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P0594Cruise Control Servo Control Circuit/Open

An open or general fault has been detected in the cruise control servo control circuit, the output that drives the actuator holding the throttle at the set speed, and cruise control is disabled.

Definition

On cable-actuated cruise systems the module drives a servo, an electric motor/clutch actuator or a solenoid-controlled vacuum unit, that pulls the throttle cable to maintain the set speed. The module monitors the servo control circuit's voltage and current against its commands and sets P0594 when it detects an open circuit or feedback that does not match. The symptom is cruise that will not engage, engages and immediately drops out, or fails to hold speed on grades. Normal throttle operation is unaffected because the driver's pedal remains fully mechanical, and the MIL typically does not illuminate.

Common causes

  • Open motor winding, clutch coil, or solenoid inside the servo actuator
  • Broken or corroded wiring between the module and the servo
  • Unplugged or corroded servo connector, often exposed to underhood weather
  • Blown fuse or open feed circuit to the servo
  • Failed servo driver stage in the cruise or engine control module

Common misdiagnoses

  • Servo replaced when a connector or fuse is the actual fault
  • Throttle cable adjusted or replaced for an electrical circuit code
  • Vehicle speed sensor replaced because cruise cannot hold speed
  • Steering-wheel switch replaced for an actuator-side fault

Troubleshooting steps

  1. 1.Inspect the servo connector and cable

    Check the connector for corrosion and backed-out pins, and verify the servo-to-throttle cable is attached with proper slack per service data.

  2. 2.Measure actuator circuit resistance

    Measure the servo motor, clutch, and solenoid circuits at the servo connector and compare each to specification; OL on any circuit confirms an internal open.

  3. 3.Verify power and ground at the servo

    With the key on and cruise master on, confirm battery voltage on the feed and a good ground drop under 0.1 volt.

  4. 4.Check control-circuit continuity

    With both ends disconnected, verify each control wire from the servo to the module reads near zero ohms with no shorts between circuits.

  5. 5.Run a functional test

    Use the scan tool's cruise servo output test, or a safe road test, to confirm the actuator strokes and holds when commanded.

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