![]() The ECM takes this information, processes it and then sends a command to the fuel injection system, telling it to change the air/fuel mixture. If the percentage of oxygen in the exhaust gas is incorrect, indicating a too rich or too lean condition, an electrical signal is sent to the ECM. Here’s a specific example of how one portion of this system operates: An oxygen sensor, located in the exhaust manifold, constantly monitors the oxygen content of the exhaust gas.The OBD system is analogous to the central nervous system in the human body: The sensors (nerve endings) constantly relay information to the ECM (brain), which processes the data and, if necessary, sends out a command to change the operating parameters of the engine (body).Note: 1996 and later OBD-II systems refer to the on-board computer as the Powertrain Control Module (PCM). The On Board Diagnostic (OBD) system consists of an Electronic Control Module (ECM) and information sensors which monitor various functions of the engine and send data back to the ECM.On-Board Diagnostic system general description Ask the parts salesman at a local auto parts store for additional information concerning availability and cost. Several tool manufacturers have released OBD-II scan tools for the home mechanic. With the arrival of the Federally mandated emission control system (OBD-II), a specially designed scanner has been developed. ![]() However, there are several exceptions where using an analog voltmeter may be necessary to test certain sensors. This dilemma does not show itself when measuring larger amounts of voltage (9 to 12 volt circuits) but if you are measuring a low voltage circuit such as the oxygen sensor signal voltage, a fraction of a volt may be a significant amount when diagnosing a problem. Because a voltmeter is hooked up in parallel with the circuit when testing, it is vital that none of the voltage being measured should be allowed to travel the parallel path set up by the meter itself. The digital multimeter is equipped with a high resistance internal circuitry (10 million ohms). Another good reason for the digital multimeter is the high impedance circuit. When working with electronic circuits which are often very low voltage, this accurate reading is most important. The analog multimeter cannot display the volts-ohms or amps measurement in hundredths and thousandths increments. A digital volt- ohmmeter is preferred over the older style analog multimeter for several reasons. A digital multimeter is necessary for checking fuel injection and emission related components (see illustration).In 1996 the government mandated OBD-II system became standard in nearly every car and truck and gave a much better indication of where and what the malfunction indicated was that had cause the check engine light to light on the dashboard. The system used in the 1988-1995 was very basic and could only provide clues as to what the malfunction was. ![]() ![]() The GMT-400 trucks and SUVs bridged the gap between the early days of computer controlled electronic fuel injection and the modern OBD-II era. This feature applies to many GM vehicles that use 4.3-liter V6, and 5.0-liter, 5.7-liter and other V8 motors, and it is taken directly from our digital online manual 24065 for the 1988-98 Chevrolet and GMC C/K 1500/2500/3500 pickup trucks (including 1999-2000 C/K Classics), Chevy Tahoe, Blazer, and Suburban, and GMC Yukon, Yukon Denali, and Suburban, 2WD or 4WD with V6 or V8 gasoline engines. All Haynes Digital Online Manuals feature everything you love about our books in an instantly accessible digital online form, that you can use via any device with an internet connection. ![]()
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