Vacuum-operated modulator valves are installed in automobile automatic transmissions for the purpose of controlling shift patterns. The modulator is connected to the engine through a combination of steel tubing and rubber hoses. When higher amounts of engine vacuum act on the modulator, the transmission begins to shift earlier and softer. As engine loads increase, vacuum diminishes, causing the transmission to shift later and more firmly. Various symptoms arise when the vacuum modulator diaphragm ruptures or when vacuum leaks develop. You can troubleshoot these symptoms to determine if your car’s modulator is faulty.
Start your car and allow the engine to warm up to its normal operating temperature. If the car is in a garage, ensure the garage door is wide open or pull the car to an outer drive. Place the transmission in the “Park” position and set the parking brake firmly.