Case IH 520579-03 - BCM – BCM: Turn light rear left - short circuit to B+

The control module has detected battery positive voltage (B+) on the rear left turn signal circuit at a time when it should not be present (e.g., when the light is commanded off), or it detects no voltage drop across the load.

• MAGNUM AFS Connect
• Steiger AFS Connect

1. 1

   Inspect the rear left turn light assembly, lens, and socket for physical damage, water intrusion, or melted contacts that could bridge the power circuit.

2. 2

   Perform a harness isolation test: Turn the ignition OFF, disconnect the wiring harness from the rear left turn light, turn the ignition ON (do not activate turn signal), and measure voltage at the harness connector's signal pin. Reading ~12V indicates a harness short; reading 0V indicates an internal light assembly short.

3. 3

   Check the BCM output pin for the rear left turn signal with the harness disconnected at the BCM. If the pin constantly outputs 12V when commanded off, the BCM's internal driver may have failed.

4. 4

   Trace the wiring from the rear fender back to the cab/BCM, looking for pinch points, chafing, or aftermarket accessories improperly spliced into the lighting circuit.

1. 1

   Power Down

   Turn off the tractor, remove the key, and disconnect the battery ground cable to prevent further electrical damage during repair.

2. 2

   Address the Light Assembly (If Faulty)

   If the diagnostic tests proved the light assembly has an internal short, unbolt the rear left turn light from the fender/cab mount. Install the replacement assembly and plug in the connector.

3. 3

   Address the Wiring (If Faulty)

   If the harness is shorted to power, open the wire loom at the identified pinch point or damaged area. Cut out the melted or chafed section of the turn signal wire (and the power wire it was shorting to). Splice in new automotive-grade wire using weather-sealed butt connectors or solder with adhesive-lined heat shrink tubing. Re-loom and secure the harness away from moving parts.

4. 4

   Reassembly

   Reconnect the BCM connectors (if removed) and the battery ground cable.

5. 5

   Clear and Test

   Turn the ignition ON. Use a diagnostic scan tool to clear the 520579-03 fault code from the BCM. Test the left turn signal to verify proper operation and ensure the code does not return.