Case IH 639-04 - IMM – IMM: J1939 Data Link Voltage Below Normal, or Shorted to Low Source

The CAN bus voltage has dropped out of its normal operating range to below 1V, indicating a physical short to ground in the wiring harness, a failed termination resistor, or an internally shorted control module.

• MAGNUM AFS Connect
• Magnum series tractors
• Steiger series tractors
• Puma series tractors
• Maxxum series tractors
• Farmall series tractors
• Axial-Flow Combines
• Patriot Sprayers

1. 1

   Verify the tractor's battery and charging system are providing a stable 12.5V (key on, engine off) to 14.0V (engine running) to rule out false CAN codes from low system voltage.

2. 2

   With the key ON, use a multimeter to measure the voltage of the CAN High and CAN Low pins at the diagnostic connector to chassis ground. Normal readings are ~2.5V to 2.7V for CAN High and ~2.3V to 2.5V for CAN Low. Readings below 1V indicate a hard short to ground.

3. 3

   Disconnect the battery and measure the resistance between CAN High and CAN Low at the diagnostic port. A healthy CAN bus reads exactly 60 ohms. ~120 ohms indicates a failed resistor or broken wire; near 0 ohms indicates wires are shorted together or to ground.

4. 4

   If the voltage is below 1V, perform a module isolation (drop-out) test by unplugging modules on that specific CAN backbone one at a time, starting with the IMM. If unplugging a module causes the CAN voltage to instantly return to ~2.5V, that module is internally shorted.

1. 1

   Disconnect Power

   Disconnect the negative battery cable before servicing the wiring harness.

2. 2

   Trace the Harness

   If the module isolation test did not reveal a faulty ECU, physically trace the CAN bus wiring harness from the IMM module back to the main splice block.

3. 3

   Inspect for Damage

   Look for pinch points, chafing against the chassis, or melted insulation (especially near the engine block, exhaust, or cab pivot points) which would cause the CAN wires to short to the metal frame.

4. 4

   Repair Wiring

   If a short is found, cut out the damaged section of wire. Solder and use OEM-approved adhesive-lined heat shrink tubing to repair the connection. Do not use cheap crimp connectors on CAN data lines.

5. 5

   Replace Components

   If a termination resistor tested bad, replace it. If the IMM module was found to be internally shorted during the isolation test, replace the module.

6. 6

   Clear Codes

   Reconnect the battery, turn the key ON, and use the Case IH Electronic Service Tool (EST) or the in-cab diagnostic display to clear the inactive fault codes. Cycle the key to ensure the code does not return.