660 Auto Electricity and Electronics Copyright by Goodheart-Willcox Co., Inc. You can use a small transistor radio to fi nd induced voltage sources. Turn the radio on and set it on the AM band, but do not tune it to a station. If the shop is equipped with fl uorescent lights, turn them off or test the vehicle out- side, away from power lines and any other sources of EMI. Move the radio around the engine compartment and under the dash with the engine running. If EMI noise (static) is present, a popping or cracking noise will be produced by the transistor radio. You can also use a car antenna cable and the car radio as a “noise sniff er.” See Figure 34-5. To correct an EMI problem, you must stop the source of the interference (replace leaking spark plug wire, use sup- pressing condenser, etc.) or shield the aff ected system’s wir- ing from the interference (reroute the sensor wire or wrap the wire with foil-type tape, for example). Using a Digital Thermometer A digital thermometer is an electronic device for measuring temperature. Measuring actual temperature can help you verify scan tool readouts and fi nd hard-to-locate problems. It is handy for advanced diagnosis of various sys- tems and components. A digital thermometer can be used to check: • Engine operating temperature. • Exhaust temperature. • Coolant temperature. • Sensor temperature. • Ambient temperature. • Air conditioning outlet temperature. You can test a temperature sensor while it is still in the engine by checking thermometer readings against sensor resis- tance when the engine is cold and aft er it warms. Touch the thermometer’s probe to the sensor to get a reading of its operat- ing temperature. Th is will let you compare sensor temperature and resistance readings with manual specifi cations. Finding Temperature-Related Performance Problems When an engine performance problem only occurs at a specifi c temperature, suspect electronic parts. Electronic circuits, especially ignition control modules, can be aff ected by temperature extremes. To check a component for problems aff ected by tem- perature, use a heat gun to warm the component or a can of freeze spray to cool the unit. If the problem occurs with the temperature change, the unit is at fault and should be replaced. See Figure 34-6. Figure 34-4. Electromagnetic interference can come from a vari- ety of sources and can cause major problems. A—This wave- form is caused by a defective alternator diode. B—This is the injector pulse waveform from the same vehicle. Note the hump pattern similar to the alternator pattern showing poor direct cur- rent. It should be a smooth line showing clean DC. (IATN) Noise from Defective Alternator Diode Alternator Diode Noise A B Figure 34-5. Radiation can upset the operation of the computer sensor signals and car radio. A—Electromagnetic interference can be caused by ignition secondary voltage, a leaking diode in the alternator, and other sources of voltage spikes or magnetic field. B—A cheap transistor radio or an extra antenna cable connected to the vehicle’s radio will “listen” or “sniff” for the source of interference. Move Antenna Cable or Small Transistor Radio Around Dash and Engine Sniffer or Antenna Cable to Radio Wire from Harness to Computer Radiation Noise B A