Lafayette 3612 Engine VNQ6800 - Caterpillar

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 g06101687
Typical example of the schematic for the CAN A data link
Illustration 2 g06101696
Typical example of the schematic for the CAN B data link
Illustration 3 g06101703
Typical example of the schematic for the CAN C data link
Table 2
Troubleshooting Test Steps Values Results
1. Use the "System Communication Status to Check the Status of the Other Devices on the Suspect CAN Data Link"
A. In the electronic service tool, click the "Diagnostics" tab on the tool bar.
B. Select the "System Communication Status" option from the drop-down list.
Component Identified
Result: One or more of the devices on the suspect data link is not visible on the electronic service tool.
Repair: Thoroughly inspect the electrical connectors for the suspect devices
Refer to Troubleshooting, "Electrical Connectors - Inspect".
Check the power supply to the suspect devices.
If necessary, refer to Troubleshooting, "Electrical Power Supply - Test".
If the fault has been resolved, return the engine to service.
If the fault is still present, proceed to Test Step 2.
Result: All devices are visible on the electronic service tool.
Proceed to Test Step 2.
2. Check the Data Link Terminating Resistance
A. Disconnect the P1 and P2 connectors from the ECM.
B. Measure the resistance between P1:34 and P1:50.
C. Measure the resistance between the P2:21 and P2:29.
D. Measure the resistance between the P1:37 and P1:38.
Between 50 Ohms and 70 Ohms
Result: The resistance is less than 50 Ohms - there is a short circuit in the harness.
Repair: Repair the connectors or the harness and/or replace the connectors or the harness. Ensure that all 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 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.
Repair: 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 110 and 130 Ohms, proceed to Test Step