Chapter 34 Advanced Diagnostics 669 Copyright by Goodheart-Willcox Co., Inc. An optical pickup test measures the output generated by the photodiodes as they are energized by the LEDs. It is also easily done with a hand-held scope probing into the sen- sor’s electrical connector. Again, refer to the service manual to fi nd the connector pin numbers for the optical pickup’s output wire. Optical sensors are used in a few distributor designs and are never used in crankshaft sensors. An optical sensor’s waveform should have straight sides and adequate voltage output. Th e upper horizontal line on the waveform should almost reach reference voltage. Th e bottom horizontal line should almost reach ground, or zero. See Figure 34-18. Remember that optical sensors are susceptible to dirt. An oil mist or a fi lm of dirt can prevent light transfer from the LEDs to the photodiodes. Again, refer to the manufac- turer’s service literature for specifi c information. Crankshaft Position Sensor Tests Figure 34-19A shows how to use a hand-held scope or advance scan tool to test a crankshaft position sensor. You can use the needle probe on the scope lead to check for an Figure 34-17. A—This scope is being used to check the signal from a Hall-effect sensor. B—Hall-effect sensor signal. The frequency of the signal should increase as engine speed increases. (Fluke) Lead Grounded Distributor or Coil Module Probe into Wire from Sensor Handheld Scope A Peak-to-peak voltages should equal reference voltage The upper horizontal lines should reach reference voltage Voltage transitions should be straight and vertical The lower horizontal lines should almost reach ground B Figure 34-18. Typical waveform generated by an optical sensor. If the shutter blade widths vary, the pulse width will also vary. (Fluke) Peak-to-peak voltages should equal reference voltage The upper horizontal lines should reach reference voltage Voltage transitions should be straight and vertical Signal pulse width may vary due to size variations in the trigger wheel window The lower horizontal lines should almost reach ground Figure 34-19. Scope testing crankshaft position sensors is similar to testing magnetic distributor sensors. A—Since crankshaft sen- sors generate their own voltage signal, connect the scope to the terminals as specified. B—Note the resulting display. Compare the waveform to the service manual description. Crank Trigger Wheel A Test Leads Wires from ECM Scope Crankshaft Sensor B