BCU 001890.04 – PTO Shaft Spline Identifier Switch Low Supply Voltage. John Deere System Advisory.

BCU 001890.04 (BCU 1890.04)

Code: 001890.04

Shortcode: 1890.04


Error code BCU 001890.04 is triggered when the Basic Control Unit (BCU) of a John Deere vehicle detects a supply voltage of less than 9 volts at the positive power supply lead of the PTO shaft spline identifier switch (B179). This low voltage condition typically indicates a short to ground within the circuit, which can affect the functionality of the switch and may impact the overall operation of the PTO system.


In response to this voltage issue, the BCU may signal a warning to the operator and could restrict PTO operations to prevent damage or operational inconsistency that could arise from inadequate power supply to the switch.


  • Inspect Wiring and Connections: Check the wiring and connections to the PTO shaft spline identifier switch for any signs of damage, wear, or evidence of shorting to ground. Pay particular attention to exposed wires, damaged insulation, or loose connections.
  • Test for Continuity and Shorts: Use a multimeter to test the continuity of the wiring from the power supply to the switch. Look for any continuity to ground that would indicate a short circuit.
  • Repair or Replace Wiring: If a short to ground is found, repair the damaged section of wiring or replace it entirely to ensure integrity of the circuit. Ensure all connections are secure and properly insulated after making repairs.
  • Verify Voltage Supply: After addressing any shorts, verify that the correct voltage is being supplied to the PTO shaft spline identifier switch under normal operating conditions.
  • Reset the BCU and Clear Diagnostic Codes: Clear any fault codes from the BCU and reset the system to ensure that the repairs have resolved the issue without other underlying problems remaining.
  • Test PTO System Operation: Conduct operational tests of the PTO system to ensure the switch is functioning correctly and the system responds appropriately under all conditions.


Regular maintenance of the electrical system, including careful inspection of wires and connections, is vital to prevent issues such as shorts to ground, which can lead to significant operational problems. Keeping all components in good working order ensures that the PTO and related systems continue to operate efficiently and reliably, supporting the productivity and functionality of your John Deere equipment.