Making Sense of Sensors: Part 2

Last time, we talked about how the computer gets the information to do its job, and I described what sensors are involved. Now we'll talk about what the computer does with this information and what it controls

In the days before computers, when you did a tune-up, a mechanic set the engine under a specific set of conditions. In other words, he adjusted the carburetor and set the timing while it was idling in the shop. It was assumed that if the engine performed well at idle, it would perform well at all speeds. This was not a bad thing, and 99 percent of the time it worked very well. With the introduction of computer-controlled engines, it is now possible to "tune" the engine to any particular condition at any point in time. It's like having a mechanic under the hood making adjustments as you drive.

The main function of the computer is fuel management. It does this by controlling the fuel injectors, or more specifically, how long the fuel injectors open. The injectors are simply a valve with a solenoid that opens and closes it. Leading to the injectors are fuel lines with high-pressure fuel. The computer determines how long the injectors stay open, thus controlling the amount of fuel used. The time the injectors are open is measured in milliseconds (thousandths of a second), anywhere from 4 to 9 milliseconds. They do this anywhere from 600 to 3000 times a minute, depending on engine speed.

The computer is not the only thing that determines how much fuel is injected into the engine; fuel pressure also factors into this equation. The higher the fuel pressure, the more fuel is injected. The Fuel Pressure Regulator regulates the fuel pressure. The computer does not control the FPR; it is controlled by engine vacuum. There is a vacuum line that goes directly from the intake manifold to the FPR. Inside the FPR is a diaphragm with a valve. At idle when engine vacuum is high, it opens the valve lowering fuel pressure. At higher RPM's when engine vacuum is low, the valve closes increasing fuel pressure.

Some computers can control the fuel pressure to a certain extent; Nissan comes to mind. The computer controls how much voltage is sent to the fuel pump. At idle, when fuel demand is low, the computer cuts the voltage going to the fuel pump in half, thus lowering fuel pressure.

Another job of the computer is to control engine timing, when the spark plugs fire. Early computer systems did have some manual timing adjustment, but with the introduction of Distributorless Ignition Systems, even that little bit is gone. Using information from the various sensors, the computer will determine the best time to fire the plugs.

I won't even begin to try and explain how the computer processes this information, any more than I can explain how the computer on my desk turns my keystrokes into words on a screen. Suffice it to say it can and it does. Stored inside the computer is a set of values. These are the base values the computer uses to compare actual conditions to. Some computers have the ability to "learn." In other words, it keeps track of the way you drive and adapts itself to those conditions. It also uses these base values to check the sensors. If a sensor goes out of range of these values, it sees it as a malfunction and stores a trouble code. It will also turn a light on in the instrument panel to alert the driver that it has found a possible problem.

In the case of a sensor malfunction, the computer may go into a "fail safe" or "limp in" mode. For example, the Crank Angle Sensor dies and no longer sends a signal to the computer. The computer sees the signal is missing and notes the malfunction code and turns on the check engine light. Now lacking that CAS, signal it goes to the base values stored in the computer and uses that value to control the ignition timing. It's not perfect, but it doesn't leave you stranded on the side of the road. Quite often the computer limits engine RPM to about 2500 RPM to prevent any possible engine damage and further alert the driver to a problem.

So what does the information from the sensors mean to the computer, and how does it affect the output? Let's go sensor by sensor and see what it means to the computer.