Without Scan Tool

ALDL Connector (Located Underdash):

READING DIAGNOSTIC TROUBLE CODES
The means of communicating with the control module is the Data Link Connector (DLC) located under the instrument panel and is sometimes covered by a plastic cover labeled "DIAGNOSTIC CONNECTOR". The DLC is used in the assembly plant to receive engine information to determine proper operation before it leaves the plant. The Diagnostic Trouble Codes (DTC's) stored in the control module memory can be displayed by counting the number of flashes of the Malfunction Indicator Lamp (MIL) "Service Engine Soon" when the diagnostic terminal of the DLC is grounded. The DLC terminal "B" (diagnostic terminal) is the second terminal from the right of the DLC top row. The terminal is most easily grounded by connecting it to terminal "A" (internal control module ground), which is located to the right of terminal "B" on the top row of the DLC.

When terminals "A" and "B" have been connected, the ignition switch must be turned to the "ON" position with the engine NOT operating. The MIL (Service Engine Soon) should flash DTC 12 three times consecutively. This would be the following flash sequence: One flash, pause, two flashes, long pause, one flash, pause, two flashes, long pause, one flash, pause, two flashes. DTC 12 indicates that the control module diagnostic system is operating properly. If DTC 12 is NOT indicated, a fault is present within the diagnostic system itself, and should be addressed by consulting an appropriate diagnostic chart.

Following the output of DTC 12, the MIL will indicate any DTC three times consecutively. However if no other codes are present will continue to repeat DTC 12. If more than one diagnostic trouble code has been stored in the control module memory, the DTC's will be displayed from the lowest to the highest with each DTC being displayed three times.


DIAGNOSTIC MODE
When the diagnostic terminal is grounded with the ignition "ON" and the engine "OFF", the system will enter the diagnostic mode. In this mode, the control module will:

- Display a Diagnostic Trouble Code (DTC) 12 by flashing the Malfunction Indicator Lamp (MIL) "Service Engine Soon" (indicating that the system is operating properly).

- Display any stored DTC's by flashing the Malfunction Indicator Lamp (MIL) "Service Engine Soon". Each DTC will be flashed three times, then DTC will be flashed again.

- Energize all control module controlled relays and solenoids except the fuel pump relay. This allows checking circuits which may be difficult to energize without driving the vehicle and being under particular operating conditions.

- Move the Idle Air Control (IAC) valve 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 MIL "SERVICE ENGINE SOON" light will indicate whether the system is in OPEN or CLOSED LOOP operation.

If the system is in OPEN LOOP operation, the light will flash approximately two and one-half times per second.

CLOSED LOOP operation is indicated by the light flashing approximately 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 the "Field Service" mode, new codes cannot be stored in the control module and the "CLOSED LOOP" timer is bypassed.