It appears you’re encountering challenges reading data from heavy trucks using OBD2, despite successfully doing so with cars. This is a common issue, and stems from key differences in diagnostic systems between light-duty vehicles (cars) and heavy-duty trucks (semi-trucks). You’ve tested various CAN speeds and message formats, which is a good starting point. Let’s delve into why you’re facing this problem and what kind of OBD2 setup is appropriate for semi-trucks.
OBD2 in Cars vs. Semi Trucks: Key Differences
While both cars and semi-trucks utilize OBD (On-Board Diagnostics) systems, the implementation and protocols differ significantly. The OBD2 standard as commonly understood and implemented in cars (passenger vehicles and light trucks) is often geared towards legislated emissions standards and uses protocols like CAN (Controller Area Network) at 12V.
Heavy-duty trucks, especially those over a certain weight class, often operate on a 24V system and adhere to different diagnostic protocols. While they might have an OBD-style connector, simply using a standard car OBD2 scanner may not work due to these variations:
- Voltage: Heavy trucks typically use a 24V electrical system, whereas cars are 12V. While some OBD2 interfaces can handle both, it’s crucial to ensure compatibility to avoid damaging your equipment.
- Protocols: Heavy-duty trucks frequently use different communication protocols beyond the standard OBD2 protocols found in cars. These include:
- SAE J1939: This is a CAN-based high-speed protocol widely used in heavy-duty vehicles for diagnostics and communication between vehicle components. It’s specifically designed for commercial vehicles.
- SAE J1708 and J1587: These are older serial communication protocols still found in some older trucks. J1708 is often used with J1587 for diagnostics.
Understanding Heavy Truck Diagnostic Protocols: J1939 and J1708/J1587
For semi-trucks, it’s essential to understand J1939 and J1708/J1587 protocols:
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J1939: This is the more modern and prevalent protocol in contemporary heavy-duty trucks. It operates on the CAN bus and defines how components communicate, including diagnostic data. Scanners designed for heavy trucks must support J1939 to read data effectively.
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J1708/J1587: These are older protocols. J1708 defines the physical layer and data link layer, while J1587 is the application layer protocol for diagnostics and data exchange. While less common in the newest trucks, they might still be present in older models.
Choosing the Right OBD2 Scanner for Semi Trucks
To successfully read data from semi-trucks, you need an OBD2 scanner specifically designed for heavy-duty vehicles. Here’s what to look for:
- Heavy-Duty Compatibility: Ensure the scanner explicitly states it supports heavy-duty trucks and commercial vehicles.
- Protocol Support: Crucially, the scanner must support SAE J1939 and ideally J1708/J1587 protocols. Standard car OBD2 scanners typically do not support these.
- Voltage Range: Verify the scanner is compatible with 24V systems. Many heavy-duty scanners are designed to handle both 12V and 24V.
- Connector Type: While most trucks use a standard OBD connector (often a SAE J1962 type), confirm compatibility or if adapters are needed.
In your research project, using a scanner that supports J1939 is paramount for reading data from MAN, Mercedes, and Volvo trucks, as these manufacturers heavily utilize this protocol. Trying generic OBD2 commands designed for passenger cars will likely not yield results. Look for diagnostic tools explicitly marketed for “heavy-duty”, “commercial vehicle”, or “J1939” applications to effectively interface with semi-trucks.
