Chapter 14 Troubleshooting Electronic Control System Problems 351 Copyright by Goodheart-Willcox Co., Inc. Caution: Never use a test light to check any electronic circuit unless specifically recommended by the manufacturer. The current flow through a test light is great enough to damage most electronic circuits. Ohmmeters, Figure 14-17, are used to check for conti- nuity or the presence of resistance. Wire resistance should be at or near zero. The resistance of an electronic part should be as specified in the service literature. Modern voltmeters, ammeters, and ohmmeters are generally combined into a single unit called a multimeter. A multimeter used on any electronic system must have high impedance, or resistance to current flow. Many solid-state components, such as ignition modules and ECMs, can be severely damaged by careless use of test lights and multi- meters. Specialized testers are often needed to check the operation of specific electronic devices or systems. Scopes can be used to detect problems in sensors and solenoids. The technician connects the scope and observes the waveform produced by the operation of the suspected device. Then the technician compares the actual wave- form with the standard waveform. The standard wave- form is the waveform the device is supposed to produce when operating properly. If the waveforms do not match, the device is usually defective. Occasionally, an incorrect waveform is caused by a related electrical problem instead of by the device itself. Therefore, even if the waveform is incorrect, it is important to make further checks before condemning the device. If you are unclear about the operation, refer to Chapter 3 for more information. Temperature sensing tools can be used to determine the exact operating temperature of the engine or transmission. The actual temperature can be compared with the sensor readings recorded by the scan tool to determine whether the sensor is sending the correct signals to the ECM. System Operation Testing Once a suspect system has been isolated, the techni- cian should determine whether that system and its compo- nents are working correctly. The easiest way to diagnose an electronic control system is by using the proper scan tool. If the transmission is operable, drive the vehicle and check the applicable scan tool readings. At the same time, observe transmission and transaxle operation. During the test drive, note the transmission or trans- axle shift pattern. Also note any slipping, harsh engage- ments, or noises. Try to match the malfunction with the scan tool readings. This will provide an indication of which gear has the problem and possible causes. With this infor- mation, you should be able to proceed to test the suspect components and systems. General Testing Information This following section explains how to test individual electronic transmission and transaxle components. Before going on the individual component testing, be sure to read the general test information presented in the following section. Types of Component Tests There are three major ways to test electronic compo- nents: visual checks, electrical tests, and testing by substitu- tion. General information on performing these tests is given in the following sections. Making Visual Checks You have already performed some visual checks as part of your preliminary diagnosis. If a problem is suspected in a specific system, check the system for disconnected or Figure 14-16. An inductive pickup allows the meter to read a greater amount of amperage without subjecting the meter itself to high current. (Fluke) Figure 14-17. Ohmmeters are commonly used to check temper- ature sensors. (Nissan) Temperature sensor Inductive pickup