Chevrolet Full Size Cars 1979-1989

Vacuum Servo


Most factory or dealer installed cruise control systems for these vehicles utilize a vacuum servo to rotate the throttle plate and hold engine speed. The servo is connected to the throttle plate by linkage, either a chain, rod or cable.


Because the vacuum servo is designed to modulate the throttle based on a vacuum signal, the diaphragm must be intact and able to hold vacuum in order to operation properly.

  1. Disconnect the vacuum hose from the servo assembly.
  3. Use a hand-held vacuum pump to apply 14 inches of vacuum to the servo. Watch the pump gauge for 1 minute and make sure that no more than 5 inches of vacuum leaks down.
  5. If no vacuum pump is available, push the diaphragm in by hand, then hold your finger tightly over the vacuum nipple and release the diaphragm. If the diaphragm is intake, it should hold in the pushed in position.
  7. Repair or replace any diaphragm which is defective or cannot hold vacuum


  1. Disconnect the negative battery cable.
  3. Remove the vacuum line from the servo assembly.
  5. Disconnect the servo linkage.
  7. Remove the servo and bracket assembly retaining bolts, then remove the servo from the vehicle. To install:
  9. Install the servo and bracket assembly, then secure using the retaining bolts.
  11. Connect the servo linkage.
  13. Install the vacuum line to the servo assembly.
  15. Connect the negative battery cable.


Chain Linkage

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Fig. Fig. 1 Chain linkage adjustment late model vehicles

Chain linkage can be found on most full size vehicles up to and including 1983 models. To adjust the chain linkage, first fully warm the engine, then check and adjust idle speed to assure the throttle is properly positioned. For most vehicles equipped with an idle stop solenoid, make sure the air conditioning is off and the solenoid is disconnected when adjusting curb idle. Shut the engine off and adjust the linkage.

For some older models, adjust the chain by loosening the jamnut on the back of the servo, then turn the servo assembly to adjust. The chain should be positioned with minimal slack, but should not hold the throttle open.

On most models, the chain is adjusted by removing the chain from the connector and making it as tight as possible, while still leaving a little slack. Again, the slack is necessary to make sure the chain does not hold the throttle open. Once the chain is adjusted, install it back to the connector.

Cable Linkage

Some Chevrolet full size vehicles are equipped with a cable linkage from the servo to the throttle. The cable linkage is usually adjusted either by jamnuts or by a servo blade which contains various adjustment holes. In either case, once the engine is fully warmed, position the cable so it is tight, with little slack, but NOT holding the throttle open. For vehicles equipped with the servo blade, if the desired hole seems to open the throttle, take the next hole on the blade in order to leave the required slack.

Rod Linkage
  1. Start and warm the engine to normal operating temperature.
  3. Make sure the idle speed is properly adjusted, then shut the engine OFF .
  5. For diesel engines, adjust the length of the rod to achieve minimum slack with the injection pump on the slow idle screw.
  7. For gasoline engines, be sure the idle speed motor is retracted and the throttle is fully closed. Install the rod on the throttle stud at the large end of the slot, then adjust the length so the stud is at the end of the slot and the rod is aligned with the hole in the servo. Install the retainer, insert the rod end through the holes and snap the retainer into place.