Twist the key to start your car, pickup truck or SUV, and you are faced with a
veritable Christmas tree of lights on the dashboard. Even entry-level cars
have warning lights indicating that the engine, anti-lock-braking system and
airbag restraint system are ready for action. One by one, these lights wink
out as sensors communicate with computers, and each system is checked and
determined to be in working order.
Once the vehicle has been started, the engine oil pressure, coolant level and
temperature, transmission status and fuel level are all sampled by sensors,
and their status is reported to the driver, sometimes using traditional analog
gauges, but more commonly through a general "systems OK" message. Some cars
alert the driver only when there is something wrong, illuminating a "check
engine" light to tell you that all is not well.
The Old Analog Days
In the old days, engine sensors and instrumentation were very simple. There
was an oil light that would illuminate when the oil pressure was low and an
alternator light that would come on when the battery wasn't receiving a proper
charge. Sometimes the gauges were the sensors. For example, the oil pressure
gauge was often attached to the engine itself with a thin oil line that ran to
the gauge in the dashboard. All of these analog gauges were quite simple, and
it was up to the driver to monitor them while driving. If something went
wrong, the driver was expected to notice an odd reading on one of the gauges.
Today's Digital Age
Today's vehicles are obviously more complicated. Some of this complication is
the result of legislated exhaust emissions and safety regulations. Some of it
is due to the vast array of power accessories and luxury conveniences on all
but the most basic cars. Digital computers now control engines. Luxury cars
have "body computers" to control luxury features. And all of these computers
rely on fast-acting, reliable sensors.
On the plus side, these digital electronic systems make it easy to constantly
monitor vital engine parameters like oil pressure, coolant temperature and
exhaust emissions, and report back to the driver when something is amiss.
Sensors continually monitor major systems, sending real-time signals to
onboard computers. And these computers keep track of everything, alerting the
driver only when something is wrong or when it is due for service.
How far has this gone? In many vehicles today, digital computers control not
only engine functions, but also everything from the climate control system to
headlight and taillight operation. More than that, many systems are now
required to store information for later access. Service Technicians can plug
their own computers into a vehicle and often quickly diagnose what is wrong,
even if the problem is sporadic. That is, of course, if everything is
operating as it should and if the Technician has been thoroughly trained and
has the proper service tools.
Too Many Wires?
All of these sensors and computers require lots and lots of electrical wires,
connections and harnesses. The sheer number of wires needed to control
everything from automatic transmissions to power windows is overwhelming. The
space occupied by the wiring and even the weight of the wiring harness is a
real problem for automotive engineers trying to package everything into
smaller spaces. For many vehicles, the solution is multiplexing.
Because there already are numerous computers in the vehicle, adding a few more
is no big deal. Now, for example, instead of running a heavy wire that carries
electrical power from the power window switch directly to the motor that
raises a window, a whole new way of controlling the window motor has been
developed. Rather than carrying the voltage from the switch to the motor, the
switch is now connected with a thin, low-voltage wire to a body-control
computer. This computer senses the activation of the switch, and then sends a
signal to the window motor, telling it to open or close the window. The window
motors all have their own power source wires, and are just waiting for a
signal to come down the multiplex line from the computer. And, of course, many
of these systems have sensors that not only tell the computer when the window
is all the way up or all the way down, but also whether there is anything
impeding its progress.
With multiplexing, it is possible for a computer to perform a variety of
interrelated functions, such as tilting the outside mirrors downward when
Reverse gear is selected, or adjusting the interior cabin temperature based
upon how quickly the engine is warming up on a chilly morning. The wide use of
so many computers also allows the engine management system to monitor an even
broader range of engine functions. But when things go wrong, it can be even
more difficult to figure out where the problem lies.
