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Diagnose Cruise Control

Copyright AA1Car

Cruise control not working? Cruise control not maintaining a constant speed? These are common problems that can prevent your cruise control system from functioning properly.

The cruise control system is designed to make highway driving less tiring. It allows you to select a speed and take your foot off the accelerator pedal. This type of cruise control is ideal for open road driving when there is little traffic, or the traffic is rolling along at a fairly constant speed.

 cruise control switch

The cruise control system maintains the speed you select when you press the SET button until you step on the brake pedal, step on the gas to pass another vehicle, select a different speed setting, or turn the system off (press the CANCEL or OFF button).

Pressing the RESUME button after braking will signal the system to resume the former speed you selected. Pressing the ACCEL or (+) button will increase the speed, while pressing the DECEL, COAST or (-) button will decrease the speed. Most cruise control systems will only operate at speeds above 25 to 30 mph, and will not activate unless your are driving faster than the minimum speed requirement.

adaptive cruise control

ADAPTIVE CRUISE CONTROL

On newer vehicles with "adaptive" cruise control, the system will maintain a predetermined distance behind the car ahead of you, and adjust the speed up or down to maintain that distance. This type of system is better suited for heavy traffic conditions where the speed of the cars around you are constantly changing. Vehicles with adaptive cruise control have a small laser or radar transmitter and receiver (distance sensor) mounted in the front to detect vehicles ahead of you. The sensor sends out a laser beam or radar signal, and when the signal bounces off a vehicle ahead and returns to the sensor, the time delay allows the cruise control module to computer the following distance.

Laser-based systems require a clear field of vision for accurate range finding, so the laser must be mounted in the grille or behind the windshield (which provides additional protection against dirt and moisture). Road splash may obscure the sensor and set a fault code. Radar-based systems, by comparison, are more expensive but can be mounted behind plastic bumper covers and are unaffected by dirt or weather conditions.

The adaptive cruise control module (which may integrated into the body control module) not only interacts with the throttle but also the brake system to speed up or slow down the vehicle as needed. This requires a lot of two-way communications, data sharing and feedback via the vehicle's controller area network system. The active cruise control module needs inputs from its range finder sensor as well as vehicle speed, throttle position and braking status so it can calculate and maintain the proper following distance. Consequently, if there are any communication faults on the CAN bus, or the vehicle has lost input from a key sensor such as the laser or radar range finder, the vehicle speed sensor, or throttle position sensor, the system can't function. The same is true if it can't communicate with the throttle control system and brakes to regulate vehicle speed. Any of these faults should set one or more codes and make adaptive cruise control unavailable until the problem has been diagnosed and repaired.

Diagnostics for adaptive cruise control systems currently require a factory scan tool and software to run system self-tests and to check the range finder's inputs. Replacement modules and range sensors are currently dealer-only parts, but that will likely change as more of these systems are produced.

 cruise control vacuum actuator

ELECTROMECHANICAL CRUISE CONTROL

On older vehicles, a vacuum actuator connected to the throttle linkage opens and closes the throttle to maintain the vehicle's speed. The actuator may be connected to the throttle with a linkage, cable or ball chain. When the actuator pulls on the linkage, it opens the throttle and increases engine speed to maintain vehicle speed as when driving up a hill. When less throttle is needed (as when descending the hill), the actuator relaxes the linkage and the vehicle coasts until throttle is again needed.

ELECTRONIC CRUISE CONTROL

On late model vehicles with electronic throttle control, the cruise control system is fully electronic. The cruise control switch on the steering wheel or steering column sends your inputs to the Body Control Module (BCM), which then passes it along to the Powertrain Control Module (PCM), unless the system is wired to send your inputs directly to the PCM via dedicated wiring or the controller area network (CAN) bus.

The PCM notes the speed setting you requested when you turned the cruise control system on. It then looks at the actual vehicle speed via the input from the transmission vehicle speed sensor(VSS). The PCM then calculates how much throttle is needed to maintain the requested speed.

The PCM then commands the actuator motor on the throttle shaft to open or close the throttle to increase or decrease engine RPM and vehicle speed so the actual speed matches the requested speed. The PCM also monitors inputs from the transmission (so it knows what gear the transmission is in), and the position of the brake pedal (so it can temporarily deactivate cruise control when you step on the brakes). Depending on the sophistication of the system, it may also look at inputs from the ABS (antilock brake) system and/or stability control system.

When the cruise control system is engaged, the PCM also monitors the position of the throttle and/or accelerator pedal. If you step on the gas to pass another car, your foot temporarily overrides the cruise control system's set speed. When you release the accelerator pedal, your car will decelerate and resume the previous speed setting.

