Although stored codes may be read with only the use of a small jumper wire, the use of a hand-held scan tool such as gm's tech 1 or equivalent is recommended. There are many manufacturers of these tools; a purchaser must be certain that the tool is proper for the intended use. If you own a Scan type tool, it probably came with comprehensive instructions on proper use. Be sure to follow the instructions that came with your unit if they differ from what is given here; this is a general guide with useful information included.
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 ecm while the engine is running. This ability has obvious diagnostic advantages; the use of the scan tool is frequently required by the diagnostic charts. Use of the scan tool provides additional data but does not eliminate the need for use of the charts. The scan tool makes collecting information easier; the data must be correctly interpreted by an operator familiar with the system.
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; comparison to either a chart of normal values or a known good vehicle reveals the problem quickly.
The ecm is capable of communicating with a scan tool in 3 modes:Normal or Open Mode
This mode is not applicable to all engines. When engaged, certain engine data can be observed on the scanner without affecting engine operating characteristics. The number of items readable in this mode varies with engine family. Most scan tools are designed to change automatically to the aldl mode if this mode is not available.ALDL Mode
Also referred to as the 10K or special mode, the scanner will present all readable data as available. Certain operating characteristics of the engine are changed or controlled when this mode is engaged. The closed loop timers are bypassed, the spark (est) is advanced and the park/neutral restriction is bypassed. If applicable, the iac controls the engine speed to 1000 rpm - 50, and, on some engines, the canister purge solenoid is energized.Factory Test
Sometimes referred to as back-up mode, this level of communication is primarily used during vehicle assembly and testing. This mode will confirm that the default or limp-in system is working properly within the ecm. Other data obtainable in this mode has little use in diagnosis.
A scan tool that is known to display faulty data should not be used for diagnosis. Although the fault may be believed to be in only one area, it can possibly affect many other areas during diagnosis, leading to errors and incorrect repair.
To properly read system values with a scan tool, the following conditions must be met. All normal values given in the charts will be based on these conditions:
The most commonly required electrical diagnostic tool is the Digital Multimeter, allowing voltage, ohmage (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; this 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 non-powered test light, a quality tachometer with an inductive (clip-on) pick up, and the proper tools for releasing gm's Metri-Pack, Weather Pack and Micro-Pack terminals as necessary. The Micro-Pack connectors are used at the ecm connector. A vacuum pump/gauge may also be required for checking sensors, solenoids and valves.