G 20 Van V6-262 4.3L VIN Z (1993)
Cruise Control Servo: Description and Operation
Servo Unit
Fig. 2 Servo Assembly
The servo, Fig. 2, consists of a vacuum operated diaphragm, a normally open solenoid valve to vent the diaphragm, a normally closed solenoid valve
connecting the diaphragm chamber to the vacuum source, and a variable inductance position sensor that provides the module with servo position data.
The servo operates the throttle in response to control module commands as follows: When the system is engaged and operating at a steady rate of speed,
both solenoid valves are closed. Vacuum is trapped in the diaphragm chamber exerting a constant force on the diaphragm and the throttle position
remains fixed.
When the vehicle is losing speed due to increased load, or when the operator seeks to increase speed through the control switch assembly, the module
energizes the vacuum valve solenoid. This increases the force exerted on the diaphragm, which in turn increases the throttle opening. When vehicle
speed increases above the pre-set cruise speed, or when the operator seeks to decrease speed through the control switch assembly, the module
de-energizes the diaphragm vent valve solenoid. This opens the valve, decreases force on the diaphragm, and allows the throttle return spring to decrease
the throttle opening.
During normal operation, the module will pulse the operation of the vent or vacuum valves, as needed, to maintain the set cruise speed. The average
duration of each pulse is 10 milliseconds. If vehicle speed drops 5 mph below set cruise speed, the module will hold the vacuum valve in the completely
open (energized) position. If vehicle speed exceeds set cruise speed by 3 mph or more, then module will hold the vent valve in the open (de-energized)
position. The module will hold the valves open until the vehicle speed matches the set cruise speed. The diaphragm vent valve will also remain in the
fully open position if the brake or clutch pedal is depressed, the ignition is switched off, or if an open circuit exists in the system feed circuit or in the
servo position sensor.