CRUISE CONTROL SYSTEM COMPONENTS

* Cruise control module (if not integrated into the PCM)
* Body Control Module (if the cruise control inputs from the switch on the steering wheel or steering column pass through the BCM before going to the PCM)
* Brake pedal switch (located at the top of the brake pedal)
* Clutch pedal switch (if the vehicle has a manual transmission)
* Vehicle speed sensor
* Throttle actuator motor (electronic speed control), or vacuum actuator (older electromechanical speed control systems)
* Cruise control indicator lamp(s) on the instrument cluster (to let you know when the system is on)

CRUISE CONTROL WON'T ENGAGE

The cruise control system will NOT engage if any of the following conditions are present:

* The transmission is in Park, Neutral, Reverse or Low gear
* The clutch is disengaged (clutch pedal depressed)
* The vehicle is traveling less and 25 to 30 mph (varies depending on year/make/model of vehicle)
* The driver is holding his foot on the brake pedal (or the brake pedal switch is showing a constant application of the brakes)
* There is no input from the vehicle speed sensor (which will also prevent the speedometer from working)
* Engine speed and/or throttle position does not match the commanded position
* Electrical voltage to the PCM or other components in the system is too low (less than 9 volts)
* The ABS or Traction Control system has been active for more than two seconds (this may vary depending the application)
* Vehicle speed is excessive (some cruise control systems are programmed NOT to work above a certain maximum speed for safety reasons (typically 90 mph or faster)
* The system's self-diagnostics has detected a fault and set a code. This will deactivate the system and prevent it from functioning until the fault is diagnosed and repaired.

CRUISE CONTROL COMMON FAULTS

Common faults that may prevent your cruise control system from switching on, engaging or maintaining a constant speed include any of the following:

* A blown fuse. A short or electrical overload in the cruise control circuit may have caused the power supply fuse to blow. Refer to your owners manual for the location of the fuse. Remove the fuse from the fuse panel to see if it has blown. If the fuse is no good, replace it with a fuse of the SAME amp rating. Do NOT substitute a fuse with a higher amp rating as this may allow too much current through the wiring, causing it to overheat and catch fire! If the new fuse blows immediately (or as soon as you turn the cruise control system on), there is an electrical short circuit. You will have to find a wiring diagram for your vehicle's cruise control system and use a voltmeter/ohmmeter to find the short. You can get electrical wiring diagrams from the vehicle manufacturer service information websites or AlldataDIY.

cruise control wiring circuit schematic

* A faulty or misadjusted brake pedal switch. Step on the brakes to see if the brake lights come on. No lights could indicate a faulty brake pedal switch if the brakes and cruise control share the same switch. On some vehicles, there may be two brake pedal switches (one for the cruise control and/or ABS system, and one for the brake lights). Also check the parking brake switch (located at the base of the parking brake handle or pedal). If the parking brake or switch is misadjusted, or the switch reads "on" all the time, it will prevent the cruise control system from engaging.

* A faulty Vehicle Speed Sensor (VSS). The cruise control system can't function without a good speed input. A bad VSS will also prevent the speedometer from working. if the speedometer is reading normally, you can probably rule this out as a possible cause.

* Faulty cruise control switch in the steering wheel or steering column, or a bad wiring connection between the switch and BCM or PCM. If some of the cruise control buttons work, but others do not, chances are the problem is in the switch. If none of the buttons work, chances are the fault is in the wiring. Repairing the wiring or replacing the switch will require removing the steering wheel. CAUTION: In late model vehicles with air bags, the air bag system must be deactivated BEFORE you work on the steering wheel or steering column to prevent accidentally triggering the air bag. Find and remove the air bag fuse (wait at least 15 minutes before starting any work so the capacitors that store energy in the air bag system has time to dissipate their electrical charge).

* Faulty cruise control module or PCM. This may or may not set a fault code. If the Check Engine Light is on, and you find a code related to the cruise control system, the code will give an indication of the nature of the fault. If a cruise control code indicates a module fault, check the power and ground connections to the module. If that is not the problem, you will need a replacement module from a car dealer or an auto parts store (if an aftermarket replacement module is available).

* Faulty vacuum actuator. If you have an older vehicle with an electromechanical cruise control system, failure of the diaphragm inside the vacuum actuator, or a leaky vacuum connection, or a faulty vacuum control solenoid can prevent the actuator from operating normally. There is no way to rebuild the unit, so it will have to be replaced if defective.

* Faulty throttle actuator on a vehicle with electronic throttle control. If the motor that opens and closes the throttle is bad, the engine probably wont' run above idle. The Check Engine light should also be on, with codes set for the throttle actuator motor or throttle control system.

Ford Cruise Control Recall

Ford issued a cruise control recall for the following vehicles:
1992 to 2003 Ford Econoline vans
1993 to 1997 F35 Motor Homes
1995 to 2002 F150 pickup trucks
1995 to 2002 Ford Explorer and Mercury Mountainer
1995 to 2003 Ford Windstar
1995 to 1997 and 2001 to 2003 Ford Ranger
1997 to 2002 Ford Expedition and Lincoln Navigator
1999 to 2003 F-Series Super Duty trucks (diesel engine only)
2000 to 2003 Ford Excursion (diesel engine only)

The speed control deactivation switch on the brake master cylinder on some of these vehicles can short out and cause an underhood fire, even when the vehicle is parked. The switch was supplied by Texas Instruments. The recall does not replace the switch but involves installing a fuse in the switch wiring to prevent a fire in case the switch fails. As the age of the vehicle goes up, so does the risk of switch failure.

Symptoms that may indicate a failing cruise control deactivation switch include problems engaging or disengaging cruise control while driving, the brake lights coming on intermittently when the brakes are not being applied, or the brake lights not coming on when the brakes are applied, or problems shifting the transmission out of Park while holding your foot on the brake pedal (the brake switch is also part of the transmission safety interlock system).

To find out more about this issue see Ford Cruise Control Recall.




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