General System Description

DESCRIPTION
The Engine Control Module (ECM) is a compact microprocessor that is the control center of the fuel injection system. It constantly looks at the information from various sensors, and controls the systems that affect vehicle performance. The ECM performs the diagnostic function check of the system.

It can recognize operational problems, alert the driver through the Malfunction Indicator Lamp (Service Engine Soon), and store a DTC or DTCs which identify the problem areas to aid the technician in making repairs. Refer to System Diagnosis for more information on using the diagnostic function of the ECM.

BASIC KNOWLEDGE REQUIRED
Before using the information found in the following procedures, there are some areas that you should be familiar with. Without this basic knowledge, you will have trouble using them.

Basic Electric Circuits
You should understand the basic theory of electricity, and know the meaning of voltage, amps, and ohms. You should understand what happens in a circuit with an open or a shorted wire. You should be able to read and understand a wiring diagram. A short to ground is referred to as a ground to distinguish it from a short between wires.

Use Of Circuit Testing Tools
You should know how to use a test light, how to connect and use a tachometer, and how to use jumper wires to by-pass components to test circuits. Care should be taken to not deform the terminal when testing.

Use Of Digital Volt-Ohm Meter (DVOM)
You should be familiar with the Digital Volt-Ohm Meter, particularly special tool J-39200 or equivalent. You should be able to measure voltage, resistance, and current and know how to use the meter correctly.

MALFUNCTION INDICATOR LAMP (MIL)
This light is on the instrument panel and has two functions:

^ It is used to tell the driver that a problem has occurred, and that the vehicle should be taken for service as soon as reasonably possible.

^ It is used by the technician to read out "Trouble Codes" to help diagnose system problems.

As a bulb and system check, the light will come "ON" with the key "ON" and the engine not running. When the engine is started, the light will turn off. If the light remains on, the self-diagnostic system has detected a problem. If the problem goes away, the light will go out in most cases after 10 seconds, but a Trouble Code will remain stored in the ECM.

When the light remains "ON" while the engine is running, or when a malfunction is suspected due to a driveability or emissions problem, a on-board diagnostic system check must be performed. The procedures for these checks are given in System Diagnosis. These checks will expose malfunctions which may not be detected if other diagnostics are performed prematurely.

DIAGNOSTIC INFORMATION
The diagnostic "Trouble Tree" charts and functional checks are designed to locate a faulty circuit or component through logic based on the process of elimination. The charts are prepared with the requirement that the vehicle functioned correctly at the time of assembly and that there are no multiple failures.

The Engine Control Module (ECM) performs a continual self-diagnosis on certain control functions. This diagnostic capability is complemented by the diagnostic procedures contained in this manual. The ECM's language for communicating the source of a malfunction is a system of Diagnostic Trouble Codes (DTCs). The DTC(s) are two or three digit numbers that can range from 12 to 51. When a malfunction is detected by the ECM, a DTC is set and the Malfunction Indicator Lamp is illuminated.

INTERMITTENT MALFUNCTION INDICATOR LAMP (MIL)
In the case of an "intermittent" problem, the "Check Engine" malfunction indicator lamp will light for ten (10) seconds and then will go out. However, the corresponding trouble code will be stored in the memory of the ECM until the battery positive voltage to the ECM has been removed. When unexpected DTC(s) appear during the DTC code reading process, one can assume that these DTC(s) were set by an intermittent malfunction and could be helpful in diagnosing the system. An intermittent DTC may or may not re-set. If it is an intermittent failure, a DTC Chart is not used. Consult the "Diagnostic Aids" on the intermittent trouble code. Diagnosis by Symptom also covers the topic of "intermittents". A physical inspection of the applicable sub-system most often will resolve the problem.

LEARNING ABILITY
The system has a "learning" ability which allows it to make corrections for minor variations in the fuel system to improve driveability. If the battery is disconnected, to clear diagnostic trouble codes or for other repair the "learning" process resets and begins again. A change may be noted in the vehicle's performance. To "teach" the vehicle, ensure that the engine is at operating temperature. The vehicle should be driven at part throttle, with moderate acceleration and idle conditions until normal performance returns.