J1708, also known as SAE J1587, is a communication protocol crucial for diagnostics in older heavy-duty trucks. This guide provides a comprehensive overview of J1708 Code, its functionality, and troubleshooting techniques for common issues.
A standard 6-pin J1708 diagnostic connector.
What is J1708 Code?
J1708 is a serial communication protocol used in heavy-duty vehicles manufactured primarily before the mid-2000s. It allows Electronic Control Modules (ECMs) to communicate with each other and with diagnostic tools. This communication enables technicians to retrieve diagnostic trouble codes (DTCs), monitor sensor data, and perform various tests. J1708 operates at a relatively slow speed of 9600 bits per second using a simple two-wire twisted pair configuration.
Troubleshooting J1708 Communication Issues
Problems with J1708 communication can prevent proper diagnostics. Here’s a step-by-step guide to troubleshoot common issues:
1. Verify Power to the Diagnostic Port
- Check the Indicator Light: When connecting your diagnostic tool, ensure its power indicator light illuminates. This confirms power to the adapter.
- Voltage Test: If the adapter doesn’t power up, check for battery voltage (around 12V) between Pin C (Power) and Pin E (Ground) on the 6-pin connector. Low or no voltage indicates a power or ground issue.
Testing for battery voltage between Pin C and Pin E.
2. Check Fuses
A blown fuse can interrupt power to the diagnostic port. Consult the vehicle’s fuse box diagram and check the fuses associated with the diagnostic connector or related circuits (e.g., radio, cigarette lighter).
3. Diagnose Adapter Communication
- Software Diagnostics: Utilize the software provided with your diagnostic adapter (e.g., Nexiq, Noregon, Diesel Laptops) to check communication between the laptop and the adapter, and between the adapter and the truck. Select the J1708 protocol for testing.
Example of diagnostic software checking J1708 communication.
- Driver Issues: If communication fails between the laptop and the adapter, try reinstalling or updating the adapter drivers.
4. Check Data Link Voltage
- Key On: Ensure the vehicle’s ignition key is in the “ON” position.
- High Voltage: Measure voltage between Pin E (Ground) and Pin A (J1708+). You should observe fluctuating voltage between 3.00V and 4.50V, indicating data transmission.
- Low Voltage: Measure voltage between Pin E (Ground) and Pin B (J1708-). Fluctuating voltage between 0.500V and 1.500V should be present.
Checking voltage fluctuations on the J1708+ line.
5. Inspect for Shorts
- Unplug Modules: Disconnect all ECMs from the network.
- Resistance Test: Check for resistance between Pin A and Pin B. A reading less than OL (Open Loop) indicates a short between the J1708+ and J1708- wires. Also, check for shorts to ground on both Pin A and Pin B using Pin E as the ground reference.
Testing for a short circuit between the J1708 data lines.
6. Identify Aftermarket Devices
Aftermarket devices (e.g., ELDs, safety systems) improperly installed can disrupt J1708 communication. Inspect for non-factory wiring or spliced connections near the diagnostic port.
J1708 in 9-Pin Connectors
J1708 is sometimes found in the industry-standard 9-pin connector, typically on pins F and G. The troubleshooting procedures remain the same as with the 6-pin connector.
J1708 signal location on a 9-pin connector.
Conclusion
Troubleshooting J1708 code issues requires a systematic approach. By following these steps and utilizing appropriate diagnostic tools, you can effectively diagnose and resolve communication problems in heavy-duty trucks equipped with the J1708 protocol. If problems persist, consult a qualified diesel technician or refer to vehicle-specific wiring diagrams.
