DRIVEABILITY & EMISSIONS
INTRODUCTION
The 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. The codes are two digit numbers that can range from 12 to 99. When a malfunction is detected by the ECM, a code is set and the "Check Engine" light is illuminated. "Check Engine" Light

This light is on the instrument panel and has the following functions.

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 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 "Diagnostic Circuit Check" must be performed. These checks will expose malfunctions which may not be detected if other diagnostics are performed prematurely.

INTERMITTENT "CHECK ENGINE" LIGHT
In the case of an "intermittent" problem, the "Check Engine" light will light for ten seconds and then will go out. However, the corresponding code will be stored in the memory of the ECM until the battery voltage to the ECM has been removed. When unexpected codes appear during the code reading process, one can assume that these codes were set by an intermittent malfunction and could be helpful in diagnosing the system.

An intermittent code may or may not reset. If it is an intermittent failure, a Diagnostic Code Chart is not used. Consult the "Diagnostic Aids" on the page of the diagnostic chart corresponding to the intermittent trouble code. The Smptoms section below also covers the topic of "Intermittents." A physical inspection of the applicable sub-system most often will resolve the problem.

READING CODES
The provision for communicating with the ECM is the Assembly Line Diagnostic Link (ALDL) connector (See Figure below). It is located under the instrument panel and is sometimes covered by a plastic cover labeled "DIAGNOSTIC CONNECTOR." It is used in the assembly plant to receive information in checking that the engine is operating properly before it leaves the plant. The code(s) stored in the ECM's memory can be read either through TECH 1 Diagnostic Computer, a hand-held diagnostic scanner plugged into the ALDL connector or by counting the number of flashes of the "Check Engine" light when the diagnostic terminal of the ALDL connector is grounded. The ALDL connector terminal "B" (diagnostic terminal) is the second terminal from the right of the ALDL connector's top row. The terminal is most easily grounded by connecting it to terminal "A" (internal ECM ground), the terminal to the right of terminal "B" on the top row of the ALDL connector. (A bent paper clip is sufficient and is a useful diagnostic 'tool' to keep in the truck.)

ALDL Connector
ALDL Connector
AGround
BDiagnostic Terminal
FT.C.C. (If Used)
HSpeed Input From Antilock Module
MSerial Data

Once terminals "A" and "B" have been connected, the ignition switch must be moved to the "ON" position, with the engine not running. At this point, the "Check Engine" light should flash Code 12 three times consecutively. This would be the following flash sequence: "flash, pause, flash-flash, long pause, flash, pause, flash-flash, long pause, flash, pause, flash-flash." Code 12 indicates that the ECM's diagnostic system is operating. If Code 12 is not indicated, a problem is present within the diagnostic system itself, and should be addressed by consulting the appropriate diagnostic chart.

Following the output of Code 12, the "Check Engine" light will indicate a diagnostic code three times if a code is present, or it will simply continue to output Code 12. If more than one diagnostic code has been stored in the ECM's memory, the codes will be output from the lowest to the highest, with each code being displayed three times.

CLEARING CODES
To clear the codes from the memory of the ECM, either to determine if the malfunction will occur again or because repair has been completed, the ECM power feed must be disconnected for at least thirty (30) seconds. The ECM power feed can be disconnected at the ECMB fuse in the fuse block. (The negative battery terminal may be disconnected, but other on-board memory data, such as preset radio tuning, will also be lost.)

NOTICE: To prevent ECM damage, the key must be "OFF" when disconnecting or reconnecting ECM power.

When using a hand-held Tech 1 Diagnostic Computer, or "Scan" tool to read the codes, clearing the diagnostic codes is done in the same manner as in the above procedure.

DIAGNOSTIC MODE
When the Diagnostic terminal is grounded with the ignition "ON" and the engine "OFF," the system will enter what is called the Diagnostic Mode. In this mode the ECM will:

  1. Display a Code 12 by flashing the "Check Engine" light (indicating the system is operating correctly).
  2. Display any stored codes by flashing the "Check Engine" light. Each code will be flashed three times, then Code 12 will be flashed again.
  3. Energize all ECM controlled relays and solenoids except fuel pump relay. This allows checking circuits which may be difficult to energize without driving the vehicle and being under particular operating conditions.
  4. The IAC valve moves to its fully extended position on most models, blocking the idle air passage. This is useful in checking the minimum idle speed.

FIELD SERVICE MODE
If the diagnostic terminal is grounded with the engine running, the system will enter the Field Service mode. In this mode, the "Check Engine" light will indicate whether the system is in "Open Loop" or "Closed Loop."

In "Open Loop" the "Check Engine" light flashes two and one-half times per second.

In "Closed Loop," the light flashes once per second. Also, in "Closed Loop," the light will stay "OFF" most of the time if the system is running lean. It will stay "ON" most of the time if the system is running rich.

While the system is in Field Service Mode, new codes cannot be stored in the ECM and the "Closed Loop" timer is bypassed.

ECM LEARNING ABILITY
The ECM 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 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.

DIAGNOSTIC CIRCUIT CHECK
After the visual/physical underhood inspection, the "Diagnostic Circuit Check" is the starting point for all diagnostic procedures or finding the cause of an emissions test failure.

The correct procedure to diagnose a problem is to follow three basic steps.

  1. Are the On-Vehicle Diagnostics working? This is determined by performing the "Diagnostic Circuit Check." Since this is the starting point for the diagnostic procedures or finding the cause of an emissions test failure, always begin here. If the On-Vehicle Diagnostics aren't working, the "Diagnostic Circuit Check" will lead to a diagnostic chart to correct the problem. If the On-Vehicle Diagnostics are working correctly, the next step is:
  2. Is there a Code stored? If a code is stored, go directly to the page below that corresponds to the numbered code. This will determine if the fault is still present. If no code is stored, then:
  3. "Scan" Serial Data transmitted bv the ECM This involves reading the information available on the Serial Data Stream with Tech 1 or one of the tools available for that purpose (Such as the Diacom package by Rinda Technologies). Information on these tools and the meaning of the various displays can be found in the succeeding paragraphs. Expected readings under a particular operating condition can be found below.

DIAGNOSTIC CODES
NOTE: All diagrams shown in the Diagnostic Code pages are of a higher resolution than that displayed - View seperately either in or out of your browser if some text is difficult to read.

PERFORMING SYMPTOM DIAGNOSTICS
The DIAGNOSTIC CIRCUIT CHECK should be performed before using this section. The purpose of this section is to locate the source of a driveability or emissions problem when other diagnostic procedures cannot be used. This may be because of difficulties in locating a suspected sub-system or component.

Many driveability related problems can be eliminated by following the procedures found in Service Bulletins. These bulletins supersede this manual. Be sure to check all bulletins related to the complaint or suspected system.

If the engine cranks but will not run, use **CHART A-3**.

The sequence of the checks listed in this section is not intended to be followed as on a step-by-step procedure. The checks are listed such that the less difficult and time consuming operations are performed before more difficult ones.

Most of the symptom procedures call for a careful visual and physical check. The importance of this step cannot be stressed too strongly. It can lead to correcting a problem without further checks, and can save valuable time. This procedure includes checking the following.

SYMPTOM DIAGNOSTICS

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