Try these checks; I don't know the date Ford went from da VAc CC sys to the Electr sys; may have been a Model Year (MY) or mid-MY change in 92?
For vac op sys
Vacuum Dump Valve disconnects the speed control whenever the brake pedal is depressed.
1. Make sure that the brake pedal is fully released and the valve's plunger is in contact with the brake pedal adapter.
2. Move the valve forward in its retaining clip until 3mm (1/8 inch) of the valve plunger is exposed.
3. Make sure that the brake pedal is still in the fully released position, against its stop.
"TRICKS OF THE TRADE" Troubleshooting TSB 90-25-12 by Ford for 86-91 Bronco, Aerostar, Econoline, Explorer, F-150, F-250, F-350, Ranger & cars
Source: by Ford via miesk5 (Al) at Miesk5's Home Page of Big Bronco Links home.comcast.net
Intermittent or Inoperative Speed Control Operation May Occur During Wet Weather Operation TSB 92-7-8 for 86-91 Bronco, Econoline, F-150, F-250, F-350 & Ranger, 91 Explorer, 86-90 Bronco II; "...May be caused by a corroded connector at the servo; Inspect the connector at the servo for corrosion to determine if replacement is required. Refer to the following procedure for service details. Disconnect the connector at the servo Blow dry the connector assembly with shop air. Clean and inspect pins for corrosion Replace any damaged or corroded pins (D1AZ-14488-E). Apply grease (F2AZ-19584-A) to both ends the connector to prevent water entry..."
Source: by Ford via Chilton
Speed control amplifier and wiring
Removal & Installation
1.Disconnect the wiring at the amplifier, located behind the instrument panel.
2.Remove the amplifier mounting bracket attaching screws or nuts and remove the amplifier and bracket.
3.Remove the amplifier from the bracket.
4.Installation is the reverse of removal.
by Ford via Chilton
1.Snap the molded cable retainer over the accelerator cable end fitting attached, to the throttle ball stud.
2.Remove the adjuster retainer clip, if installed, from the adjuster mounting tab.
3.Insert the speed control actuator cable adjuster mounting tab in the slot provided in the accelerator cable support bracket.
4.Pull the cable through the adjuster until a slight tension is felt without opening the throttle plate.
5.Insert the adjuster cable retainer clip slowly, until engagement is felt, then, push it downwards until it locks in position.
Removal & Installation
for later MY and from 93 to 96..
Do the stoplamps light when the brake pedal is depressed? If not, check the stop lamp circuit fuse, stop lamps, wiring and stop lamp switch.
The speed control system also interacts with the brake system, and with the clutch system on manual transmission vehicles. Speed control requires electrical continuity through the stoplamps (and clutch switch on manual transmission vehicles) in order to operate. Proper stoplamp operation is required if the system is to disengage with brakes. Proper clutch switch operation is also required on manual transmission vehicles if the system is to disengage with clutch pedal actuation. Stoplamp and clutch switch circuit diagnosis is detailed in Section 37-05 of the appropriate Shop Manual.
Does the horn work? If not, check the horn circuit fuse, horn relay and horn circuit wiring.
* Check for unseated connectors at the speed control amplifier. The amplifier location varies by carline. Refer to the appropriate Shop Manual, Section 37-05, for location.
* Look for loose or unseated connector pins.
* Check for broken wires at the connectors.
SERVO: Check for the following items.
* Disconnected or cut vacuum hose from the servo to the manifold source.
* Loose or disconnected electrical connector, or broken wire.
* Loose, or cracked plastic elbow at the servo.
* Disconnected, or loose, dump valve hose at the servo.