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SAE J1939 Simplified: The Ultimate (2024) Guide

Updated at 17 May, 2022

— Master SAE J1939 with the ultimate 2024 guide! Demystify protocols, dive into CAN bus & more. Includes a beginner's starter kit. Unlock expertise now!

SAE J1939 Simplified: The Ultimate (2024) Guide
Welcome to our simple guide on SAE J1939. It's the backbone of communication in the world of heavy-duty vehicles. This system has revolutionized how electronic components in these machines communicate with each other. The SAE J1939 protocol isn't exclusive to trucks and buses. It's also found in industrial machines, farming equipment, and even marine vessels. Let's jump in and see what makes this system so special for vehicles and machines.

SAE J1939 is a common system in heavy-duty vehicles that sets the rules for how their electronic parts talk to each other over the CAN bus. Users prefer using the SAE J1939 system because it's fast, trustworthy, and can share lots of data.

In this guide, we'll break down how SAE J1939 works. Whether you're new or just need more info, this is for you.

We'll cover how J1939 is different from CAN and why you might want to use J1939. Plus, we'll get into the technical bits like its code and performance.

We've aimed to keep this simple. Keep reading, as we've also included a beginner's guide to SAE J1939 at the end of the article.


Outline of a J1939 connector pinout with nodes labeled, illustrating the concept of J1939.

What is SAE J1939?

SAE J1939 is the go-to protocol for ensuring smooth communication within the electronic systems of heavy-duty vehicles. It lays down the rules for how these electronic components talk to each other over the CAN bus.

A standout feature of the J1939 protocol is the capability in managing complex data flows across multiple ECUs. By setting a universal communication blueprint, it paves the way for innovators to seamlessly introduce new tech solutions in vehicles. What's more, J1939 stands resilient against the rough conditions typical of robust vehicles.

Stack it against other protocols, like CANopen, and J1939 shines even brighter. It not only supports a broader span of data speeds, ranging from 250 kbps to 1 Mbps, but its 29-bit identifier also offers an expansive message addressing arena.

Diagram showing two ECUs connected through CAN with notes on SAE J1939 protocol and CAN network basics.

Breakdown of J1939 Protocol Facts

Diving into heavy-duty vehicle communication, the SAE J1939 protocol holds power. Here's a breakdown of its essential characteristics;

  • Predominantly Used: Backbone in heavy-duty vehicles such as trucks, buses, and construction equipment.

  • Identifier: Boasts a 29-bit extended CAN ID for diverse communication needs.

  • Origin: An innovation of the Society of Automotive Engineers (SAE), ensuring industry-wide standards.

  • Speeds: Offers data transmission ranging from a brisk 250 kbps to a rapid 1 Mbps.

  • Architecture: An elevated protocol built upon the foundational CAN system.

  • Wiring Choice: Relies on the shielded twisted-pair wiring for enhanced signal integrity.

  • Message Identity: All J1939 messages come with an 18-bit PGN for easy categorization.

  • Signal Nomenclature: In J1939 lingo, signals are termed as SPNs.

  • Adaptability: Thanks to its structure, J1939 can easily integrate with various diagnostics tools and software.

  • Safety and Reliability: Designed to withstand and operate efficiency in the rigorous conditions of heavy machinery environments.

  • Future-Ready: With its capability to handle complex data streams, J1939 is equipped for future innovations in vehicle electronics.

Informational graphic explaining SAE J1939 used in vehicle communication with CAN, highlighting key facts and specifications.
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J1939 vs. CAN: Which is Better?

While J1939 surpasses CAN in terms of advancement, there are times when CAN might be more fitting.

For simpler data transmission needs, CAN could offer adequate performance at reduced costs. CAN often finds its place in smaller networks or situations where utmost reliability isn't crucial. Yet, for most heavy-duty vehicles, J1939 is favored for its robust functions, ability to manage complex data streams, and resilience in tough settings.

Real-life Scenarios of J1939 vs. CAN

J1939 for Industrial:

If you run a logistics company with a fleet of heavy-duty trucks, maintaining seamless communication between numerous onboard system is crucial. To ensure cargo safety, route optimization, and fuel efficiency, the robustness of J1939 is indispensable, catering to your complex operational demands.

CAN for Local:

On the other hand, let's say you operate a local delivery service using a handful of compact vans to transport goods within the city. Your tech needs are relatively straightforward - perhaps monitoring fuel consumption or tacking delivery routes. In such cases the straightforward and cost-effective CAN protocol aligns perfectly with your business scale and requirements.

J1939 Code in Detail

Exploring into the complexity of the J1939 code reveals its importance in automotive communication. Let's break it down:

  • J1939 Code Structure: The J1939 uses a notable 29-bit identifier. This identifier is crucial for directing messages' priority and endpoint.

    • The initial 3 bits emphasize message priority during decision-making, with the utmost priority marked as '0'. It's common to find vital control messages, such as torque control communication between the engine and transmission, processing higher priority. In contrast, non-urgent messages like road speed have lower priority values.

