John Deere 1890.03 – BCU: PTO Shaft Spline Identifier Switch (Component B179) - Supply Voltage Too High

The Basic Control Unit (BCU) has detected a supply voltage exceeding normal limits (typically exceeding 16 volts, or a 5-volt reference circuit reading too high) at the positive power supply lead of the rear PTO shaft spline identifier switch. This usually indicates a short to power in the wiring harness or an internal short within the switch itself.

• 6110R
• 6120R
• 6130R
• 6145R
• 6155R
• 6175R
• 6195R
• 6215R
• 6230
• 6330
• 6430
• 6110M
• 6120M
• 6125M
• 6130M
• 6135M
• 6140M
• 6145M
• 6155M
• 6175M
• 6195M
• 6M-1654
• 6M-1854
• 6M-2104
• 5080R
• 5090R
• 5100R
• 5080RN
• 5090RN
• 5100RN
• 6165J
• 6185J
• 6205J
• 6210J
• 1654
• 1854
• 2054
• 2104
• 6130-6930 series

1. 1

   Inspect the wiring harness and connectors leading to the PTO shaft spline identifier switch (B179) located at the rear PTO housing for chafed wires, melted insulation, or exposed copper.

2. 2

   Disconnect the switch and use a multimeter to test the continuity of the wiring from the BCU to the switch connector, checking for a short to power (voltage on the signal/supply line exceeding the specified 5V or 12V baseline when the key is ON).

3. 3

   Measure the voltage at the switch connector under normal operating conditions to confirm if the high voltage is coming from the harness (BCU/wiring issue) or if the switch is internally shorted.

1. 1

   Disconnect Power

   Turn off the ignition and disconnect the battery to prevent electrical damage during repairs.

2. 2

   Access the Switch

   Locate the B179 PTO Shaft Spline Identifier Switch at the rear PTO assembly.

3. 3

   Repair Wiring (If Applicable)

   If your multimeter tests revealed a short to power in the harness, trace the harness back toward the BCU. Repair the damaged section of wiring or replace the affected harness pigtail. Ensure all splices are sealed with heat-shrink tubing.

4. 4

   Replace the Switch (If Applicable)

   If the wiring tests perfectly but the code persists (or if the switch fails an internal resistance test), the switch is internally shorted. Unbolt the old switch from the PTO housing, install the new switch, and reconnect the harness.

5. 5

   Reassemble and Clear Codes

   Reconnect the battery. Use a John Deere diagnostic tool (Service ADVISOR) or the tractor's onboard diagnostic display to clear the BCU 001890.03 fault code.

6. 6

   Operational Test

   Start the tractor and engage the rear PTO. Verify that the BCU correctly identifies the spline configuration and that the fault code does not return.