CM2150 Electronic Control System – 019-165   Data Link Circuit, SAE J1939

The OEM J1939 datalink circuit is located in the OEM wiring harness.

The purpose of this datalink is to allow the ECM to communicate with the vehicle control-operated systems such as the transmission controllers, traction control system, etc.

The traditional OEM J1939 datalink circuit is described as a shielded twisted pair and includes the wires connected to the J1939 datalink positive (+) pin, the J1939 datalink negative (-) pin, and the J1939 (shield) pin in the OEM harness.

On newer vehicles and equipment, OEM’s can utilize an OEM J1939 datalink circuit that is described as an unshielded twisted pair (UTP). The unshielded twisted pair (UTP) J1939 datalink does
not include the J1939 (shield) pin and
only includes the J1939 datalink positive (+) pin and the J1939 datalink negative (-) pin in the OEM harness.

The ECM will broadcast public datalink messages on the OEM J1939 datalink when the ECM is powered and the keyswitch is in the ON position. The ECM will stop broadcasting on the OEM datalink when the keyswitch is turned to the OFF position.

The Society of Automotive Engineers (SAE) J1939 has strict guidelines that
must be followed for successful communication. Understanding some fundamentals about SAE J1939 will help make sure these guidelines are followed.

The main component of an SAE J1939 system is a backbone harness. The harness can be up to 40 meters [131 feet] in length. The backbone harness is terminated at each end with a 120 ohm resistor.

A maximum of thirty different devices can be attached to the SAE J1939 backbone at once. Each device, such as the datalink adapter, is connected to the backbone through a stub, which can be up to 1 meter [3.3 ft] in length. The stub connector is a 3-pin plug.

The terminating resistor caps (1)
must be in place on the OEM backbone harness plugs (2) to maintain proper communication. Each resistor is 120 ohms and can be located in a removable cap.

NOTE: On applications with an Electronic VGT Actuator one of the terminating resistors is manufactured into the Electronic VGT actuator and is non-serviceable.

Some OEMs will choose to provide a complete SAE J1939 backbone harness. If this is supplied, connection to the INSITE™ electronic service tool is accomplished by a 9-pin datalink connector (1), Part Number 3162848.

NOTE: Some OEM’s place a 9-pin connector in the cab, but do
not connect all of the pins to support J1939 protocol.

To check for the OEM J1939 backbone, turn the keyswitch to the OFF position. Measure the resistance from the SAE J1939 datalink positive (+) pin to the SAE J1939 datalink negative (-) pin of the 9-pin Deutsch™ connector.

The multimeter
must read between 50 and 65 ohms for the INSITE™ electronic service tool to be able to establish communication with the ECM.

If the OEM does
not supply the J1939 backbone harness to the 9-pin connector, the
only way to establish J1939 communication with the ECM is through either the bench communication setup or the engine communication setup. Refer to Procedure
022-999.

NOTE: The typical SAE J1939 connector will be a 9-pin connector.

WARNING  Batteries can emit explosive gases. To reduce the possibility of personal injury, always ventilate the compartment before servicing the batteries. To reduce the possibility of arcing, remove the negative (-) battery cable first and attach the negative (-) battery cable last.

CAUTION  The leads must fit tightly in the connector without expanding the pins in the connector otherwise the connector will be damaged.

Disconnect the batteries.

Disconnect the OEM harness connector from the ECM. Turn the keyswitch to the OFF position.

Insert a test lead into the SAE J1939 datalink positive (+) pin of the OEM harness connector, and connect it to the multimeter probe. Insert the other test lead into the SAE J1939 datalink positive (+) pin of the 9-pin Deutsch™ connector, and connect it to the multimeter.

Measure the resistance. The multimeter
must show a closed circuit (10 ohms or less).

If the circuit is
not closed, repair or replace the OEM harness. Refer to the OEM troubleshooting and repair manual for the procedures.

Insert the multimeter lead into the SAE J1939 datalink negative (-) of the OEM harness connector. Touch the other lead to the SAE J1939 datalink negative (-) pin of the 9-pin Deutsch™ connector. Measure the resistance. The multimeter
must show a closed circuit (10 ohms or less)

If the circuit is
not closed, repair or replace the OEM harness. Refer to the OEM troubleshooting and repair manual for the procedures.

If the values are correct, the circuit
must still be checked for a short circuit to ground and a short circuit from pin to pin.

Remove the lead from the SAE J1939 datalink negative (-) pin of the OEM harness connector and insert it into the SAE J1939 datalink (shield) pin, if the shield pin is available.

If the J1939 datalink circuit is an unshielded twisted pair (UTP), the shield pin will
not be provided.

If the shield pin is provided, measure the resistance from the SAE J1939 datalink (shield) pin of the OEM harness connector to the SAE J1939 datalink (shield) pin of the 9-pin Deutsch™ connector.

