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J1587/J1708 and J1939 data line diagnostics

By Larry Turay, MACS contributor

J1587/J1708 and J1939 data line diagnostics

Modern heavy-duty equipment is basically comprised of computers with operating units attached. In short, it is a multiplexing system that replaces traditional power distribution devices with ECUs that communicate electronically over the vehicle datalink. The electronic control units govern power distribution to the vehicle electrical loads by monitoring inputs, such as sensors and switches; they supply power to outputs, such as lighting air conditioning, gauges, and indicators.

Multiplexing reduces the number of interconnecting wires and allows for more precise control of the electrical system by enabling multiple control or diagnostic commands to communicate on a two-wire datalink or network.

The multiplexing system serves three main functions: It transmits multiple electronic messages through the same wire at the same time; it performs tasks and monitors components simultaneously; it uses electronic control units to operate the system, interpreting different messages being transmitted on the same wire. Most vehicles with multiplexing will have two datalinks in common, the J1587/1708 datalink and the J1939 datalink, which are the primary datalinks for chassis and powertrain control. This will be a two-part article, this month we will look at the J1587/J1708 Datalink.

(Check back in November for part 2)

The J1587 datalink is a low-speed datalink that communicates information between the electronic control units on the vehicle. The J1587 is also referred to as J1708. J1708 refers to the SAE standard for the physical part of the datalink, such as wiring and electronic components. J1587 refers to the SAE standard for messaging protocol that communicates on the J1708 network. In the context of vehicle repair, the terms J1708 and J1587b are used interchangeably. The J1587 pins will be in the 9-pin diagnostic connector at Pin F, which is dark green color J1587 + and Pin G, which is orange color J1587.

Symptoms of a malfunctioning J1587 Datalink may include the following conditions: gauges at working; ICU displays no J1587; no Eng. or no ABS; ABS and check engine lamp on; inability to retrieve faults codes from an ECU; the off-board diagnostic tool will at connect to the vehicle; one or more ECUs missing from the diagnostic tools J1587 ECU list. Before diagnosing the J1587/J1708 datalink, check the fuses and the battery voltage and be sure the ECU connections are secure on the datalink, as well.

 If the off-board diagnostic tool will connect to the vehicle, perform the following tests: Voltage Tests: Check the connector on the problem ECU for proper fit, with key OFF, remove the connector for the problem ECU; with the ignition key turned ON, measure voltage between the J1587 + and J1587- terminals on the connector — the reading should be between 3 to 4 VDC. Additionally, measure voltage between the positive post on the battery and J1587+ terminal — reading should be 6 to 11 VDC. Measure the voltage between the positive battery post and J1587- terminal — voltage should be 9 to 13.5 volts.

 If the voltage is NOT within this range, perform a Resistance Test: with the key in the OFF position, remove the connector from the problem ECU, measure the resistance between the J1587+ and J1587- terminals — the value should be 3K to 18K Ohms. Measure the resistance between the J1587+ terminal and the vehicle ground — the value should be more than 1K Ohms. Measure the resistance between the J1587 – terminal and the vehicle ground — the value should be more than 1K Ohms.

 If the resistance is not within this range, Test the J1587 junction block: locate the J1587 junction block connector, and with the off-board diagnostic tool connected, find the J1587/J1708 ECU list, then disconnect a junction block connector. When the connector is removed, the ECU will drop off the list. When a connection is removed and the ECU will not drop off the list, perform a continuity test on the circuit to find the wiring fault. Repair the affected harness.

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