Quick Answer
J1708 is an older, slower serial protocol (9,600 bps) largely superseded on modern trucks, while J1939 is the current CAN bus based standard (250–500 kbps) used for high speed communication between engine, transmission, brake, and body ECUs on virtually every heavy duty truck built since the early 2000s. Most "CAN bus diagnostics" work today is J1939 work, with J1708 support kept mostly for legacy or hybrid systems.
Quick Comparison
- Speed: J1708 runs at 9,600 bps; J1939 runs at 250 or 500 kbps — roughly 25–50x faster.
- Physical layer: J1708 uses RS-485 serial; J1939 uses CAN bus (ISO 11898).
- Typical use today: J1939 handles nearly all engine, transmission, ABS, and body control communication on current heavy duty trucks. J1708 mostly survives as a legacy display/diagnostic link on older or mixed fleet equipment.
- Message structure: J1939 organizes data into standardized Parameter Groups (PGNs) and Suspect Parameter Numbers (SPNs), giving diagnostic tools a consistent way to decode fault data across manufacturers.
What Is J1708?
SAE J1708 is a serial communication standard developed in the 1980s for heavy duty vehicles, paired with the J1587 protocol that defines the actual message content. It was the backbone of truck diagnostics for roughly two decades and is still present on some older fleet equipment and certain body builder add ons, but it has been steadily phased out in favor of CAN based J1939 since the early 2000s.
What Is J1939?
SAE J1939 is the current standard for heavy duty vehicle networks, built on the CAN 2.0B physical layer. It defines how ECUs — engine, transmission, ABS/EBS, instrument cluster, aftertreatment, and body control modules — broadcast and receive standardized messages over a shared bus. Its structured Parameter Group Number (PGN) and Suspect Parameter Number (SPN) system is what lets a diagnostic tool decode a fault code consistently whether it's reading a Volvo, Scania, or Freightliner.
Why the Difference Matters on the Bench
Getting the protocol wrong at the connection stage is one of the most common reasons a diagnostic tool fails to communicate with a truck at all. A tool configured for J1939-only won't see data on a J1708-only legacy bus, and vice versa. Most modern dealer level tools (VOCOM2+, SDP3, DAVIE) auto negotiate, but aftermarket generic scanners sometimes require the protocol to be selected manually — which is worth checking first if you're getting a "no communication" error rather than assuming it's a wiring fault.
How to Tell Which One You're Working With
The fastest check is the diagnostic connector pinout and your scan tool's connection log — most tools will report which protocol it linked on. As a rule of thumb: anything built in roughly the last 15–20 years is running J1939 for its primary engine/transmission/brake communication, with J1708 (if present at all) relegated to legacy add on modules.
Key Takeaways
- J1939 is CAN bus based and roughly 25–50x faster than the older serial J1708 standard.
- Nearly all modern heavy duty trucks communicate primarily over J1939.
- A "no communication" error is often a protocol mismatch, not a wiring fault — check what your tool is actually configured to look for.
- J1939's standardized PGN/SPN structure is what makes fault codes readable consistently across truck brands.
Frequently Asked Questions
Yes — most modern dealer level and quality aftermarket tools support both and will auto detect which protocol the vehicle is running.
Largely, yes, for new vehicles. It still appears on older equipment and certain legacy add on modules, but J1939 has been the primary standard for engine and driveline communication for roughly two decades.
The most common causes are a protocol mismatch, a wiring/connector fault, or the ECU being asleep — check the tool's protocol setting before assuming it's a hardware problem.