The 1987 and later logic module uses the servo to control the throttle when the speed control feature is active. It is basically a large diaphragm with a cable attached to the center and two solenoids. One solenoid provides vacuum to the diaphragm, which pulls the cable to open the throttle, the other vents the diaphragm and closes the throttle. The logic module uses these solenoids to increase and decrease throttle, based on the changing load and speed of the vehicle. It uses the reserve vacuum in the power brake booster to provide vacuum during low vacuum (or boost). After passing, you may notice that it takes some time for the speed control to "grab" the throttle again. This is because the reserve vacuum in the booster has been depleted and it must build up again before the servo can function. This problem can be eliminated by adding a vacuum reserve canister in the vacuum line to the servo (after the check valve).
For 1986 and earlier models, the cruise control servo is responsible for its own control, as opposed to the logic module controlling it. The outputs of the speed control switch are sent directly to the servo and the speedometer cable also feeds through it on the way to the speedometer.
For 1987 and later models, when the logic module is turning on or off either solenoid, it momentarily checks the output of the solenoid driver to see if it is responding the way it should. If it does not see about 0V when on (open circuit) or 12V when off (short circuit), a fault code 34 is stored. No fault codes are stored if problems occur with the servo on earlier models.
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Updated 12/19/2003.
Copyright © 1996-2003 Russ W. Knize.