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DTC P0594: Speed Control Servo Power Circuit: Testing

  1. Turn the ignition off. Disconnect the Speed Control Servo harness connector. Turn ignition on, engine not running. Using a 12-volt test light connected to ground, probe the Speed Control Brake Switch Output terminal in the Servo Harness connector. It is necessary to PRESS and HOLD the Speed Control Switch in the ON position while checking for voltage. Does the test light illuminate brightly? If yes, go to next step. If no, go to step  3.
    2. WARNING: When the engine is operating, do not stand in a direct line with the fan. Do not put your hands near the pulleys, belts or fan. Do not wear loose clothing.
  2. The conditions that set the DTC are not present at this time. The following may help in identifying the intermittent condition. With the engine running at normal operating temperature, monitor the DRBIII® parameters related to the DTC while wiggling the wiring harness. Look for parameter values to change and/or a DTC to set. Review the DRBIII® Freeze Frame information. If possible, try to duplicate the conditions under which the DTC was set. Refer to any Technical Service Bulletins (TSB) that may apply. Visually inspect the related wire harness. Look for any chafed, pierced, pinched, or partially broken wires. Visually inspect the related wire harness connectors. Look for broken, bent, pushed out, or corroded terminals. Were any of the above conditions present? If yes, repair as necessary. If no, test is complete.
    3. 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. Use Miller Special Tool (8815) when performing diagnosis at PCM harness connectors.
  3. Turn the ignition off. Disconnect the PCM harness connectors. Disconnect the Brakelight Switch harness connector. Measure the resistance of the Speed Control Power Supply circuit from the Brakelight Switch harness connector to the appropriate terminal of the Miller Special Tool. Is the resistance below 5.0 ohms? If yes, go to next step. If no, repair the open/high resistance in the Speed Control Power Supply circuit between the PCM and Brakelight Switch.
  4. Measure the resistance between ground and the Speed Control 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 Speed Control Power Supply circuit.
  5. Measure the resistance between ground and the Speed Control 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 Speed Control Brake Switch Output circuit. If no, go to next step.
  6. Measure the resistance of the Speed Control Brake Switch Output circuit from the Brakelight Switch harness connector to the Speed Control Servo harness connector. Is the resistance below 5.0 ohms? If yes, repair the open/high resistance in the Speed Control Brake Switch Output circuit.
  7. Reconnect the Speed Control Servo harness connector. Turn ignition on, engine not running. Using a 12-volt test light connected to ground, probe the Speed Control Power Supply circuit in the Brakelight 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 Brakelight Switch. If no, go to next step.
    8. NOTE: Before continuing, check the PCM harness connectors for corrosion, damage or terminal push out. Repair as necessary.
  8. There are no possible causes remaining, replace PCM. Program the new PCM. See PROGRAMMING .
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