What is the Cruise Control Vacuum Reservoir all about?

When setting your cruise control speed, you don’t consider the behind-the-scenes actions which take place. The engine creates vacuum pressure when the sucking action of pistons in the down stroke draw air into the cylinders. The vacuum pressure is harnessed to operate other features such as cruise control. A servo acts as a vacuum operated switch to hold the vehicle speed steady when cruise control is set. A constant supply of vacuum ensures the servo maintains its position. When there is more demand for vacuum than the engine can supply immediately, operations cease to function as they should. The servo will not stay in the set position, and the cruise control speed will drop or surge to excessive speeds.

To moderate vacuum and to store a cache of vacuum when supply runs low, an air bladder known as the cruise control vacuum reservoir is installed in the system. When your engine is started, there is no initial vacuum and it needs to ‘charge’ the lines and vacuum reservoir. As it builds vacuum, the air bladder has negative pressure inside, or vacuum, that is stored up. When the engine RPMs increase and the engine vacuum drops, the cruise control vacuum reservoir has a storage of vacuum to maintain steady operation of the functions that need it. If the vacuum reservoir has a leak or is cracked, the vacuum pressure will not remain constant and the cruise control will possibly surge to excessive speeds or steadily lose speed.

Keep in mind:

• Cruise control is not the only system that uses the engine’s vacuum. Power brakes also employ the engine vacuum and heater controls at times.

How it's done:

• The cruise control vacuum reservoir is verified that it needs to be replaced. The vacuum line is then disconnected from the reservoir.
• The defective Cruise Control Vacuum Reservoir is removed.
• The new Cruise Control Vacuum Reservoir is installed.
• The vacuum line is connected to the reservoir. The Cruise Control Vacuum Reservoir is tested for operation by starting and checking for leaks.
• The vehicle is road tested for proper operation of the Cruise Control.

Our recommendation:

Your cruise control vacuum reservoir is typically hidden behind the front bumper where it is inaccessible for inspection. It does not require maintenance. If you suspect the vacuum reservoir is cracked or leaking, have it checked by one of our expert technicians and repaired if necessary.

What common symptoms indicate you may need to replace the Cruise Control Vacuum Reservoir?

• Cruise control surges to speeds well above the set cruising speed
• Cruise control will not sustain speed and drops well below the set speed
• Nausea-inducing surge can be felt when the cruise is set
• Other vacuum-operated features such as power brakes, air conditioning, and heater control are functioning improperly

How important is this service?

Cruise control is a convenience feature, so its operation is not crucial. If the cruise control vacuum reservoir needs to be replaced, you can do so at your leisure.