The multimeter
must show a closed circuit (10 ohms or less). If the circuit is
not closed, repair or replace the OEM harness. Refer to the OEM troubleshooting and repair manual for the procedures.

If the (shield) pin is provided, measure the resistance from the SAE J1939 datalink (shield) pin of the 9-pin Deutsch™ connector to the engine block ground. The SAE J1939 datalink shield
must be grounded to the vehicle battery ground. The multimeter
must show a closed circuit (10 ohms or less). If the circuit is
not closed, refer to the OEM troubleshooting and repair manual for repair instruction.

If more than 10 ohms are measured in any of these steps, there can be an open circuit in the SAE J1939 datalink positive (+) pin, the SAE J1939 datalink negative (-) pin, or the SAE J1939 (shield) pin, or the polarity is
not correct. There can also be an open circuit from the datalink (shield) pin to vehicle battery ground.

If the values are correct, the SAE J1939 datalink positive (+) pin and the datalink negative (-) pin
must still be checked for a short circuit to ground. The SAE J1939 datalink positive (+) pin, the datalink negative (-) pin, and the datalink (shield) pin
must still be checked for a short circuit from pin to pin.

CAUTION  The leads must fit tightly in the connector without expanding the pins in the connector otherwise the connector will be damaged.

Disconnect the OEM harness connector from the ECM. Insert a test lead into the SAE J1939 datalink positive (+) pin of the OEM harness connector and connect it to a multimeter probe. Touch the other multimeter probe to the engine block ground.

Measure the resistance. The multimeter
must show an open circuit (100k ohms or more).

If the circuit is
not open, repair or replace the OEM harness. Refer to the OEM troubleshooting and repair manual.

Remove the test lead from the SAE J1939 datalink positive (+) pin and insert it into the SAE J1939 datalink negative (-) pin. Measure the resistance from the SAE J1939 datalink negative (-) pin of the OEM harness connector to the engine block ground. The multimeter
must show an open circuit (100k ohms or more).

If the circuit is
not open, repair or replace the OEM harness. Refer to the OEM troubleshooting and repair manual.

CAUTION  The leads must fit tightly in the connector without expanding the pins in the connector otherwise the connector will be damaged.

Disconnect the OEM harness connector from the ECM.

Insert a test lead into the SAE J1939 datalink positive (+) pin of the OEM harness connector and connect it to the multimeter probe. Insert the other test lead into another pin in the connector of the OEM harness and connect it to the other multimeter probe.

Measure the resistance from the SAE J1939 datalink positive (+) pin to the first pin in the connector. The multimeter
must show an open circuit (100k ohms or more).

If the circuit is
not open, repair or replace the OEM harness. Refer to the OEM troubleshooting and repair manual.

Measure the resistance from the SAE J1939 datalink positive (+) pin of the OEM harness connector to all other pins in the connector, one at a time. The multimeter
must show an open circuit (100k ohms or more) at all pins, except the J1939 datalink negative (-).

If the circuit is
not open, repair or replace the OEM harness. Refer to the OEM troubleshooting and repair manual.

Remove the test lead from the J1939 datalink positive (+) pin and insert it into the J1939 datalink (shield) pin of the OEM harness connector, if the shield pin is available

NOTE: If the J1939 datalink circuit is an unshielded twisted pair (UTP), the (shield) pin will
not be provided. If the shield pin is
not provided, the datalink negative (-) pin
must still be checked for a short circuit to the other pins.

Insert the other test lead into another pin in the connector. Measure the resistance.

The multimeter
must show an open circuit (100k ohms or more).

If the circuit is
not open, repair or replace the OEM harness. Refer to the OEM troubleshooting and repair manual.

Measure the resistance from the SAE J1939 datalink (shield) pin, if available, to all other pins in the connector, one at a time. The multimeter
must show an open circuit (100k ohms or more).

If the circuit is
not open, repair or replace the OEM harness. Refer to the OEM troubleshooting and repair manual.

Remove the test lead from the SAE J1939 datalink (shield) pin and insert it into the SAE J1939 datalink negative (-) pin of the OEM harness connector. Insert the other test lead into another pin in the connector. Measure the resistance.

The multimeter
must show an open circuit (100k ohms or more).

If the circuit is
not open, repair or replace the OEM harness. Refer to the OEM troubleshooting and repair manual.

Measure the resistance from the SAE J1939 datalink negative (-) pin of the OEM harness connector to all other pins in the connector. The multimeter
must show an open circuit (100k ohms or more) at all pins, except the J1939 datalink positive (+) pin.

If the circuit is
not open, repair or replace the OEM harness. Refer to the OEM troubleshooting and repair manual.

Connect all the components after the repair is complete.