Understanding the Controller Area Network (CAN) bus is crucial for modern automotive diagnostics. When troubleshooting communication issues, measuring the CAN bus voltage and resistance is a common step. However, it’s essential to use the correct tools and understand the specifications to avoid misdiagnosis. This article will clarify the expected ohm and voltage specifications for CAN bus high and low lines in OBDII systems.
Decoding CAN Bus Voltage Readings
Attempting to measure CAN bus voltage with a standard multimeter can lead to inaccurate readings and misinterpretations. The CAN bus operates using high-speed digital pulse signals, which a multimeter is not designed to capture correctly. To accurately assess CAN bus voltage, an oscilloscope is required.
In a healthy CAN bus system, both CAN High and CAN Low lines idle at approximately 2.5V. When transmitting data, the voltage levels fluctuate to represent dominant and recessive bits. During a dominant bit transmission:
- CAN High: Voltage rises to approximately 3.5V.
- CAN Low: Voltage drops to around 1.5V.
Therefore, if you were to incorrectly measure with a multimeter, especially on a lightly loaded bus, you might see readings close to 2.5V for both lines, which can be misleading if you expect a significant difference based on a static voltage concept. Readings like 2.09V on CAN High and 2.75V on CAN Low, as observed with a multimeter, are not typical static levels but rather artifacts of the multimeter’s averaging of the pulse signal.
Interpreting CAN Bus Resistance Specifications
Measuring the resistance across the CAN bus lines with the battery disconnected is a valuable diagnostic step. The expected resistance between CAN High (pin 6) and CAN Low (pin 14) at the OBDII Data Link Connector (DLC) should be approximately 60 ohms. This value arises from the two 120-ohm terminating resistors typically located at each end of the CAN bus network in a vehicle. These resistors are wired in parallel, resulting in a total resistance of 60 ohms (1/(1/120 + 1/120) = 60).
A reading of 45.6 ohms, slightly lower than the ideal 60 ohms, might raise concern. While ideally, the resistance should be close to 60 ohms, slight deviations are often acceptable, especially in passenger vehicles with relatively short CAN bus线路. Factors such as additional modules or slight variations in component tolerances can cause minor differences.
A significantly lower resistance reading could indicate a short circuit or additional termination resistors being present on the network. In contrast, a significantly higher or infinite resistance reading suggests an open circuit or a missing termination resistor, which can disrupt CAN bus communication.
In summary, while precise 60-ohm resistance is the target, minor deviations are not always indicative of a problem. Experience shows that systems can often tolerate resistance values slightly below 60 ohms without significant issues, particularly in shorter CAN bus implementations. However, it is crucial to investigate significantly low or high resistance readings further to pinpoint potential faults within the CAN bus network.
