674 Auto Electricity and Electronics Copyright by Goodheart-Willcox Co., Inc. closed, both oxygen sensors should show about the same change in voltage. Typical readings rise above 0.8 volts and then fall below 0.2 volts with an average change of about 0.45 volts. If either sensor is lazy (slow) and fails to change voltage with the snap of the engine throttle, replace that oxygen sensor. ECM Scope Tests A scope can be used to check the output pulses leav- ing an electronic control module. You can measure and observe the pulses going to fuel injectors, solenoids, and servo motors and compare them to good traces of voltage or current. You can also check the reference voltage being sent to sensors, Figure 34-29. Since ECM testing varies and is complex, always refer to the service manual for detailed instructions. You must compare your test waveform or voltage to known correct values. If the ECM fails to produce a good pulse or reference voltage, it should be replaced. A bad ECM will oft en fail to send data to your scan tool. Flight Record Tests A fl ight record test stores the sensor waveform in the scope’s memory when a problem occurs. For example, when trying to check an intermittent problem, connect the hand- held scope to the sensor and test drive the vehicle. When the problem occurs, press the storage button on the scope. Th e scope or scan tool will then store a picture of the sensor output for analysis, Figure 34-30. Ignition System Scope Patterns A vehicle’s ignition system produces wide fl uctuations in voltage. When an ignition system is functioning properly, these voltages will be within specs for the specifi c make and model vehicle. A component with higher-than-normal resistance (open spark plug electrical connection, for example) would be indicated on the scope as a higher-than-normal voltage trace. Th e high resistance would produce a high voltage spike. A shorted component (fouled spark plug) would have Figure 34-29. A—A scope will also check reference voltage going to sensors and the control pulses from the ECM to the actuators. Compare readings and waveforms to service manual specifica- tions. B—Reference voltages should meet specifications and the waveforms should reflect smooth DC voltage. (Fluke) Scope Probe to Service Manual Recommended Wire Computer or ECM A B Figure 34-30. The flight recorder feature is used on some small handheld scopes. Connect to a suspect sensor using long test leads and place the scope on the seat during a test drive. A—Normal, consistent signal from a magnetic sensor. B—When the problem occurs, the scope will store a picture of the sensor signals. Note how each signal varies, possibly from intermittent open sensor coil windings or loose mounting. (Fluke) A B