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The following background information is related to this procedure:The CAN data links are also known as J1939 data links. A data link is an industry standard for sending data between different devices in the same application.High speed data is transferred via the data links. The data links cannot be accurately tested without complicated equipment. The data links require a resistance of 60 Ohms between the two wires to transmit the data correctly. This resistance is made up of two 120 Ohm resistors. The two resistors are known as "Terminating Resistors". The terminating resistors should be at opposite ends of a data link circuit. If this resistance is not present, then the data will be intermittent or unreadable.Note: The wiring for a J1939 data link is a shielded twisted pair cable. If the wiring is damaged, the replacement type must be shielded twisted pair cable.
Illustration 1 g03680024
Schematic diagram for the CAN data link
Illustration 2 g03239957
Typical view of the pin locations on the P1 connector
(16) CAN 0+
(36) CAN 0-
Table 2
Troubleshooting Test Steps Values Results
1. Inspect Electrical Connectors and Wiring
A. Turn the keyswitch to the OFF position.
B. Thoroughly inspect the connectors in the circuit for the CAN data link. Refer to Troubleshooting, "Electrical Connectors - Inspect" for details.
C. Perform a 45 N (10 lb) pull test on each of the wires that are associated with the CAN data link.
D. Check the harness for abrasion and pinch points from the keyswitch to the Electronic Control Module (ECM).
Loose connection or damaged wire
Result: There is a fault in a connector or the wiring.
Repair the connectors or the harness and/or replace the connectors or the harness. Ensure that all of the seals are correctly in place and ensure that the connectors are correctly connected.
Use the electronic service tool to clear all logged diagnostic codes and verify that the repair eliminates the fault.
Result: All connectors, pins, and sockets are correctly connected and/or inserted and the harness is free of corrosion, of abrasion and of pinch points.
Proceed to Test Step 2.
2. Check the Data Link Terminating Resistance
A. Disconnect the P1 connector from the ECM.
B. Measure the resistance between the P1:16 and P1:36.
50 Ohms to 70 Ohms
Result: The resistance is less than 50 Ohms. There is a short circuit in the harness.
Repair the connectors or the harness and/or replace the connectors or the harness.
Ensure that all of the seals are correctly in place and ensure that the connectors are correctly connected.
Use the electronic service tool in order to clear all logged diagnostic codes and then verify that the repair eliminates the fault.
Result: The resistance is between 110 and 130 Ohms. One of the terminating resistors may have failed.
Locate the two terminating resistors and remove the two terminating resistors from the harness. Depending on the application, one or both of the terminating resistors may be located in other ECMs on the data link.
Measure the resistance of the two terminating resistors.
If one of the terminating resistors is incorrect, replace the faulty terminating resistor.
If the two terminating resistors are between 50 and 70 Ohms, proceed to Test Step 4.
Result The resistance is greater than 150 Ohms. There may be a break in the harness.
Proceed to Test Step 3.
3. Check the Data Link Wiring
A. Disconnect each of the connectors that connect other devices on the data link.
B. Use a multimeter to measure the resistance between P1:16 to each of the CAN+ pins on other devices on the CAN 0 data link.
C. Use a multimeter to measure the resistance between P1:36 to each of the CAN- pins on other devices on the CAN 0 data link.
Less than 5 Ohms
Result: At least one of the resistance measurements is greater than 5 Ohms.
Repair the connectors or the harness and/or replace the connectors or the harness.
Ensure that all seals are correctly in place and ensure that the connectors are correctly connected.
Use the electronic service tool to clear all logged diagnostic codes and verify that the repair eliminates the fault.
Result: All resistance measurements are less than 5 Ohms.
Proceed to Test Step 4.
4. Check the Other Devices on the J1939 Data Link
A. Use the appropriate service tools in order to diagnose other devices on the data link.
Other devices
Result: The other devices are not working correctly.
Use the appropriate service tools in order to repair other devices on the data link.
Use the electronic service tool to clear all logged diagnostic codes and verify that the repair eliminates the fault.
Result: The other devices are operating correctly.
Repeat this test procedure from Test Step 1.