expand compartment 115 by pushing plate 118 toward the left a release valve 121 is normally biased by a compression spring 122 to connect pressure compartment 115 to outer atmospheric pressure upon energizing release valve solenoid 84 release valve 121 contacts a valve seat 123 of housing 113 a diode 124 connected across solenoid 84 shunts surges which might be applied on transistor 71 or solenoid 84 a modulator valve 125 in pressure compartment 115 is normally biased by a compression spring 128 to close a vacuum nozzle 126 connected with intake manifold 130 of the engine on the automobile and opens an atmospheric pressure nozzle 127 upon energizing modulator valve solenoid 72 modulator valve 125 is forced against compression spring 125 to open vacuum nozzle 126 in pressure compartment 115 and close atmospheric pressure nozzle 127 by which flexible diaphragm 114 is forced to compress pressure compartment 115 with negative vacuum in compartment 115 an end of a chain 131 is connected with rivet 117 through its hole 116 the other end of chain 131 is connected with a lever 133 which drives throttle valve 132 a tension spring 134 normally forces lever 133 toward closure of throttle valve 132 by closing main switch 57 base current flows to transistor 73 from power source 56 through main switch 57 release valve.