670 Auto Electricity and Electronics Copyright by Goodheart-Willcox Co., Inc. output signal without disconnecting wires. Th is scope will show both AC output and a trace for voltage signal variations. Note that this testing method would also work on engine block-mounted magnetic crankshaft position sensors. Note: Some electrical connectors are sealed and do not allow easy probing. You may need to install a test connector or jumper wires between the two halves of the connector to probe sensor voltages. When reading the sensor waveform, make sure the peak voltage levels are equal to each other. If one is short or missing, inspect the trigger wheel for a broken tooth. Peak- to-peak voltage levels should be within specifi cations. See Figure 34-19B. As with any sensor, reference voltages, wiring, and other criteria will vary. If in doubt, always refer to the service manual for the vehicle being tested to get accurate electrical values. Throttle Position Sensor Tests To scope test a throttle position sensor (TPS), con- nect the test leads to the sensor output wire and to ground. Voltage should still be fed to the sensor from the ECM. Move the throttle opened and closed. Th e TPS waveform should show a smooth curve, without any spikes. See Figure 34-20. Manifold Absolute Pressure Sensor Tests A scope can also be used to test the operation of both analog and digital manifold absolute pressure (MAP) sen- sors. Accelerate the engine and note the changes in airfl ow signals going to the ECM. Compare the amplitude and shape of the waveform to known good patterns. Th is is shown in Figure 34-21. Mass Airfl ow Sensor Tests To test analog or digital mass airfl ow sensors using a scope, probe the connector as recommended in the service manual. Compare your scope readings to factory specifi ca- tions and known good readings. See Figure 34-22. Knock Sensor Testing To test a knock sensor with a scope, connect the scope test leads to the sensor. Th en, tap the engine next to the sen- sor with a small hammer or a wrench. See Figure 34-23A. Th is should make the sensor produce a signal that is similar to the one shown in Figure 34-23B. Another way to check a knock sensor and the ECM is to measure ignition timing while tapping the engine next to the sensor. Th e ECM should retard ignition timing when you tap the engine. Figure 34-20. Throttle position sensor can also be checked with a scope. A—Probe through the wires or use jumpers so power can be connected to the sensor. B—Potentiometer, or variable resistor, TPS should produce a smooth curve as the throttle is moved opened and closed. Spikes indicate a sensor problem. C— Switching-type TPS should produce a good square wave without ringing. (Fluke) Throttle Position Sensor (TPS) A Scope Probe to Correct Terminal Ground Test Jumpers Defective TPS Pattern Peak Voltage Indicates Wide Open Throttle (WOT) Spikes in a Downward Direction Indicate a Short to Ground or an Intermittent Open in the Resistive Carbon Strips Voltage Decrease Identifies Enleanment (Throttle Plate Closing) Minimum Voltage Indicates Closed Throttle Plate DC Offset Indicates Voltage at Key On, Throttle Closed Voltage Increase Identifies Enrichment B Throttle at Position Other Than Closed (Not Necessarily Wide Open Throttle) Ringing May Indicate Worn Contacts or Loose Throttle Return Springs Reference Voltage Throttle Opening and Voltage Transitioning Transitions Should Be Straight and Vertical Throttle Plate Closed To ensure proper results from your test, verify the type of sensor under test C