Cruise control is a system that automatically controls the speed of a motor vehicle; it takes over the throttle of the car to maintain a steady speed set by the driver. The system improves driver comfort in steady traffic conditions. In modern designs, the cruise control may need to be turned on before use. When depressing the brake or clutch pedal the system is disabled so the driver can change the speed without resistance from the system. The system is operated with controls easily within the driver's reach, usually with two or more buttons on the steering wheel spokes. The driver must bring the vehicle up to the desired speed and use a button to set the cruise control to the current speed. Generally, the cruise control takes its speed signal from internal speed pulses produced electronically by the vehicle and the vehicle will maintain the desired speed using the electronic systems. Cruise control often includes a memory feature to resume the set speed after braking. When the cruise control is engaged, the throttle can still be used to accelerate the car, but once the pedal is released the car will then slow down until it reaches the previously set speed. On the latest vehicles fitted with electronic throttle control, cruise control can be easily integrated into the vehicle's engine. Modern “adaptive” systems include the ability to automatically reduce speed when the distance to a car in front, or the speed limit, decreases. The cruise control systems of some vehicles incorporate a “speed limiter” function, which will not allow the vehicle to accelerate beyond a pre-set maximum. Possible causes of failures The automobile engine compartment is an unfavorable environment in which to expect sensitive electronics to operate reliably thus any of the following changes could result in cruise control system failure: 1-When it is hot, dirty, humid and vibration levels are high; 2-When electromagnetic Interference (EMI) levels are high; 3-There can be appreciable shock loadings that may damage PCB wiring or affect electrical contacts; 4-Electrical connectors can become dislodged or distorted 5-Electrical contacts be subject to fretting fatigue; 6-Wiring, if not properly restrained, can be rapidly damaged by vibration-induced fretting; 7-Servicing of the vehicle, involving the removal and replacement of components, may result in displacement or damage to wiring or unwitting damage to connectors or sometimes the wrong connections being made. In such a hostile environment, intermittent electrical, mechanical and electronic faults in the cruise control system would scarcely be surprising. Such malfunctions might be permanent once-only events, requiring the replacement of a “dead” module, or they might be intermittent, occurring randomly 8-Sensors, switches, connectors and wiring : a speed sensor or its wiring may fail; electrical switches may fail to open or fail to close; electrical switches may become mispositioned, loose or fall off; multiplexed switches, where several switches communicate with a control module over a single wire, may not switch correctly in the presence of a high-resistance joint or earth contact electrical connectors may fail open circuit or short circuit; electrical switches or wiring may overheat causing damage; slip ring connections (to steering wheel switches, for example) may become intermittent or fail; earthling connections may become intermittent or fail; presence or absence of extreme cold/ heat, moisture, pollution, road salt etc. may play a significant role here. presence of monocrystalline tin whiskers resulting from the use of lead free solder that may cause intermittent shorts. The Cruise Control Module : electronic components may fail; vibration, shock or thermal cycling; may cause intermittent open or short circuits on PCBs; moisture and surface contamination may cause electrical tracking across insulating surfaces, in turn causing : the speed reference signal to drift, either up or down; a high gain amplifier or integrator, or digital equivalent, to drift into saturation; logic may be affected by transient signals/noise; logic may lock on or lock off. a microprocessor may get into an endless processing loop The throttle actuator There are different kinds of actuator used ( electro-pneumatic and electro-mechanical) but they are all essentially power amplifiers, converting a small control signal into throttle movement. The result of an input signal, whatever its source, will be movement of the throttle. Spurious control signals may derive from many sources including : RF noise at the input; false signals from a malfunctioning cruise control module; stray potentials resulting from perhaps poorly earthed components elsewhere in the engine compartment; wiring faults. The actuator has mechanical elements that have the potential to jam in any position from fully closed to fully open. Generally speaking, fault mechanisms may be divided into two kinds: external to the cruise control/actuator modules (external fault mechanisms) internal to the cruise control/actuator modules (internal fault mechanisms) A fault may arise from the interplay of a wide variety of combinations of external and internal fault mechanisms. Sudden acceleration Sudden acceleration is also described by a variety of other terms including “sudden unexplained acceleration”, “uncontrolled acceleration” ; in an NBC TV report (NBC News/MSNBC, Feb. 10, 1999), as “vehicles taking off on their own”. The failure is presented of a vehicle event in which the throttle moves, apparently uncommanded, of its own accord, rather mysteriously, to the fully open position, without the driver pressing the accelerator pedal. In such a situation the driver is not able to exercise any kind of control, except by switching off the ignition or braking, which itself may be dangerous. For example: Some of the alleged sudden acceleration incidents in Ford Explorers reported in the UK by the Channel 4 program in the Autumn of 2000 appear to be outside the NHTSA definition, since (a) they mostly took place with the vehicle already moving at speed and (b) in some cases the vehicle was brought to a halt without causing an accident and it could be argued that the driver had managed to retain control of the vehicle. __