    • The subsequent bit, reserved for futuristic applications, is typically set to 0 when messages are dispatched. Following this, there's a data page selector bit, enhancing the usability of Parameter Group Numbers (PGNs).

  • Message Routing with the J1939 Code: The Protocol Data Unit (PDU) format plays a critical role here. It decides if a message is broadcasted or targets a specific address. The PDU-specific (PS) field, integral to addressing messages, fluctuates based on the PF value. Specifically:

    • If the PF ranges between 0 and 239, the message is address-centric (PDU1), and the PS field embeds the destination address.

    • For PF values spanning 240 to 255, broadcasting is the route chosen (PDU2), with the PS field integrating a group extension.

    • The J1939 code's versatility is further showcased through its support for diverse message types, including broadcasts (sent universally to all ECUs), direct point-to-point interactions, and overarching global messages that anchor the network.

  • Efficiency in J1939 Data Traffic: The J1939 protocol isn't just about communication; it's about smart communication. It offers an array of filtering methods, such as source address, message ID, and parameter group filters. These approaches pare down unnecessary data traffic, bolstering efficiency and reducing data conflict chances.

  • Performance Metric of J1939: The J1939 code is gauged using two key performance yardsticks. Message latency, the span between a message's dispatch and its reception, and bus utilization, which calculates the time percentage the network dedicates to data transmission.

With this understanding of the J1939 code, optimizing a J1939 network becomes a achievable feat.

Breakdown of a 29-bit CAN identifier, detailing priority, data page, PDU format and specific fields, and source address in SAE J1939.

Troubleshooting and Resolving J1939 Code Issues

Addressing "how to fix J1939 code" in a vehicle typically involves a systematic approach.

Begin by using a diagnostic tool to read and interpret the specific J1939 fault code. Once identified, consult the vehicle's service manual for troubleshooting steps related to that code. It's essential to inspect the associated sensors, wiring, and connectors for any visible damage or wear. In many cases, resolving the issue may require replacing faulty components or repairing damaged wiring. After repairs, clear the code using the diagnostic tool and test-drive the vehicle to ensure the problem is fully resolved

Effective Use of J1939 Diagnostic Requests

With the advancement of telematics and IoT in vehicles, the use of J1939 diagnostics and request messages is anticipated to grow. As vehicles become more interconnected, the ability to selectively request data will become even more crucial, which will ensure efficient data transmission and effective vehicle monitoring.

Many J1939 messages are broadcasted over the CAN bus, but some are only transmitted when prompted. This prompted data often encompasses J1939 diagnostic trouble codes (DTCs). This is how it works:


Overview of PGN 59904 request in J1939 with details on data bytes and priority for DM2 diagnostic messages.

  • J1939 request are initiated using a unique message: PGN 59904.

  • This distinct message contains only 3 data bytes.

  • Within these bytes, the desired PGN is specified.

  • J1939 DM2 diagnostic alert can be part of the specified PGN.

  • The request message operates at priority 6.

  • Its primary function is to seek specific data from an ECU.

  • Once an ECU receives this message, it evaluates if it holds the requested data.

  • If the data is present, the ECU prepares a response.

  • Diagnostic tools or monitoring systems typically trigger this request.

  • The ECU's response is dispatched in a separate message using the PGN from the initiating request.


Graphic representation of J1939 protocol for on-request diagnostic trouble codes (DTCs) retrieval process.


J1939 SPN: Decoding Suspect Parameter Numbers in CAN

In the realm of CAN bus communication and vehicle diagnostics, J1939 SPN stands out prominently. It's an identifier for specific signals within CAN message data bytes. Within J1939, SPNs are categorized by Parameter Group Numbers (PGNs) and depict a range of data, from engine speed to temperature and beyond.

To put it simply, the J1939 SPN pinpoints a distinct data signal, characterized by its bit start position, offset, unit, and bit length. This specification lets the receiving end correctly decode the data, ensuring the precise extraction of information from the CAN message.

Consider the J1939 SPN as a specific tag on vehicle data files.

Imagine you have a folder filled with different vehicle performance reports. Each report might be about oil pressure, engine speed, or fuel efficiency. The J1939 SPN acts like the title of each report. So, when data indicates a reading of 3,000 RPM, the J1939 SPN is the tag that says 'Engine Speed', letting anyone reviewing the data know exactly what that 3,000 RPM pertains to.

Line drawing of a tractor with labeled J1939 network nodes, illustrating data communication points.

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PGN Message Board: The Heart of SAE J1939 Communication

The PNG message board plays a crucial role in the SAE J1939 standard. Acting as a unique identifier, the PGN clusters related data parameters, ensuring streamlined message transmission. This 18-bit subset is part of the broader 29-bit extended CAN ID.

The PGN structure is divided into:

  • Data page (1-bit): Determines which data page the PGN belongs to.

  • Reserved (1-bit): Kept for potential future uses in the protocol.

  • PDU format (8-bit): Defines the message's overall structure and purpose.

  • PDU specific (8-bit): Provides detailed addressing or group extension information.

