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DTC P0594: Speed Control Servo Power Circuit: Testing

  1. Turn the ignition off. Disconnect the speed control servo harness connector. Ignition on, engine not running. It is necessary to PRESS and HOLD the speed control switch in the ON position while checking for voltage. Using a 12-volt test light connected to ground, probe the (V40) S/C brake switch output terminal in the servo harness connector. Does the test light illuminate brightly? If yes, see INTERMITTENT CONDITION  under SELF-DIAGNOSTIC SYSTEM. If no, go to next step.
    2. CAUTION: Do not probe the PCM harness connectors. Probing the PCM harness connectors will damage the PCM terminals resulting in poor terminal to pin connection. Install Miller Special Tool (8815) to perform diagnosis.
  2. Turn the ignition off. Disconnect the PCM harness connectors. Disconnect the brake lamp switch harness connector. Measure the resistance of the (V32) S/C power supply circuit from the brake lamp switch harness connector to the appropriate terminal of Miller Special Tool (8815). Is the resistance below 5.0 ohms? If yes, go to next step. If no, repair the open/high resistance in the (V32) S/C power supply circuit between the PCM and brake switch.
  3. Turn the ignition off. Disconnect the PCM harness connectors. Disconnect the brake switch harness connector. Measure the resistance between ground and the (V32) S/C power supply circuit in the brake switch harness connector. Is the resistance below 100 ohms? If yes, go to next step. If no, repair the short to ground in the (V32) SIC power supply circuit.
  4. Turn the ignition off. Disconnect the S/C servo harness connector. Disconnect the brake switch harness connector. Measure the resistance of the (V40) S/C brake switch output circuit from the brake switch harness connector to the S/C servo harness connector. Is the resistance below 5.0 ohms? If yes, go to next step. If no, repair the open/high resistance in the (V40) S/C brake switch output circuit.
  5. Turn the ignition off. Disconnect the speed control servo harness connector. Disconnect the brake switch harness connector. Measure the resistance between ground and the (V40) S/C brake switch output circuit at the speed control servo harness connector. Is the resistance below 100 ohms? If yes, repair the short to ground in the (V40) S/C brake switch output circuit. If no, go to next step.
  6. Disconnect the brake lamp switch harness connector. Reconnect the PCM harness connectors. Ignition on, engine not running. Using a 12-volt test light connected to ground, probe the (V32) speed control power supply circuit in the brake lamp switch harness connector. It is necessary to HOLD the cruise control switch in the ON position to get an accurate reading. Does the test light illuminate brightly? If yes, replace the brake lamp switch. If no, go to next step.
  7. Before continuing disconnect the PCM harness connector and check the related wiring terminals for corrosion, damage or terminal push out. Repair as necessary. Pay particular attention to all power and ground circuits. If there are no possible causes remaining, replace and program the PCM. See PROGRAMMING .
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