Diagnostic equipment information

Check the operation of the fuel injection and emission control system components using a digital multimeter (DMM). A digital multimeter is preferred for several reasons. First, analog meters make it difficult (sometimes impossible) to read results to the hundredths or thousandths of a unit, while such accuracy is especially important when testing circuits that include electronic components. Second, the internal circuit of a digital multimeter has a very high impedance (the internal resistance of the meter is 10 megohms). Since a voltmeter is connected in parallel with the circuit under test, measurement accuracy is higher when less leakage current flows through the meter itself. This factor is not significant when measuring relatively high voltages (9 to 12 V), but becomes critical when diagnosing components that output low-voltage signals, such as the oxygen sensor, where measurements are in fractions of a volt.

The most convenient tool for diagnosing engine management systems of modern vehicles is a handheld scan tool. First-generation scan tools are used to read trouble codes from OBD-I systems. Before use, verify that the scan tool is compatible with the model and model year of the vehicle being tested. Some scan tools are multifunctional, with the ability to change the cartridge depending on the vehicle being diagnosed (Ford, GM, Chrysler, etc.), while others are tied to requirements of regional authorities and are intended for use in certain regions of the world (Europe, Asia, USA, etc.).

With the introduction of second-generation On-Board Diagnostics (OBD-II) systems that meet the requirements of the latest environmental legislation, scan tools of special design began to be produced. Some manufacturers have released scan tools intended for use by home mechanics; ask at automotive accessory stores. In principle, trouble codes stored in the self-diagnostic system memory can be read using a jumper wire installed between specific terminals of the 16-pin diagnostic connector.