The most commonly required electrical diagnostic tool is the digital multimeter, allowing voltage, ohms (resistance) and amperage to be read by one instrument. The multimeter must be a high-impedance unit, with 10 megohms of impedance in the voltmeter. This type of meter will not place an additional load on the circuit it is testing; which is extremely important in low voltage circuits. The multimeter must be of high quality in all respects. It should be handled carefully and protected from impact or damage. Replace batteries frequently in the unit.
Other necessary tools include an unpowered test light, and electrical leads capable of back-probing electrical terminals without damaging them. A vacuum pump/gauge is also needed for checking some sensors, solenoids and valves.
On all 1996-98 models, an OBD II compliant scan tool must be used to retrieve the ECM Diagnostic Trouble Codes (DTCs). There are many manufacturers of these tools; a purchaser must be certain that the tool is proper for the intended use. A suitable good quality Data Scan Tool (DST) should come with comprehensive instructions on its proper use.
The scan tool allows any stored codes to be read from the ECM memory. The tool also allows the operator to view the data being sent to the computer control module while the engine is running. This ability has obvious diagnostic advantages; as the use of the scan tool is frequently required for component testing. The scan tool makes collecting information easier; however, an operator familiar with the system must correctly interpret the data.
An example of the usefulness of the scan tool may be seen in the case of a temperature sensor, which has changed its electrical characteristics. The ECM is reacting to an apparently warmer engine (causing a driveability problem), but the sensor's voltage has not changed enough to set a fault code. Connecting the scan tool, the voltage signal being sent to the ECM may be viewed; and comparison to normal values or a known good vehicle reveals the problem quickly.