Automotive Computer Sensors COLUMN BY KEN BUIE This month we want to look at sensors that create and send variable frequency and digital pulse signals to the computer. Just as the changing volt- age of a TPS sensor indicates changing throttle position, a changing frequency signal from a Ford MAP sensor indi- cates changing engine load. There are three types of signal inputs a computer receives: 1. Direct Current (DC) Signals ¢ Battery and ignition power feeds e Analog sensor signals ¢ Ground circuit 2. Alternating Current (AC) Signals e Permanent Magnet (PM) Generators used for: cam and crank position, vehicle speed, wheel speed e Engine knock sensor 3. Digital Pulse Signals ¢ Hall Effect sensor signals ¢ Distributor reference pulses ¢ Ford MAP sensor signal ¢ Some GM MAF sensors These different types of signals require different methods and equip- ment to measure them. As in all computer testing, it is important to have a good Digital Voltmeter (DVM), a high-impedance testlight, a regulator 12-volt testlight and, if at all possible, a lab type oscilloscope (lab scope). A lab scope is nothing more than a voltmeter that displays voltage with a line or trace on a picture tube, rather than using a meter or numbers. Lab scopes have the same high-impedance characteristics of a DVM so they are safe to use when testing computer sys- tems. Many new style lab scopes have the ability to record and play back sig- nals. These are called digital storage oscilloscopes. (DSO). The purpose of this article is not to get too deep into scope pattern analy- sis or electronic theory. We could spend hours discussing RMS voltage, Duty Cycle, Pulse Width Modulation, and so on. This month, all we want to do is introduce you to a few basic scope patterns and sensors that send frequency signals rather than analog voltage signals. Scope Patterns See Figures 1 and 2 on page 14. In Figure 1 we are using examples of what you would see on a scope screen. Some engine analyzers have a built-in lab scope feature that lets you use your ignition scope as a lab scope. Some lab scopes can be a little confus- ing when you first use them, but in time you’ll become comfortable with setting it up to measure different types of voltage signals. Figure 1l-a shows that the vertical axis (up and down) represents voltage and the horizontal axis (left to right) indicates time. The line at the bottom of the scope is the zero voltage reference line. Any voltage above the zero line is positive volts, any voltage below the zero line is negative volts and any volt- age at the zero line is ZERO volts. Figure 1-b represents a DC voltage that starts out near one volt and climbs to five volts. This is typical of watching the voltage change on a TPS. The volt- age starts out low and as the throttle is depressed, the voltage increases. This is also called an analog voltage signal because it varies smoothly over a continuous range rather than as separate discrete steps. Figure 1-c represents an AC voltage signal. This is the type of voltage a vehi- cle speed sensor puts out. If you were awake in high school science class, you’ll remember that an AC signal is called Alternating Current because the voltage is constantly ALTERNATING from positive to negative. The trace that is displayed above the zero line in Figure 1-c hits a peak at five volts. The trace below the zero line bottoms out at-five volts. This is actually read as 10 volts AC. The thing to remember about AC signals is they are repetitive. This means they continu- ously cycle back and forth to produce the AC voltage. This back and forth cycling can be fast or slow and still pro- duce a constant voltage. The frequency of this back and forth cycling can also be measured. Figure 1-d (page 14) also represents a 10 volt AC signal. The difference between the two AC signals is that 1-d has a faster, or higher, frequency than 1-c. Frequency is the rate at which a wave form is repeated. The basic unit of frequency is Hertz (Hz), which is one cycle per second. If the left to right time measurement of our scope screen is one second, Figure 1-c has a frequency of three Hz, while Figure 1-d has a frequency of six Hz. Many times an AC voltage must be converted to a DC voltage. This is done by the use of a signal converter. A signal converter looks at the top peaks of the AC signal and then “squares” them up. If the AC signal in Figure 1-c was converted to a DC signal, it would look like Figure 2-a (page 14). The signal in Figure 2-a is a digital pulse signal. The frequency of this signal is the same as the AC signal in Figure 1-c. Digital Pulse Signals The pattern in Figure 2-b has the same voltage amplitude as Figure 2-a but has a faster, or higher, frequency. To get an idea of the frequencies we’re talking about, refer to the chart in Figure 3. Hertz is also represented as kHz (kilohertz) or thousands of hertz and MHz (megahertz) or millions of hertz. So a signal that measures 3000 hertz can also be represented as three