BCU 001890.03 – PTO Shaft Spline Identifier Switch High Supply Voltage. John Deere System Advisory.

BCU 001890.03 (BCU 1890.03)

Code: 001890.03

Shortcode: 1890.03


Error code BCU 001890.03 is triggered when the Basic Control Unit (BCU) of a John Deere vehicle detects a supply voltage exceeding 16 volts at the positive power supply lead of the PTO shaft spline identifier switch (B179). Such high voltage levels can indicate problems with the vehicle’s electrical charging system, such as a defective battery or generator (alternator), which are responsible for maintaining appropriate voltage levels throughout the vehicle’s electrical circuits.


The BCU may issue a warning to the operator and could potentially limit operations to prevent damage to the electrical systems and components directly affected by the high voltage.


  • Inspect Battery and Charging System: Check the battery for signs of damage or failure. Test the battery’s voltage while the engine is off and running to assess whether it is holding and properly regulating charge. Similarly, evaluate the generator’s output to ensure it is not exceeding normal charging voltages, which typically range between 13.8 and 14.5 volts.
  • Check Voltage Regulator: The voltage regulator may also be faulty if it allows higher-than-normal voltages to be supplied to the vehicle’s systems. Test or replace the voltage regulator if it is found to be malfunctioning.
  • Examine Wiring to B179 Switch: Inspect the wiring and connections leading to the PTO shaft spline identifier switch for any signs of damage, corrosion, or improper connections that might contribute to voltage irregularities.
  • Replace Defective Components: Replace the battery, generator, or voltage regulator if any of these components are determined to be defective.
  • Reset the BCU and Clear Diagnostic Codes: After repairs or replacements are made, reset the BCU to clear the diagnostic codes. Verify that the repair was successful by measuring the voltage at the B179 switch to ensure it is within the specified range.
  • Test System Functionality: Conduct a system functionality test to ensure that all PTO-related operations are performing correctly under normal operating conditions.


Maintaining stable and appropriate voltage levels is crucial for the reliable operation of all electronic components in modern agricultural equipment. Regular monitoring and maintenance of the battery, generator, and other charging system components can prevent voltage issues that may lead to equipment malfunction or damage. Ensuring these systems are functioning correctly helps keep your John Deere machinery operating safely and efficiently.