Chapter 34 Advanced Diagnostics 667 Copyright by Goodheart-Willcox Co., Inc. Voltage Voltage is shown on the scope screen along the vertical (up and down) axis, or scale. Voltage values are given on the right and left borders of the screen. See Figure 34-14. With the controls set on kV, the numbers on the screen represent kilovolts. One kV equals 1000 volts 5 kV equals 5000 volts etc. If a line on the scope screen extends from zero to 7 kV, the scope is reading 7000 volts. If the scope is set to read 0 to 10 volts for checking the ECU and its sensors, a line fi ve divisions tall would indicate 5 volts. Similarly, a waveform fi ve divisions tall would be a reading of 5 volts peak-to-peak (from the top of the positive trace to the bottom of the negative trace). Voltage is the most commonly used value on a scope screen. As voltage increases, the trace line on the scope moves up. As voltage drops, the trace line moves down a proportionate amount. Time Time is given on the horizontal scale of the scope screen in degrees, milliseconds, or duty cycle. Diff erent scales may be given on the bottom of the screen for four-, six-, or eight-cylinder engines. Th ese scales are calibrated in degrees of distributor rotation. Degrees may also be given as a percentage, for quick reference to any number of cylinders. Th e scope screen may also have a milliseconds scale for measuring actual time. Th is makes it possible to measure how long each spark plug fi res in milliseconds. A certain amount of time is needed to properly ignite and burn the air-fuel mixture. Sweep Rate Scope sweep rate is the frequency or time division shown on the screen during each test. Th e sweep rate adjustment aff ects the horizontal, or time, measurement. Th e scope sweep rate must be set to match the waveform frequency to be analyzed. Sweep rate is commonly given in milliseconds (ms). A low sweep rate will compress the waveform, and too much information will be shown at once. A high sweep rate will expand the waveform, and only a small section of the complete duty cycle will be displayed. Trial-and-error adjustment of sweep rate is commonly used. Th e sweep rate knob, or sweep knob, (time/division) on the scope is turned until the desired waveform is dis- played on the screen. Compare the waveform pattern on the scope to a known good pattern. Reading Scope Patterns To read a scope pattern, inspect the waveform for abnormal shapes (high or low voltages, incorrect dwell or time periods). Since there are so many variations of elec- tronic waveforms, refer to the scope operating manual or another reference. Locate an illustration of a good scope pattern for the particular system or components and com- pare it to the test pattern. When analyzing a square wave, there are several things you should check. Th ey include: • Base line—the reference line, or zero volts. • Rising edge (leading edge)—where the square wave goes from zero to high voltage. • On-time (high-time)—where the square wave stays at maximum voltage. • Trailing edge (or falling edge)—the drop in voltage back to zero. • Off -time (low-time)—where the square wave stays on the base line. • Amplitude—determined by the vertical distance from the base line to the high-time. Note that for DC square waves, amplitude and peak-to-peak voltage equal the same value. You can inspect these sections of the waveform to determine if there is a problem. Some common problems that can aff ect a square wave include: • Low or high resistance in the circuit or its components. • Faulty electronic circuit. • Circuit contaminated by moisture. When analyzing sine waves, check the following: • Analog peak-to-peak voltage—Is the waveform volt- age strong from top to bottom? • Analog wave shape—Is the trace normal for a known good component? • Analog wave frequency—Is the distance between waves normal? • Analog wave smoothness—Is there unwanted hair or static on sine wave? Figure 34-14. Scales on an oscilloscope screen allow you to mea- sure voltage and time accurately. A—0 to 25,000 volt scale. B—0 to 50,000 volt scale. C—Scale for measuring time in milliseconds. D—Scale for measuring in degrees. (Sun Electric Corporation) A B C D