Case IH 639-03 - IMM – IMM: SAE J1939 Data Link Voltage Above Normal, Or Shorted To High Source

The CAN bus communication lines are experiencing a voltage that exceeds the normal operating range (above 5V). This is typically caused by a wiring short circuit where the CAN lines are touching a higher voltage power supply (such as a 12V battery line or a 5V sensor reference line), which disrupts network communication between modules.

• MAGNUM AFS Connect
• Magnum
• Puma
• Steiger
• Maxxum
• Optum

1. 1

   Inspect the CAN bus wiring harness and connectors leading to the IMM for chafing, melted insulation, or water ingress that could cause a short to a power source.

2. 2

   With the system powered, use a multimeter to check the voltage supply on the CAN lines to verify if it is abnormally high (exceeding the standard 5V range).

3. 3

   Disconnect the battery and measure the resistance between the CAN High and CAN Low wires. A healthy CAN bus should read approximately 60 ohms.

4. 4

   Check the integrity of the insulation and cable connections to ensure the CAN lines are not shorted to the battery positive (B+).

1. 1

   Power Down

   Turn off the machine and disconnect the battery to prevent electrical damage during testing.

2. 2

   Isolate the Short

   Disconnect the CAN connector at the IMM. Measure the resistance of the CAN lines to ground and to the power supply lines to locate the short circuit.

3. 3

   Inspect the Harness

   Trace the wiring harness from the IMM back to the main CAN backbone, looking for pinch points, rubbed-through wire loom, or damaged connectors.

4. 4

   Repair Wiring

   If a short to a high voltage source is found, repair the damaged section of the wiring harness using proper splicing techniques and heat-shrink tubing, or replace the damaged connector.

5. 5

   Check Terminators

   Ensure the CAN terminator resistors are intact and functioning properly, as damaged terminators can cause network voltage anomalies.

6. 6

   Reassemble and Clear

   Reconnect the battery and the IMM. Turn the key on and use the tractor's AFS Pro display or a diagnostic service tool to clear the stored 639-03 fault code. Verify that the code does not return and that module communication is restored.