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P0593Cruise Control Multi-Function Input "B" Circuit High

The cruise control Multi-Function Input B circuit is stuck at a high voltage, indicating an open circuit or a short to voltage, so button presses never reach the module and cruise is inoperative.

Definition

With the switch at rest, the Multi-Function Input B circuit sits at the module's pull-up reference, and pressing a button pulls the voltage down through a specific ladder resistance. P0593 sets when the circuit stays at or above the reference even while buttons are pressed, which means the pull-down path is broken by an open circuit, or the line is shorted to a higher voltage source. Common breaks are in the clockspring, which flexes with every steering input, or at column connectors. The result is cruise buttons that do nothing; there is no driveability effect and the MIL generally remains off.

Common causes

  • Broken clockspring conductor from steering wheel rotation wear
  • Open in the signal or switch-ground wire in the column harness
  • Unplugged or backed-out terminal at the switch pod or column connector
  • Open circuit inside a failed cruise switch
  • Signal wire shorted to an ignition or battery voltage circuit

Common misdiagnoses

  • Switch pod replaced when the clockspring circuit is broken
  • Control module replaced for a simple open in the column wiring
  • Brake switch replaced because no cruise function works
  • Whole steering wheel assembly replaced instead of testing the circuit

Troubleshooting steps

  1. 1.Verify the stuck-high state

    Back-probe Input B at the module and press each button; if the voltage never leaves the pull-up reference, the pull-down path is open.

  2. 2.Jumper at the switch connector

    Bridge the signal to its return through a resistor matching one button's spec at the switch connector; if the module now sees the button, the switch is bad, otherwise the fault is toward the module.

  3. 3.Test the clockspring

    Check continuity through the clockspring circuits while slowly rotating the wheel lock to lock, since a fatigue break often only opens at certain wheel angles.

  4. 4.Check the switch return/ground

    Verify the switch's ground or return wire has continuity to its termination point, as an open return mimics an open signal.

  5. 5.Check for a short to power

    If the measured voltage exceeds the normal pull-up reference, disconnect the module and hunt for a feed from another powered circuit in shared harness runs.

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