Case IH 520594-03 - BCM – BCM: Short circuit to B+ (Battery Positive) on the license plate light circuit

The control module is detecting 12V (battery voltage) on the ground or control wire of the license plate light when it should not be present. This is typically caused by the wiring harness chafing and shorting against a live power wire, an internal short within the lamp assembly, or a fault within the module itself.

• MAGNUM AFS Connect
• Magnum (AFS Connect series)
• Puma (130–240)
• Maxxum (110–145)
• Farmall (75A–115A, 85C–120U)
• JX series
• Axial-Flow series (5130, 6130, 7130, 7230, 8230, 9230)

1. 1

   Perform a diagnostic pre-scan to confirm the code is active. Clear the code and cycle the exterior lights to see if the fault returns immediately (indicating a hard short) or intermittently.

2. 2

   Inspect the license plate light assembly (usually roof-mounted or located on the rear fender/platform) for physical damage, water ingress, or melted plastic.

3. 3

   Turn the ignition OFF (terminal 15 off) and disconnect the license plate lamp connector. Turn the ignition ON and use a multimeter to check the voltage on the harness pins. If the ground or control wire shows ~12V (B+) when the circuit should be off, there is a short to power in the harness.

4. 4

   Wiggle the wiring harness along its routing path (especially near pinch points, cab entry points, and hinges) while monitoring the multimeter to see if the short is intermittent.

5. 5

   If the harness tests fine (no voltage present when disconnected), the short circuit is likely internal to the license plate light assembly itself.

1. 1

   Disconnect Power

   Turn off the ignition and disconnect the negative battery terminal to prevent further electrical damage during repair.

2. 2

   Access the Lamp

   Locate the license plate light assembly on the rear of the cab roof or rear fender. Remove the mounting screws holding the lamp or the support bracket.

3. 3

   Inspect the Connector

   Unplug the electrical connector and inspect the pins for corrosion, melting, or bridging, which can cause a short.

4. 4

   Repair Wiring

   If the diagnostic tests indicated a short in the harness, trace the wire back toward the BCM. Locate the chafed wire (often where it rubs against the frame or shares a loom with a power wire) and repair it using weather-sealed butt connectors and heat shrink tubing.

5. 5

   Replace the Lamp

   If the lamp assembly is internally shorted or heavily corroded, install a new license plate light assembly. Plug in the connector and secure the mounting screws.

6. 6

   Reassemble and Test

   Reconnect the battery. Turn the ignition on, activate the exterior lights, and verify the license plate light illuminates correctly.

7. 7

   Clear Fault Codes

   Use a diagnostic tool to clear the 520594-03 fault code from the BCM and verify it does not return.