Computers have become a vital part of the modern motor vehicle, and they can help you limp home in a crisis, but sometimes their sensors fail for the strangest reason.
The computers, also known as processing units, fitted to modern cars are usually very reliable, but the sensors less so. Many of them are affected by heat and vibration so that they often fail from old age or physical damage. Sometimes the part they are monitoring fails. In both cases they tend to send signals to the computer that the latter cannot interpret.
LIMP HOME MODE
Most manufacturers solve this problem by including a “limp home mode” in the software. When the computer gets a signal that is outside the normal range it is programmed to deal with, it will assume a value that will work.
On the first computer-controlled engines the fuel mixture was the only area that was controlled and if the relevant sensor sent a weird signal the computer would simply tell the carburettor to supply a rich mixture. The engine would still run but the fuel consumption would be excessive.
The computer would also cause an amber check engine light to appear on the dashboard and store a fault code in its memory. The owner’s manual would advise the driver to have the engine checked by a dealer.
On modern cars the computer is far more sophisticated. The check-engine light might come on for a temporary fault that the computer can handle and will go out again if the relevant sensor starts to send normal signals again.
The limp home mode is designed to take over control calibrations of the engine when a serious problem – one that would otherwise make a car undriveable or damage the engine – occurs. This mode will cause a check-engine light or some other warning light to appear.
This type of control is made easier by employing a drive-by-wire throttle control. The throttle pedal is no longer connected to the throttle valve by a cable. Instead, an electronic position control unit at the throttle pedal is connected to an electric stepper motor at the throttle valve.
A sophisticated feedback system is employed to ensure that there is some relationship between what the driver demands from the engine and the amount of air being inhaled. The engine computer can interfere with these signals to tell the throttle how to behave, and in the limp home mode the throttle will be instructed to stay at a fast idle so the car can be driven slowly to a workshop.
In some engines the control computer will shut down the signal to the throttle position motor, allowing a set of springs at the throttle to cause a fast idle.
Sometimes the relationship between the throttle pedal movement and throttle valve movement is controlled to such an extent that pedal sensitivity can be changed between driving on the road and driving off the road.
There are often a number of computers on board to control not just the fuel/air mixture but also the spark timing, the ABS brakes, the air suspension, the headlights, the entertainment, the steering, the gearbox and the air conditioning system.
The ABS system will usually have a separate computer. There will be a warning light that shows a steady or flashing ABS. If the ABS light comes on it usually means there is fault in the ABS component of the brakes (the brakes will still work, but ABS will be inactive).
If there is a leak in the brake system or some other serious fault, a separate brake light will normally come on. This means the brakes need immediate and serious attention.
A separate computer that gets inputs from “accelerometers” in various parts of a car controls the airbags. Their input will enable the computer to decide whether that bump against a post was serious enough to cause the airbag to deploy. In a real crash it will also tighten the seatbelts a few milliseconds before deploying the airbags.
If the airbag light comes on it normally means the airbag will not deploy. It does not mean the airbag could deploy at any moment.
Automatic gearboxes often have their own computers. Many are so sophisticated that they use inputs from the airbag computer’s accelerometers and the throttle position sensor to determine what driving mood you’re in. If you feel like driving vigorously it will stay in each gear until the revs are high, but if you’re in a relaxed mode it will change quite soon into a higher gear.
Gearboxes often have a limp home mode of their own. If there is a serious gearbox problem it may allow you to select only reverse or second gear to get you to a workshop slowly.
The failure of other components, such as air suspension, often results in a slow speed limp home mode. One can be sure that, as time goes by, the number of reasons for a limp home mode will increase.
The computer will record diagnostic trouble codes even if the warning light goes out after a while. These can be recalled using diagnostic equipment and will normally indicate which sensor has given an out-of-spec reading. The mechanic can then decide what repairs are necessary. He can also delete these codes by resetting the computer.