Speed Control: Overview Dodge Dakota III

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

1. Stepping on the brake pedal
2. Depressing the OFF switch
3. Depressing the CANCEL switch.
4. Depressing the clutch pedal (if equipped).

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

1. An indication of Park or Neutral
2. A rapid increase rpm (indicates that the clutch has been disengaged)
3. Excessive engine rpm (indicates that the transmission may be in a low gear)
4. 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)
5. 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.

Scheme 1

Actual engine vacuum, a vacuum reservoir (2) equipped with a one-way check valve, and vacuum lines (3), (4) AND (5) 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 lines to engine (3), (4) AND (5) 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. Disconnect vacuum line at reservoir 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 (2) 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.

DESCRIPTION

The speed control servo cable is connected between the speed control vacuum servo diaphragm and the throttle body control linkage. This cable is used only with 3.7L/4.7L engines.

This cable causes the throttle control linkage to open or close the throttle valve in response to movement of the vacuum servo diaphragm.

Scheme 2

Scheme 3

Scheme 4

Scheme 5

1. Disconnect negative battery cable at battery.
2. Remove air intake tube at top of throttle body. The accelerator cable must be partially removed to gain access to speed control cable.
3. Hold throttle in wide open position. While held in this position, slide throttle cable pin (1) from throttle body bellcrank.
4. Using a pick or small screwdriver, press release tab (2) to release plastic cable mount from bracket. Press on tab only enough to release cable from bracket. If tab is pressed too much, it will be broken. Slide plastic mount (4) towards right side of vehicle to remove throttle cable from throttle body bracket.
5. Using finger pressure only, disconnect servo cable connector (2) at throttle body bellcrank pin by pushing connector off bellcrank pin towards front of vehicle. DO NOT try to pull connector off perpendicular to the bellcrank pin. Connector will be broken.
6. Slide speed control cable plastic mount (2) towards right of vehicle to remove cable from throttle body bracket.
7. Remove servo cable from servo. Refer to «SERVO-SPEED CONTROL»(ref-247660-S26340466952007020100000) .

A speed control servo is used only with 3.7L V-6 or 4.7L V-8 engines.

The Powertrain Control Module (PCM) controls the solenoid valve body. The solenoid valve body controls the application and release of vacuum to the diaphragm of the vacuum servo. The servo unit cannot be repaired and is serviced only as a complete assembly.

Power is supplied to the servo's by the PCM through the brake switch. The PCM controls the ground path for the vacuum and vent solenoids.

The dump solenoid is energized anytime it receives power. If power to the dump solenoid is interrupted, the solenoid dumps vacuum in the servo. This provides a safety backup to the vent and vacuum solenoids.

The vacuum and vent solenoids must be grounded at the PCM to operate. When the PCM grounds the vacuum servo solenoid, the solenoid allows vacuum to enter the servo and pull open the throttle plate using the cable. When the PCM breaks the ground, the solenoid closes and no more vacuum is allowed to enter the servo. The PCM also operates the vent solenoid via ground. The vent solenoid opens and closes a passage to bleed or hold vacuum in the servo as required.

The PCM duty cycles the vacuum and vent solenoids to maintain the set speed, or to accelerate and decelerate the vehicle. To increase throttle opening, the PCM grounds the vacuum and vent solenoids. To decrease throttle opening, the PCM removes the grounds from the vacuum and vent solenoids. When the brake is released, if vehicle speed exceeds 30 mph to resume, 35 mph to set, and the RES/ACCEL switch has been depressed, ground for the vent and vacuum circuits is restored.

Two separate switch pods operate the speed control system. The steering-wheel-mounted switches use multiplexed circuits to provide inputs to the PCM for ON, OFF, RESUME, ACCELERATE, SET, DECEL and CANCEL modes. Refer to the owner's manual for more information on speed control switch functions and setting procedures.

The individual switches cannot be repaired. If one switch fails, the entire switch module must be replaced.

When speed control is selected by depressing the ON, OFF switch, the PCM 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 approximately 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

1. Stepping on the brake pedal
2. Depressing the OFF switch
3. Depressing the CANCEL switch.

The speed control can be disengaged also by any of the following conditions

1. An indication of Park or Neutral (auto. trans.)
2. The VSS signal increases at a rate of 10 mph per second (indicates that the co-efficient of friction between the road surface and tires is extremely low)
3. Depressing the clutch pedal (manual trans.).
4. Excessive engine rpm (indicates that the transmission may be in a low gear)
5. The VSS signal decreases at a rate of 10 mph per second (indicates that the vehicle may have decelerated at an extremely high rate)
6. If the actual speed is not within 20 mph of the set speed

The previous disengagement conditions are programmed for added safety.

Once the speed control has been disengaged, depressing the ACCEL switch restores the vehicle to the target speed that was stored in the PCM's RAM

Note. Depressing the OFF switch will erase the set speed stored in the PCM's RAM.

If, while the speed control is engaged, the driver wishes to increase vehicle speed, the PCM is programmed for an acceleration feature. With the ACCEL switch held closed, the vehicle accelerates slowly to the desired speed. The new target speed is stored in the PCM's RAM when the ACCEL switch is released. The PCM 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 ACCEL switch.

The PCM also provides a means to decelerate without disengaging speed control. To decelerate from an existing recorded target speed, depress and hold the COAST switch until the desired speed is reached. Then release the switch. The ON, OFF switch operates two components: the PCM's ON, OFF input, and the battery voltage to the brake switch.

Scheme 6

1. Disconnect and isolate negative battery cable from battery.
2. Remove airbag module. Refer to «RESTRAINTS - SERVICE INFORMATION»(ref-247645) .
3. Unplug electrical connector.
4. Remove speed control switch mounting screw (1) and remove switch from steering wheel.

The vacuum reservoir (2) is located under the windshield wiper cowel (1).

The vacuum reservoir is used to supply the vacuum needed to maintain proper speed control operation when engine vacuum drops, such as in climbing a grade while driving. A one-way check valve is used in the vacuum line between the reservoir and the vacuum source. This check valve is used to trap engine vacuum in the reservoir. On certain vehicle applications, this reservoir is shared with the heating/air-conditioning system. The vacuum reservoir cannot be repaired and must be replaced if faulty.