Abstract: An accessory is provided for a vehicle having a transmission vacuum modulator and an engine equipped with a supercharging device such as a turbocharger that temporarily boosts intake manifold pressure above atmospheric pressure. The accessory normally provides a communication path for use in coupling a manifold port to a vacuum modulator port so that variations in intake manifold pressure serve as an input signal to the vacuum modulator. The accessory provides for comparing intake manifold pressure with a reference pressure, and, when the manifold pressure exceeds the reference pressure, the communication path is interrupted.