What J1708 Is and Which Trucks Use It
J1708 is an SAE standard that defines the physical communication layer for an older heavy-duty vehicle serial network. It specifies the wiring type, data rate (9,600 bits per second), and electrical characteristics of the two-wire serial bus used for module communication on commercial vehicles from approximately the early 1990s through the mid-2000s. J1708 is the transport layer; J1587 (the diagnostic messaging standard) runs on top of J1708 just as J1939 application messages run on top of CAN.
Trucks that use J1708 for primary diagnostic communication include pre-2007 emissions-era engines: Cummins ISX and ISM (pre-2007), Detroit Series 60, Caterpillar 3406E and C15, older International engines, and many ABS and instrument cluster modules from the same era. These engines remain in commercial service and still generate J1708 fault codes accessible only with tools that support the J1708/J1587 protocol.
J1708 vs. J1939: The Main Differences
J1708 operates at 9,600 bps; J1939 operates at 250 kbps — J1939 is approximately 26 times faster, which allows J1939 networks to carry the high-bandwidth live data (multiple sensor values per second from dozens of modules) that modern diagnostics and safety systems require. J1708 was designed when trucks had few electronic modules and low data bandwidth needs; J1939 was developed to handle the complexity of modern emissions, ABS, and telematics systems.
The two networks use different connectors — J1708 systems typically use an older 6-pin or 9-pin connector design specific to their era. The J1939 9-pin SAE connector (the current standard diagnostic port) is separate from J1708 connector designs. A tool that reads J1939 from the current 9-pin port cannot automatically read J1708 data — it requires the appropriate legacy adapter and J1708 protocol support in the software.
Diagnosing J1708 Codes on Trucks Currently in Service
Diagnosing J1708 faults requires: a scan tool with J1708/J1587 protocol support (not a J1939-only tool), the appropriate legacy adapter cable, and the correct fault code reference. Professional heavy-duty tools (Noregon DLA+, Dearborn Group DG Technology, NEXIQ USB-Link) support both protocols. Older dedicated tools (Cummins INSITE versions compatible with pre-2007 engines, older Detroit DiagnosticLink) also support J1708.
When an older truck is brought in for diagnosis and the technician's J1939-only tool returns 'no modules found' or cannot read codes, J1708 is often the reason. The modules are communicating on the J1708 network; the J1939 tool has no visibility into that network. This is a particularly common situation for ABS modules on older trucks, which may have continued to use J1708 even when the engine ECM moved to J1939.
J1708 Tools and Protocol Support in Shops Today
Not all modern diagnostic shops maintain J1708-capable tools — some have transitioned entirely to J1939 equipment as their older truck population decreased. For a fleet with older equipment, confirming that a shop has J1708 tool capability before sending a pre-2007 truck for diagnosis avoids discovering the capability gap at drop-off.
Fleet maintenance programs that include older trucks in their active inventory should document the communication protocol requirements for each vehicle — specifically identifying which trucks need J1708 support for full diagnostics. This documentation helps maintenance planners select shops and tools appropriately and ensures that fault codes from J1708 modules are not missed because the diagnostic tool only reads J1939.
Related Pages
Sources
- SAE J1939 Standards Collection SAE International · official · accessed 2026-05-05 · confidence medium
Source: SAE International, SAE J1939 Standards Collection. This page paraphrases factual fields only and is not a substitute for the original document.
Open source
FAQ
Can a modern J1939 scan tool read fault codes from an older truck that uses J1708?
Only if the scan tool includes J1708 support. Many professional heavy-duty scan tools include both J1939 and J1708/J1587 compatibility with appropriate adapters. A J1939-only tool will not communicate with J1708 modules. Check the tool's protocol support documentation before working on older trucks.
Is J1708 still in use on trucks that are currently on the road?
Yes. While no longer used on new trucks, J1708 is still the communication standard on trucks manufactured before the full transition to J1939 (approximately mid-2000s) that remain in service. Some legacy components on transitional-era trucks (older ABS modules, engine controllers) still communicate via J1708 even on otherwise newer trucks.
Can J1708 and J1939 run simultaneously on the same truck?
Yes. Many trucks from the late 1990s through approximately 2007–2010 used both networks simultaneously. Older engine controllers or ABS modules used J1708, while newer additions used J1939. Gateway modules that bridged both networks were common. A complete diagnostic on these trucks requires a tool that can read both.