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Computer in your Auto
How
your cars computer works
Sensors,
actuators
ECM
Troubleshoot
your car with its computer
Cars have changed dramatically over the past few decades. The good
news is that computers do a lot to help your car run smoother and mom
efficiently. The bad news is that if the computer fails, most car owners
haven't a due what to do about it.
Understanding how the computer on your car works can help you figure out where
to start-or at least who to ask. A computer is simply a machine. It computes.
On or off? It just does it so fast that it can check the condition of millions
of things before you finish this sentence.
Computers are found throughout modern cars. They check fuel,
spark, heat, speed, emissions, electrical signals, internal engine parts, and
much more. Most of what car computers do is control the fuel and ignition
systems.
Computer control systems have only three stages: input, processing, and output.
Input checks to see how a part is doing, processing compares it against what it
should be doing, and output makes any needed changes.
The temperature of coolant is checked and compared against what engineers have told
the computer it should be. Then an adjustment is made to the flow of coolant to
correct the temperature. The same is true for engine speed, manifold heat,
exhaust emissions, and bunches of other systems throughout your car: test,
verify, correct.
Sensors
The boss is the Electronic Control Module, or ECM.(call it an Electronic
Control Unit. It knows what conditions should be, and it's what can tell your
your mechanic-if things aren't going well. Getting a Sense of Things
Sensors monitor the various conditions throughout your car. They check
temperature, electricity, heat, movement, and other physical things. Sensors
then turn that information into an electrical signal that is sent back to the
ECM.
Let's get more specific.
An oxygen (OZ) sensor tests the oxygen level in the engine's exhaust gasses. It
then turns this information into an electrical signal and sends it to the ECM:
Now, sensors don't work all the time. They are off when your car isn't running,
of course, but they may also be off until certain things happen. For example,
the oxygen sensor might be programmed to not wake up until the exhaust
temperature reaches 600° Fahrenheit. A temperature sensor says, wake
up!" and the oxygen sensor grabs and goes to work testing the oxygen.
Until the sensor goes to work, the circuit is considered "open," and
once it starts working, the circuit is "closed." Think of the circuit
as a loop that must be closed to work.
There are many other types of sensors that are in your car. An engine speed
sensor sends information about engine speed (revolutions per minute, or rpm) to
the ECM. Even tell the ECM the position of the individual pistons and the
crankshaft!
A vehicle speed sensor does just that: measures the speed of the car. It senses
the rotations of the car's transmission parts and does some quick calculating
to tell you the speed in miles per hour (mph).
A knock or detonation sensor measures abnormal engine vibrations caused by
advanced ignition timing. The signal is used to retard the timing and correct
the condition.
Some sensors in your car aren't as smart. The only answers they can send back
to the ECM are yes or no. But they, too, serve. Common passive sensors measure
the coolant temperature, engine air flow, manifold air temperature, and throttle
position sensor.
In addition, there are numerous switching sensors throughout your car. Is the
door open or closed? Is the transmission in reverse or not? Et cetera.
In days of yore, all the wires for these sensors made the car's engine
compartment and dashboard look like electric spaghetti. Fortunately, automotive
designers have since figured out a way of carrying most of these sensing
signals over just one or two wires.
A single wire can carry many sensor reports. This is called (as it is in radio
broadcasting) multiplexing, or MUX.
How does all this info get from the radiator, manifold, and other places to the
control center? The wire with all these reports is the data bus. Each sensor
gets a specific amount of time, and in a particular order, to make his/her/its
report. Electronic signals move so fast that many hundreds of reports can be
made in a second.
Some cars use fiber optics rather than copper wires to carry all these sensor
reports. Fiber optics eliminate interference from other electrical devices.
ECM
The electronic control module, or ECM, is where all the decisions are made.
That's why it's a good idea to know how it works and what to do if it
doesn't.
The ECM has five easy-to-understand sections:
1.The input section converts the sensor's signals to a form the computer can
use.
2.The memory section stores data until it is needed by the computer.
3.The logic section does the arithmetic calculations.
4.The control section manages everything.
5.The output section converts the computer's output signal to a form that the
control section can use to do the job.
The ECM remembers what sensors tell it as well as what the car's designer
taught it at ECM school. Think of it as short-term and long-term memory. What's
really cool about today's cars is that the long-term memory (PROM or EPROM, for
you acronym nuts) can be changed after the car has left the factory. In fact,
you can have your car's ECM reprogrammed to have more power or more efficiency.
Actuators
We've been introduced to the tattlers (sensors) and the boss (ECM), but who
does all the work? the actuators. An actuator simply converts the electrical
signal from the ECM to physical movement. Typical movement includes opening,
closing, pushing, or pulling. For example, a fuel injector is an actuator that
opens up to allow the fuel into the throttle body or cylinders. If the car has
a carburetor, a mixture control solenoid adds air as needed to make the optimum
fuel-air mixture.
In some cases, the car needs to recycle exhaust gases for cleaner
burning of the fuel. The exhaust gas recirculation (EGR) valve opens and closes
as mandated by the ECM. There are other actuators throughout the modem car. In
all cases, they convert an electronic signal into work. Some of them use vacuum
power to move things. Others, like the electric fan relay, pass along a signal
to another part that does the actual work.
Trouble
So today's computerized car is really quite simple in principle. Sensors check
things, the ECM makes a decision based on what it knows should happen, and
actuators make it happen. But what can you do if things don't work as they
should? What can the car's owner do without a gazillion gadgets to test the
computer?
First, the service manual for your car will probably tell you many things. It
will probably give you a logic diagram to follow.