The speed control system is operated by the use of a cable and a vacuum controlled servo. Electronic control of the
speed control system is integrated into the Powertrain Control Module (PCM). The controls consist of two steering
wheel mounted switches. The switches are labeled: ON/OFF, RES/ACCEL, SET, COAST, and CANCEL.

The system is designed to operate at speeds above 30 mph (50 km/h).

5.7L V-8 Engine

The speed control system is fully and electronically controlled by the Powertrain Control Module (PCM). A cable
and a vacuum controlled servo are not used. This is a servo-less system. The controls consist of two steering
wheel mounted switches. The switches are labeled: ON/OFF, RES/ACCEL, SET, COAST, and CANCEL.

The system is designed to operate at speeds above 30 mph (50 km/h).

OPERATION

When speed control is selected by depressing the ON switch, the PCM (the ECM if equipped with a diesel engine)
allows a set speed to be stored in its RAM for speed control. To store a set speed, depress the SET switch while
the vehicle is moving at a speed between 35 and 85 mph. In order for the speed control to engage, the brakes
cannot be applied, nor can the gear selector be indicating the transmission is in Park or Neutral.

The speed control can be disengaged manually by:

•

Stepping on the brake pedal

•

Depressing the OFF switch

•

Depressing the CANCEL switch.

•

Depressing the clutch pedal (if equipped).

SPEED CONTROL

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NOTE: Depressing the OFF switch or turning off the ignition switch will erase the set speed stored in the
PCM (the ECM with a diesel engine).

For added safety, the speed control system is programmed to disengage for any of the following conditions:

•

An indication of Park or Neutral

•

A rapid increase rpm (indicates that the clutch has been disengaged)

•

Excessive engine rpm (indicates that the transmission may be in a low gear)

•

The speed signal increases at a rate of 10 mph per second (indicates that the coefficient of friction between
the road surface and tires is extremely low)

•

The speed signal decreases at a rate of 10 mph per second (indicates that the vehicle may have decelerated
at an extremely high rate)

Once the speed control has been disengaged, depressing the RES/ACCEL switch (when speed is greater than 30
mph) restores the vehicle to the target speed that was stored in the PCM (the ECM with a diesel engine).

While the speed control is engaged, the driver can increase the vehicle speed by depressing the RES/ACCEL
switch. The new target speed is stored in the PCM (the ECM with a diesel engine) when the RES/ACCEL is
released. The PCM (the ECM with a diesel engine) also has a

tap-up

feature in which vehicle speed increases at

a rate of approximately 2 mph for each momentary switch activation of the RES/ACCEL switch.

A “tap down” feature is used to decelerate without disengaging the speed control system. To decelerate from an
existing recorded target speed, momentarily depress the COAST switch. For each switch activation, speed will be
lowered approximately 1 mph.

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SPEED CONTROL

DIAGNOSIS AND TESTING

DIAGNOSIS AND TESTING - VACUUM SUPPLY TEST

3.7L / 4.7L Engines

Actual engine vacuum, a vacuum reservoir (9) equipped with a one-way check valve, and vacuum lines (2) are used
to supply vacuum to the speed control servo.

1. Remove rubber air tube between throttle body and air filter housing to gain access to speed control servo.

2. Disconnect vacuum hose at speed control servo and install a vacuum gauge into the disconnected hose.

3. Start engine and observe gauge at idle. Vacuum gauge should read at least ten inches of mercury.

4. If vacuum is less than ten inches of mercury, determine source of leak. Check vacuum line to engine for leaks.

Also check actual engine intake manifold vacuum. If manifold vacuum does not meet this requirement, check for
poor engine performance and repair as necessary.

5. If vacuum line to engine is not leaking, check for leak at vacuum reservoir (9). Disconnect vacuum line at res-

ervoir and connect a hand-operated vacuum pump to reservoir fitting. Apply vacuum. Reservoir vacuum should
not bleed off. If vacuum is being lost, replace reservoir.

6. The reservoir (9) is equipped with an integral one-way check valve to retain a certain amount of vacuum within

the reservoir. Verify operation of one-way check valve and check it for leaks.

1 - Air Cleaner Box
2 - Vacuum Line
3 - Speed Control Servo
4 - Cross Support
5 - Electrical. Connector
6 - Servo Mounting Bracket
7 - Servo Mounting Bolt
8 - Vacuum Reservoir Mounting Bolt
9 - Vacuum Reservoir

SPEED CONTROL

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