John Deere 1890.04 – RPT (Rear PTO Control Unit) or BCU (Basic Control Unit): PTO Shaft Spline Identifier Switch, Voltage Too Low
Issue description
The control unit detects a supply voltage of less than 9 volts (or a low 5-volt reference) at the positive power supply lead of the PTO shaft spline identifier switch, typically indicating a short-to-ground within the circuit.
Applies to these machines
51 machines- 6110R
- 6120R
- 6130R
- 6145R
- 6155R
- 6175R
- 6195R
- 6215R
- 5080R
- 5090R
- 5100R
- 5080RN
- 5090RN
- 5100RN
- 6230
- 6330
- 6430
- 6530
- 6630
- 6830
- 6930
- 6105M
- 6110M
- 6120M
- 6125M
- 6130M
- 6135M
- 6140M
- 6145M
- 6155M
- 6175M
- 6195M
- 6M-1654
- 6M-1854
- 6M-2104
- 6140R
- 6150R
- 6170R
- 6190R
- 6210R
- 6090MC
- 6090RC
- 6100MC
- 6100RC
- 6110MC
- 6110RC
- 7210R
- 7230R
- 7250R
- 7270R
- 7290R
Check the cause
Visually inspect the PTO Shaft Spline Identifier Switch (B179) for physical damage, disconnections, or loose mounting.
Inspect the wiring harness leading to the switch for wear, damaged insulation, corrosion, or exposed wires shorting to the chassis (ground).
Test continuity of the wiring from the BCU/RPT power supply to the switch using a multimeter, specifically checking for a short to ground.
Turn the key on and verify if the voltage supplied to the switch is being pulled below the required threshold (9V or 5V depending on the system).
Monitor live data from the switch using John Deere Service ADVISOR to see if it transmits accurate spline configuration data to the control software.
Repair steps
Isolate the Circuit
Disconnect the battery or ensure the machine is powered down before repairing electrical lines.
Repair Wiring (If shorted)
If a short-to-ground was found during diagnostics, repair the damaged section of the wiring harness or replace the affected harness section. Ensure all new connections are secure and properly insulated.
Replace the Switch (If faulty)
If the wiring tests perfectly but the voltage drops at the sensor or the sensor fails to read the spline, remove and replace the PTO Shaft Spline Identifier Switch (B179).
Software Update
Ensure the RPT or BCU control software is updated to the latest version, applying any OEM patches that address known switch malfunction logic.
Clear Codes and Calibrate
Reconnect power, use a diagnostic tool to clear the 001890.04 fault code from the BCU/RPT, and reset the system.
Operational Test
Safely engage the rear PTO system to ensure the switch correctly identifies the spline and the system operates without throwing the code again.
Parts you might need
(1)Rear PTO Shaft Spline Identifier Switch (B179)
Ensure the replacement switch is compatible with your specific control unit (BCU vs. RPT) and matches the physical spline configuration of your tractor's PTO housing.
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