When diagnosing Controller Area Network (CAN bus) issues in vehicles, it’s common to measure resistance at the OBD2 port and expect a reading of around 60 ohms. This often leads to questions, especially when considering that CAN bus systems typically use 120 ohm termination resistors. Let’s clarify why this is the case and how to correctly interpret CAN bus measurements.
The Role of 120 Ohm Termination Resistors in CAN Bus Systems
CAN bus is a robust communication standard widely used in automotive and industrial applications. It relies on differential signaling over a twisted pair of wires, designated as CAN High (CANH) and CAN Low (CANL). To ensure signal integrity and prevent reflections that can disrupt communication, termination resistors are crucial.
These resistors, typically 120 ohms, are placed at each end of the CAN bus network. Their purpose is to absorb the signal energy at the end of the bus, preventing it from bouncing back and interfering with subsequent signals. Without proper termination, signal reflections can cause data corruption and communication errors.
Why 60 Ohms at the OBD2 Port?
The OBD2 port is commonly used as a convenient access point to the vehicle’s CAN bus network. Pins 6 and 14 of the OBD2 connector are designated for CAN High and CAN Low respectively. When you measure resistance between these pins with a multimeter on a vehicle with the ignition off (and battery ideally disconnected for safety, though the original article mentions disconnected battery), you are effectively measuring the resistance of the entire CAN bus termination network as seen from that point.
In most automotive CAN bus setups, there are typically two 120 ohm termination resistors located at opposite ends of the network. When you measure at the OBD2 port, you are measuring these two 120 ohm resistors in parallel. The formula for resistors in parallel is:
1/R_total = 1/R1 + 1/R2 + … + 1/Rn
In this case, with two 120 ohm resistors:
1/R_total = 1/120 + 1/120 = 2/120 = 1/60
Therefore, R_total = 60 ohms. This is why a reading of approximately 60 ohms at the OBD2 port between pins 6 and 14 is considered normal and indicates that the CAN bus termination is likely intact.
Measuring CAN Bus Voltage: Why Multimeters Can Mislead
The original question highlights confusion about voltage readings on CAN bus lines using a multimeter. It’s important to understand that CAN bus signals are not static DC voltages but rather high-speed, pulsed digital signals. A standard multimeter is not designed to accurately capture these dynamic signals.
While a multimeter might show some voltage readings on CAN High and CAN Low pins (as seen in the original example: CAN bus Hi pin 6 2.09 V, CAN bus Low pin 14 2.75 V), these readings are often misleading averages and do not represent the actual signal characteristics.
To properly analyze CAN bus voltage signals, an oscilloscope is essential. An oscilloscope can display the waveform of the CAN bus signals, allowing you to observe the voltage levels, signal shape, and timing. In a healthy CAN bus:
- Recessive State (Idle): Both CAN High and CAN Low lines should ideally sit around 2.5V.
- Dominant State (Data Transmission): CAN High voltage rises to approximately 3.5V, and CAN Low voltage drops to around 1.5V. The difference between CAN High and CAN Low is what represents the data.
A multimeter simply cannot capture these rapid voltage fluctuations and differential signals accurately.
Resistance Measurement Deviations: When to Worry
The original article also mentions a resistance reading of 45.6 ohms, lower than the expected 60 ohms. While ideally, the resistance should be close to 60 ohms, slight deviations are often acceptable, especially in passenger vehicles with relatively short CAN bus lengths.
Factors that might cause slight variations in resistance readings include:
- Meter Accuracy: Multimeters have tolerances, and slight inaccuracies in the meter itself can contribute to minor deviations.
- Component Tolerances: The 120 ohm termination resistors themselves have manufacturing tolerances, meaning their actual resistance might be slightly above or below 120 ohms.
- Additional Components: In some complex vehicle networks, there might be additional components or circuitry connected to the CAN bus that could slightly affect the overall resistance reading.
However, significantly lower resistance readings (well below 60 ohms) could indicate:
- Extra Termination Resistors: While less common in standard setups, additional termination resistors could be present, lowering the overall resistance.
- Short Circuits: In rare cases, a short circuit somewhere in the CAN bus wiring could artificially lower the resistance reading. This is a more serious issue that would likely be accompanied by communication problems.
Readings significantly higher than 60 ohms (or even open circuit) would suggest problems such as:
- Missing Termination Resistor: One or both termination resistors might be faulty or disconnected.
- Open Circuit in Wiring: A break in the CAN bus wiring would result in a higher resistance reading.
Conclusion
In summary, understanding the 120 ohm CAN termination resistors and the expected 60 ohm reading at the OBD2 port is crucial for diagnosing CAN bus issues. Remember:
- 60 ohms at OBD2 (pins 6 & 14) is the expected resistance due to parallel 120 ohm termination resistors.
- Use an Oscilloscope for accurate CAN bus voltage signal analysis, not a multimeter.
- Slight deviations in resistance readings are often acceptable, but significant deviations warrant further investigation into potential wiring or component issues.
By understanding these principles, you can more effectively diagnose and troubleshoot CAN bus problems in vehicles.