Outlined in SAE J1939/71, PGNs facilitate efficient communication between heavy-duty vehicles and their electronic systems.

The significance of the PGN Message Board is evident in its ability to manage the transmission of J1939 messages across the network. It ensures messages are directed accurately, enhances network stability, and by groping parameters, it refined transmission and reduces traffic.

Decoding the J1939 Connector Pinout Essentials

The J1939 connector is distinctively designed, featuring circular shape and consisting of 9 pins. To integrate an embedded system into a vehicle, correct wiring and the appropriate connector are necessary. It's crucial for anyone involved with heavy-duty vehicles and their electronic components to be familiar with the J1939 pinout.

Here's a brief breakdown of the J1939 pinout:

Top view line drawing of an SAE J1939 connector detailing the pin configuration and labels for vehicle communication.

Side view line drawing of an SAE J1939 connector with multiple pins for vehicle diagnostics.

  • PIN A: Battery (-) - Represents the negative terminal of the battery.

  • PIN B: Battery (+) - Connects to the positive terminal of the battery.

  • PIN C: CAN_H - The high side of the CAN communication bus.

  • PIN D: CAN_L - The low side of the CAN communication bus.

  • PIN E: CAN_SHLD - The shield for the CAN bus, providing protection against interference.

  • PIN F: SAE J1708 (+) - The positive side of the SAE J1708 communication bus.

  • PIN G: SAE J1708 (-) - The negative side of the SAE J1708 communication bus.

  • PIN H: Reserved for OEM or alternate bus CAN_H - Typically set aside for proprietary use or alternate high CAN.

  • PIN J: Reserved for OEM or alternate bus CAN_L - Designated for proprietary applications or alternate low CAN.

From 2016 onwards, a green-colored J1939 connector was launched, phasing out the previous black variant.

This updated model boasts a superior data reading speed of 500 kb/s, mitigating potential harm to truck adapters and diagnostic devices - an issues prevalent with its predecessor.

Beyond the connector, the J1939 protocol contains a transport protocol for dispatching extensive messages that exceed the capacity of a single frame. The protocol's message filtering capability enables ECUs to sift through unnecessary messages, focusing on those applicable to their specific roles. This selective reception minimizes network data congestion, enhancing overall efficiency.

Detailed line art of a large mining haul truck, illustrating its industrial design and structure.

Exploring the J1939 Transport Protocol: Functions and Implications

The SAE J1939 protocol prominently facilitates communication within electronic systems of heavy-duty vehicles. Besides the commonly cited PGN and SPN, the J1939 transport protocol manages the complex transmission of multi-frame messages.

Here's a breakdown of the J1939 transport protocol:

  • It defines the process of disassembling, conveying, and reconstituting data packets across frames.

  • The protocol has two main modes:

    • Broadcast Announce Message (BAM) intended for widespread network communication.

    • Connection mode targeting a specific device.

For data dispatch, the source ECU dispatches an introductory BAM packet to organize the data transmission, dictating the PGN for the multi-frame message, byte count, and packets to be dispatched. This is succeeded by up to 255 data packets, where the initial data byte signifies the sequence order. The ceiling for bytes in a multi-packet message stands at 1785 bytes (7 bytes x 255).

Diagram explaining the structure of a J1939 Broadcast Announce Message (BAM), showing PGN identifier, data bytes, and packet sequence.

Heavy-duty vehicles often engage in J1939 data logging to enhance efficiency and cost-effectiveness. Live diagnostic streaming via J1939 data equips technicians to gauge vehicle health on-the-fly. Utilizing Wi-Fi CAN loggers, vehicle's performance metrics are accessible, fostering predictive maintenance based on J1939 insights. Moreover, J1939 transport protocol can act as a vehicle's Blackbox.

History of SAE J1939

  • 1994 - First released docs were J1939-11, J1939-21, and J1939-31

  • 2000 - The top-level document was published

  • 2000 - CAN became a part of the J1939 standard

  • 2001 - Former standards J1708 and J1587 being replaced by J1939

Timeline graphic showcasing key milestones in the history of the SAE J1939 standard from 1994 to 2001 with relevant standard documents.

Heavy-duty telematics usage is on the rise and is projected to grow even further. With manufacturers transitioning from the traditional CAN bus to CAN FD, we anticipate a parallel shift from SAE J1939 to SAE J1939 FD.

Leveraging J1939 with AutoPi TMU for Advanced Use Cases

By incorporating the AutoPi Telematics Unit (TMU) with the J1939 protocol, businesses can unlock a myriad of advanced use cases. With the integration of AutoPi TMU, predictive maintenance becomes significantly more attainable, ensuring that potential issues are addressed even before they become problematic. This not only reduces downtime but also aids in prolonging the lifespan of heavy-duty vehicles.

Furthermore, by harnessing the insightful data collected, fleet managers and operators can gain a deeper understanding of vehicle performance and usage patterns. This, in turn, enables a more accurate and efficient workflow, ensuring optimal utilization of resources and enhanced operational efficiency.

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