Case IH 518504-03 - BCM – BCM: Cab recirculation blower motor control signal - short circuit to B+

The control module is detecting an unexpected high voltage (battery voltage / 12V) on the control signal circuit that regulates the cab's HVAC recirculation blower fan. This is typically caused by a wiring short, a failed internal component in the blower motor, or a module error.

• MAGNUM AFS Connect
• Magnum 340
• Magnum 400 AFS Connect Tier 3
• Magnum 400 AFS Connect Tier 4
• Steiger AFS Connect

1. 1

   Inspect the wiring harness running from the BCM to the cab roof HVAC unit for chafed insulation, pinched wires, or areas where the control signal wire might be rubbing against a 12V power supply wire.

2. 2

   Disconnect the plug at the recirculation blower motor and the BCM to check for water ingress, corrosion, or bent pins that could bridge the control circuit to a power pin.

3. 3

   With the battery disconnected and the harness unplugged at both ends, test the control signal wire for continuity to the B+ (power) wires using a multimeter.

4. 4

   If the wiring harness tests good, verify if the short to B+ is internal to the recirculation blower motor's circuitry or resistor block.

5. 5

   Check if the tractor has the latest BCM software, as Case IH has issued Product Improvement Programs (PMPs) for Magnum AFS Connect tractors involving BCM 12V software updates.

1. 1

   Power Down

   Turn off the tractor and disconnect the negative battery cable to prevent electrical damage during service.

2. 2

   Access the Blower

   Remove the necessary cab roof panels or headliner components to access the HVAC recirculation blower motor assembly.

3. 3

   Isolate the Fault

   Disconnect the electrical connector from the blower motor. Reconnect the battery temporarily and turn the key to the ON position. Use a multimeter to check the control signal pin on the harness side. If it still reads battery voltage (B+) when it shouldn't, the wiring harness must be repaired or replaced.

4. 4

   Replace the Motor

   If the harness voltage is correct, the short is internal to the blower motor. Remove the mounting screws/isolators and extract the faulty recirculation blower motor. Install the new motor assembly.

5. 5

   Reassembly

   Reconnect the blower motor electrical plug and reinstall the cab roof panels.

6. 6

   Clear Codes

   Reconnect the battery, turn the ignition on, and use the AFS Pro display or a diagnostic service tool to clear the 518504-03 fault code from the BCM.

7. 7

   Test Operation

   Run the HVAC system through all fan speeds to ensure the recirculation blower operates correctly and the fault code does not return.