Cruise Control Servo Control Circuit/Open
The PCM has detected an electrical malfunction with the cruise control servo control circuit.
Code Set Parameters
The PCM works in conjunction with the cruise control module on most vehicle applications to control vehicle speed automatically. If the PCM detects that vehicle speed cannot be controlled, it initiates a self test of the entire cruise control system. This code is stored if the PCM detects an abnormal voltage/resistance level in the cruise control servo control circuit.
You will likely not be able to set the vehicle speed control if this code is stored in the PCM. A service engine soon lamp may or may not be illuminated along with this code. Some models require multiple drive cycles (usually 3) for a service engine soon lamp to be illuminated. In other models a service engine soon lamp is not illuminated for cruise control codes. Blown fuses are also likely to accompany this code.
The most likely cause of this code setting condition is a failed servo control unit. Many late model vehicles utilize a cruise control module with an integrated servo. Shorted or open circuitry in the cruise control buttons is also a possible cause of this malfunction. Liquid spilled onto cruise control components has proven problematic in the past. Other causes may include shorted or open wiring, damaged or corroded connectors, and blown fuses (although a blown fuse is usually a reaction to a more severe problem).
Be sure to check for blown fuses after replacing faulty components. Often multiple components are replaced because of a blown fuse.
- The PCM uses vehicle speed sensor data to help regulate vehicle speed when the cruise control is activated
- In most vehicles, a small servo motor is attached to the throttle linkage
- Voltage received from the PCM and the cruise control module prompt the servo motor to open or close the throttle plate as needed
- A steel cable inside of a plastic housing is often used between the cruise control servo and the throttle linkage
- If the desired speed is less than the actual speed, the throttle plate will be opened and if the actual speed is more than the desired speed, the throttle plate will be allowed to close
- When the throttle plate is opened it causes engine RPM and vehicle speed to increase in order for the vehicle to reach the desired speed
- When the throttle plate is closed, engine RPMs decrease as does vehicle speed
- Other vehicles are equipped with a drive by wire throttle system that uses no accelerator cable
- These types of vehicles control vehicle speed with the same actuator motor which opens and closes the throttle plate during normal driving.
Several tools will be instrumental in successfully diagnosing this code
- A suitable OBD-II scanner (or code reader) and a digital volt/ohmmeter will be most helpful in performing a successful diagnosis
Begin with a visual inspection of all wiring and connectors
- Repair or replace damaged, disconnected, shorted, or corroded wiring, connectors, and components as necessary
- Always retest the system after repairs are completed to ensure success.
If all system wiring, connectors, and components (Including fuses) appear to be in normal working order, connect the scanner (or code reader) to the diagnostic connector and record all stored codes and freeze frame data
- This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
- Continue by clearing the code and operating the vehicle to see if it returns
- This will help to determine whether or not the malfunction is intermittent
After the codes are cleared, test drive the vehicle to see if the code returns
- If the code fails to immediately return, you may have an intermittent condition
- Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can be made
Vacuum operated cruise control systems are designed to operate using a combination of vacuum from the engine and an electronic cruise control module
- A constant supply of engine vacuum is supplied to the cruise control servo solenoid
- When the cruise control module commands the cruise control to set, the servo solenoid opens allowing vacuum to the servo
- The cruise control module then uses a signal from the driver controls to regulate the flow of vacuum from the engine and adjust or maintain vehicle speed with the throttle linkage
- Check vacuum supply hoses visually and replace any hoses that are questionable
- If hoses appear normal, perform vacuum pressure test and compare your findings with manufacturer’s specifications and repair vacuum lines or engine components as required
- Test the one way valve by attempting to blow air through it from both directions
- You should only be able to blow through it from one direction
- Replace the one way vacuum valve as needed.
Begin the diagnosis by performing a careful visual inspection of al visible wiring harnesses, linkages, cables, electrical connectors and fuses
- Repair or replace damaged or corroded components as needed
- Reset the PCM and retest the cruise control system.
If the system parts seem to be in proper working order, suspect a faulty cruise control switch
- Replace the switch then test drive the vehicle and operate the cruise control system to ensure that the repair was successful
If the code returns, or the cruise control fails to set at the desired speed, raise the drive wheels and safely secure the vehicle
- Using a helper inside the vehicle, place the vehicle in drive and allow the speedometer to reach a speed sufficient to set the cruise control (usually 25 to 35 mph)
- Disconnect the electrical connector from the cruise control switch and test for voltage using the digital volt/ohmmeter
- Compare your findings with manufacturer’s specifications and replace the switch if readings coincide.
If there is no voltage or ground signal at the switch, disconnect the PCM and cruise control module connectors and do a continuity test on the circuitry between the switches in the passenger compartment, the relevant fuse panel, and the PCM
- Compare your findings to manufacturer’s specs and repair open or shorted circuits as required
- Clear any codes that are present and retest the system.
If the continuity test reveals no malfunctions, perform a resistance test on the cruise control on/off switch and buttons using the digital volt/ohmmeter
- Remember to leave the cruise control module and PCM disconnected before testing circuit resistance
- Replace cruise control switches as required
- Clear the codes and retest the system