Case IH 517678-03 - BCM – BCM: Trailer brake lamps - short circuit to B+

The Failure Mode Indicator (FMI) "03" universally designates a "Voltage above normal, or shorted to high source". This means the control module has detected that the circuit supplying power to the trailer brake lamps is unintentionally shorted to battery voltage (B+). This condition can cause the trailer brake lights to stay illuminated constantly, blow a fuse, or trigger a circuit protection shutdown within the module.

• MAGNUM AFS Connect
• Magnum AFS Connect 180
• Magnum AFS Connect 200
• Magnum AFS Connect 220
• Magnum AFS Connect 240
• Magnum AFS Connect 250
• Magnum AFS Connect 280
• Magnum AFS Connect 310
• Magnum AFS Connect 340
• Magnum AFS Connect 380
• Magnum AFS Connect 400

1. 1

   Isolate the tractor from the trailer by disconnecting the trailer plug from the tractor's rear socket. If the fault code goes inactive or clears, the short circuit to B+ is located in the trailer's wiring harness.

2. 2

   Inspect the tractor's 7-pin trailer connector for severe corrosion, water intrusion, dirt, or bent pins that could be bridging the brake lamp circuit pin to a constant 12V power pin.

3. 3

   Trace and visually inspect the wiring harness from the rear trailer socket forward to the BCM. Look for pinched wires, chafed insulation rubbing against the chassis, or melted wires.

4. 4

   With the brake pedal released and the trailer disconnected, use a multimeter to check the brake lamp pin on the tractor's socket. If it shows 12V (battery voltage), there is an active short to power in the tractor's harness or a stuck relay/internal BCM fault.

1. 1

   Disconnect power

   Disconnect the tractor's battery negative terminal before performing electrical repairs to prevent further module damage.

2. 2

   Clean or replace the socket

   If the short is isolated to debris or corrosion inside the 7-pin trailer socket, attempt to clean it with electrical contact cleaner. If the terminals are deformed or the housing is melted/cracked, cut the wires back to clean copper and wire in a replacement OEM 7-pin socket.

3. 3

   Repair the wiring harness

   If a short is found in the wiring harness between the BCM and the rear socket, cut out the damaged section of wire. Splice in a new section of wire of the same gauge using weather-sealed (heat-shrink) butt connectors.

4. 4

   Repair trailer wiring

   If the tractor tests fine but the code returns when the trailer is connected, inspect and repair the trailer's brake light circuit, specifically looking for crossed wires near the trailer plug or inside the trailer's junction box.

5. 5

   Clear codes and test

   Reconnect the battery. Turn the key on and use the AFS Pro display or an electronic diagnostic tool to clear the inactive BCM fault codes. Depress the brake pedal several times to verify the code does not return and the trailer brake lamps operate normally.