John Deere CAB1865 – CAB: Illogical Ignition Switch Feedback

The Cab Control Unit (CAB) has detected an illogical combination of inputs from the ignition key switch. This occurs when the controller receives signals indicating that the key is in multiple positions at once (for example, the 'start' circuit and the 'accessory' circuit being active simultaneously). This is an invalid state for the switch and points to an internal failure of the ignition switch itself or a short circuit in the associated wiring.

1. 1

   Access the onboard diagnostic display to confirm CAB 1865 is the active code and note any other active fault codes that may be related.

2. 2

   Carefully visually inspect the ignition switch's electrical connector and the surrounding wiring harness for signs of damage, such as melted plastic, corroded terminals, or loose wires.

3. 3

   Perform a key switch continuity test: Disconnect the negative battery terminal, remove dash panels to access the back of the switch, unplug the switch, and use a multimeter to verify continuity between terminals (B, S, I/IGN, A, G) at each key position. OFF: No continuity between B and any other. ACCESSORY: Continuity between B and A. RUN/ON: Continuity between B, I, and A. START: Continuity between B and S, and often B and I. Connections should be exclusive to the intended key position without unexpected combinations.

4. 4

   If the switch passes the continuity test, reconnect the battery and perform a voltage test. Check for battery voltage at the ignition (I) terminal in the 'RUN' position and the start (S) terminal in the 'START' position to rule out voltage drops or wiring issues.

1. 1

   Disconnect the battery

   Ensure the negative battery terminal is disconnected.

2. 2

   Remove the old switch

   After accessing the rear of the switch, take note of the wiring connector's orientation. Unscrew the retaining nut on the front or release the snap-in tabs to remove the switch.

3. 3

   Install the new switch

   Position the new ignition switch in the dashboard and secure it with its retaining nut or by snapping it into place.

4. 4

   Connect wiring

   Firmly plug the wiring harness connector into the back of the new switch.

5. 5

   Reconnect the battery

   Reconnect the negative battery terminal.

6. 6

   Test operation

   Turn the key to each position (Accessory, Run, Start) to confirm that all functions work correctly and that the tractor starts as expected. Check the diagnostic display to ensure the CAB 1865 fault code does not reappear.