GM Bonneville/Eighty Eight/LeSabre 1986-1999

General Information


Since the computer control module is programmed to recognize the presence and value of electrical inputs, it will also note the lack of a signal or a radical change in values. It will, for example, react to the loss of signal from the vehicle speed sensor or note that engine coolant temperature has risen beyond acceptable (programmed) limits. Once a fault is recognized, a numeric code is assigned and held in memory. The dashboard warning lamp: CHECK ENGINE or SERVICE ENGINE SOON (SES), will illuminate to advise the operator that the system has detected a fault. This lamp is also known as the Malfunction Indicator Lamp (MIL).

More than one code may be stored. Keep in mind not every engine uses every code. Additionally, the same code may carry different meanings relative to each engine or engine family.

In the event of an computer control module failure, the system will default to a pre-programmed set of values. These are compromise values which allow the engine to operate, although possibly at reduced efficiency. This is variously known as the default, limp-in or back-up mode. Driveability is almost always affected when the ECM enters this mode.


See Figures 1 and 2

On most early models, the stored codes may be read with only the use of a small jumper wire, however the use of a hand-held scan tool such as GM's TECH-1® or equivalent is recommended. On some 1994 vehicles (equipped with a 16-pin Data Link Connector), and all 1995-99 models, an OBD-II compliant scan tool must be used. 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.

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Fig. Fig. 1: A TECH 1®, or equivalent scan tool is recommended for reading trouble codes

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Fig. Fig. 2: Inexpensive scan tools, such as this Auto Xray®, are available to interface with your General Motors vehicle

The scan tool allows any stored codes to be read from the ECM or PCM 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; the use of the scan tool is frequently required for component testing. 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 normal values or a known good vehicle reveals the problem quickly.


See Figure 3

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-impe