For friends engaged in automotive electronics or related fields, the ability to quickly assess the operational status of the CAN bus is a very practical skill. Today, we will delve into the normal voltage range of the CAN bus and how to conduct basic fault diagnosis through voltage measurements.
1. Normal Voltage Range of the CAN Bus
The CAN bus transmits information through the voltage difference between two signal lines: CAN_H (high) and CAN_L (low).
Core Concept Diagram:
Voltage Change Diagram (Voltage to Ground)
Idle State (No Signal):
CAN_H │────── 2.5V ──────
CAN_L │────── 2.5V ──────
Voltage Difference = 0V
Active State (Signal Present):
CAN_H │── 3.5V ──
│
CAN_L │── 1.5V ──
Voltage Difference = 2V
In actual measurements, what we typically see with a multimeter is the effective value or average value:
Static Voltage Normal Range:
- Voltage of CAN_H to Ground: 2.5-2.7V (usually around 2.6V)
- Voltage of CAN_L to Ground: 2.3-2.5V (usually around 2.4V)
- Key Indicator: The sum of both voltages stabilizes around 5V
2. What Does the Actual Waveform Look Like?
The ideal CAN signal waveform is as follows:
Typical CAN Signal Waveform
Voltage │
3.5V │ ■■■■■ CAN_H
│ ■ ■
2.5V │■■■■■ ■■■■■■
│ ■ ■
1.5V │ ■■■■■ CAN_L
└───────────────────→ Time
│ Dominant Bit │ Recessive Bit │ Dominant Bit │
From the waveform, it is clear to see:
- The voltage changes of CAN_H and CAN_L resemble a “seesaw”
- A voltage difference of 2V is formed during the dominant bit period
- During the recessive bit period, the bus returns to 2.5V
3. How to Conduct Actual Measurements, Taking the Engine as an Example
Recommended Measurement Location: 📍 OBD-II Interface (most convenient, but the OBD pinout varies by product, this is just for demonstration):
- Pin 6: CAN_H
- Pin 14: CAN_L
Measurement Steps:
- Set the multimeter to the DC voltage range
- Connect the black probe to ground (OBD interface ground pin or vehicle/component ground, depending on the actual application)
- Use the red probe to measure pins 6 and 14 respectively
- Power on the vehicle but do not start the engine
Expected Measurement Results:
Actual Measurement Value Example Table
┌──────────┬──────────┬──────────┐
│ Measurement Point │ Theoretical Value │ Normal Range │
├──────────┼──────────┼──────────┤
│ CAN_H │ 2.6V │ 2.5-2.7V │
├──────────┼──────────┼──────────┤
│ CAN_L │ 2.4V │ 2.3-2.5V │
├──────────┼──────────┼──────────┤
│ Voltage Sum │ 5.0V │ 4.9-5.1V │
└──────────┴──────────┴──────────┘
4. Troubleshooting Voltage Anomalies
Common Abnormal Voltage Patterns:
Fault Situation Analysis Diagram
Situation 1: Short to Ground
CAN_H │── 0V ──
CAN_L │── 0V ── → Check for short circuit in the wiring
Situation 2: Short to Power
CAN_H │── 12V ──
CAN_L │── 12V ── → Check for power crosstalk
Situation 3: Abnormal Termination Resistor
CAN_H │── 1.8V ──
CAN_L │── 3.2V ── → Check the termination resistor
Systematic Troubleshooting Method:
-
Termination Resistor Check
- Measure the resistance between CAN_H and CAN_L
- Normal Value: About 60Ω (two 120Ω termination resistors in parallel)
- Abnormal Value: 120Ω (missing one termination resistor) or ∞ (open circuit)
Isolation Method
Fault Location Process:
Start → Measure Voltage Anomaly → Disconnect CAN Nodes One by One
↓
Voltage Returns to Normal? → Yes → The Last Disconnected Node is Faulty
↓
No → Continue Disconnecting the Next Node
↓
All Nodes Checked → Inspect the Wiring Itself
Basic Wiring Check
- Check if the CAN wires are shorted to ground/power
- Check if the wiring harness is broken
- Check if the connectors are making good contact
Mastering these basic voltage knowledge and troubleshooting ideas can help you quickly locate the problem direction when facing CAN network communication faults, avoiding blind replacement of parts. I hope these contents presented through diagrams can help you better understand the working principle of the CAN bus!
Feel free to share your experiences with CAN network fault cases in the comments section, and let’s learn from each other!