668 Auto Electricity and Electronics Copyright by Goodheart-Willcox Co., Inc. Computer System Scope Tests An oscilloscope can be used to help you find com- puter system problems. When the scan tool does not find anything and you still have performance problems, you may need to check sensor and computer signals with a scope. Distributor Pickup Coil Scope Tests An oscilloscope can also be used to check the output signal of a distributor pickup coil. It will not only measure voltage, it will also show the shape of the signal leaving the pickup coil. Magnetic Sensor Tests A magnetic sensor test is done by measuring the output voltage from the sensor with the engine cranking. With a magnetic sensor, connect the scope primary leads to the pickup coil. Set the selector to primary and the pri- mary height control to 40 volts. Adjust the pattern length to minimum. With the engine cranking, an AC (alternating current) signal about 1.5 volts peak-to-peak should be gen- erated, Figure 34-15. A hand-held scope is shown reading the signal from a magnetic sensor in Figure 34-16. Hall-Effect Sensor Tests A Hall-eff ect sensor test is best done by checking the sensor’s output waveform with an oscilloscope. Without disconnecting the circuit reference voltage, probe the output wire at the sensor connector, Figure 34-17A. Th e service manual will give pin numbers for probing. A Hall-eff ect sensor waveform should switch rapidly, have vertical sides, and have the specifi ed voltage output (typically about 4 to 5 volts peak-to-peak). Th e top of the square wave should reach reference voltage and the bottom should almost reach ground, or zero. Signal frequency should change with engine cranking speed or engine rpm, Figure 34-17B. Hall-eff ect pickups can be found in distributors and some crankshaft position sensors. Since specifi cations vary for Hall- eff ect sensors, refer to the service manual for that vehicle. Optical Sensor Tests An optical sensor can also be tested with an oscillo- scope. You can probe the output wires from the sensor and compare the waveform to specifi cations. Figure 34-15. Typical waveform from a magnetic distributor pickup. (Fluke) Minimum peak levels should be equal to each other. If one is shorter than the other, look for a chipped or bent tooth on the trigger wheel. Maximum peak levels should be equal to each other. If one is shorter than the other, look for a chipped or bent tooth on the trigger wheel. The waveform signature is created from the unique shape of the trigger wheel tooth, passing the pickup coil. Figure 34-16. A hand-held scope can be used to check for magnetic type sensor output signals. A—With the trigger wheel spinning, the magnetic sensor should produce an AC sign wave as shown. B—If the scope shows a straight line, no signal is leaving the sensor, and it is bad. A B