The video explains the limitations of this test, as it only checks the resistors in the network and may miss faults elsewhere. It also highlights the importance of disconnecting the battery and ensuring the network is 'asleep' for accurate readings. The presenter compares an expensive Fluke multimeter with a cheaper model and emphasizes the use of an oscilloscope for more precise diagnostics.

Takeaways

• 😀 You can test the CAN Bus network with a multimeter even if you don't own an oscilloscope.
• 😀 Before performing a resistance check on the CAN Bus, make sure the network is asleep, ideally by disconnecting the battery.
• 😀 The resistance check for CAN Bus should read around 60 ohms when everything is working correctly.
• 😀 High-speed CAN Bus uses two 120 ohm resistors connected in parallel inside control modules, and a good reading is around 60 ohms.
• 😀 The resistance check is limited to checking only two parts of the network, where the resistors are located, and doesn't fully guarantee the network's health.
• 😀 The resistance check doesn't tell you if the entire network is functional; other modules without resistors may have issues that won't show up on the resistance test.
• 😀 To check for faults, you must inspect each module on the network individually, especially if the resistance is high (kilo ohms) at a specific module.
• 😀 The resistance check is good for detecting short circuits between CAN high and CAN low, or to ground or positive.
• 😀 If a fault is detected, the resistance will change (e.g., a short will show close to zero ohms).
• 😀 Even cheaper multimeters, like those costing around 20 pounds, can give similar results to high-end ones like the Fluke meters.
• 😀 While the resistance check can help detect basic faults, an oscilloscope is recommended for more detailed and accurate CAN Bus diagnostics.

Q & A

What is the purpose of the resistance check on a CAN bus network?

The resistance check on a CAN bus network helps to assess the integrity of the network, specifically checking if the CAN high and CAN low lines are properly connected and whether there are any open circuits or short circuits.

Why is it important to ensure the CAN bus network is asleep before performing a resistance check?

It's important because if the network is awake, the resistance readings can be misleading, potentially causing you to troubleshoot a non-existent problem. Disconnecting the battery helps to ensure the network is asleep.

What resistance value should you expect between CAN high and CAN low if the network is healthy?

A healthy CAN bus network should show a resistance of around 60 ohms between CAN high and CAN low, which reflects the two 120-ohm resistors connected in parallel in the control units.

What does a reading of 120 ohms on the CAN bus resistance check indicate?

A reading of 120 ohms typically indicates that one of the 120-ohm resistors is either missing or has an open circuit, suggesting a fault in that part of the network.

Can the resistance check identify faults in all parts of the CAN bus network?

No, the resistance check only measures the parts of the network where the 120-ohm resistors are located, usually in the control modules. It won't detect faults in other sections of the network.

What would you expect if there is an open circuit in one of the control modules?

If there is an open circuit in one of the control modules, you would likely see a high resistance reading in the kilo-ohms range, but it may not show up in the resistance check if the fault is far from the testing point.

How can the resistance check still be useful if it doesn't detect all faults in the CAN bus network?

The resistance check is useful for detecting short circuits between the CAN lines, as well as checking for shorts to ground or positive voltage. It's a quick way to spot basic issues in the network.

What are the potential issues that the resistance check might miss in a CAN bus network?

The resistance check may miss issues like damaged wiring or faults in control modules that do not have the 120-ohm resistors, and it won't identify problems in areas of the network beyond the resistors.

How does the resistance reading change when a module like the ABS or EKP is disconnected?

Disconnecting a module like the ABS or EKP can cause the resistance reading to jump to 120 ohms, indicating that the resistor in that module is no longer part of the circuit.

What other testing equipment could be used for diagnosing the CAN bus network?

An oscilloscope, such as the PicoScope 2000 series, can provide more detailed insights into the network’s performance by showing waveform data, which can help diagnose more complex issues.