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Ford Cruise Control - How it works


The Amplifier (Amp) is the "brains" of the system. It simply senses road speed from the Vehicle Speed Sensor (VSS) and opens and closes the throttle to try and maintain that road speed.


The Servo contains a large diaphragm along with two solenoids and a feedback potentiometer. The diaphragm is attached by a cable to the carburetor or throttle body linkage. Engine vacuum is supplied to the Servo through a vacuum check valve which only allows vacuum to go one way.

The two solenoids are for Vacuum and Vent. One side of each solenoid is connected to battery positive with the key on. To activate a solenoid, the Amp grounds out the other side of it.

When the Amp wants to open the throttle, it activates the Vent solenoid which blocks it off. It then pulses the ground on the Vacuum solenoid to apply vacuum to the diaphragm. The diaphragm pulls the throttle open a little more each time the Vacuum solenoid is pulsed.

The Amp keeps activating the Vacuum solenoid until the desired throttle opening is reached. The Amp also uses the feedback potentiometer to determine how much the throttle is moving and bases its adjustments on that.

Even if the Vacuum solenoid stops applying vacuum, the diaphragm will hold its position. It won't release the vacuum until the vent solenoid ground is released. Then the vacuum gets dumped to the atmosphere.

If the Amp just wants to let the throttle go a small amount (going too fast for example) it will momentarily ground the Vent solenoid to dump a small amount of vacuum.

Vehicle Speed Sensor

The VSS is connected to the speedometer cable. Most of the later models had the speedo cable plugged INTO the speed sensor. On this type, the gear on the end of the sensor is driven by the output shaft in the transmission, and the sensor in turn drives the speedo cable.

The sensor acts as a little generator. A magnet spins in a coil of wire to generate an AC voltage.


The system switches are built into the steering wheel cover. Later models have ON\OFF, SET\ACCELL, COAST, and RESUME buttons. Some of the earlier systems did not have the RESUME.

The switches have a ground circuit and also get 12 volts through the coil of the horn relay. By the way, unlike most other brands, Fords during the years that this cruise was used only had a horn relay if they had cruise. We used to get calls all the time from people looking for the horn relay on non-cruise equipped cars.

One wire goes from the switches to the Amp. I'll refer to it as the "switches" wire in this explanation.

Here's how the switches operate:
The ON button when pressed puts positive (about 12 volts) to the "switches" line.
The OFF button when pressed grounds out the "switches" line.
For the other functions, the buttons switch a resistor between ground and the "switches" line. The Amp senses the different resistances and performs the necessary function.

Other Parts

Brake Light Switch

The Amp has a wire coming from the brake lights. It senses the brake light voltage when the operator steps on the brakes and disengages the cruise control.

Note: The Amp is also set to look for a resistance to ground through the brake light bulbs. As a safety feature, if it doesn't see this ground, it won't operate because it thinks the wire to the brake light switch may be broken.

Bottom line is that if both brake light bulbs are burned out, the cruise won't work. I've seen it happen a few times.

Clutch Switch

Vehicles with manual transmissions also have a switch that opens when the clutch is pushed in. This keeps the engine from revving up.

The clutch switch is wired in with the brake light switch so that it breaks the circuit to the brake light bulbs. Due to the safety feature mentioned with the brake lights above, the systems disengages.

Vacuum Dump Valve

Later systems have a vacuum dump valve on the brake pedal. A vacuum line is connected from the valve to the Servo diaphragm. When the brake pedal is pushed, vacuum in the Servo is released (dumped).

This is another safety feature. In the earlier models without this valve, when the brake light switch went bad and you were using the cruise, the more you tried to stop the car the more the Amp tried to accelerate.

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Copyright 1997-2005 Dali Design

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