Diagnostic Circuit Check

Diagnostic Circuit Check:

Wiring Diagram - Diagnostic Circuit Check:

DIAGNOSTIC CIRCUIT CHECK


Circuit Description:
The purpose of the "Scan" diagnostic circuit check is to 1) make sure the "Service Engine Soon" light works, 2) the ECM is operating and can recognize a fault, and 3) to determine if any codes are stored. This is the starting point for any diagnosis. It is important to understand the "Scan" tool operation and limitations. More information on SCAN testers can be found under TESTING PROCEDURES.
When the engine is started, the ECM grounds terminal "G" to turn out the "Service Engine Soon" light. The ECM alternately grounds and opens terminal "G" to flash a code.


Test Description: Numbers below refer to circled numbers on the diagnostic chart.
1. Checks for the proper operation of the "Service Engine Soon" light. With the key "ON," "scan" tool not connected and the engine not running, the light should be "ON" steady.

2. Grounding the test terminal will flash a Code 12 and any stored trouble codes. The light must go "ON" and "OFF" for a proper code. If the light goes from "bright" to "dim," this is not considered a code. See CHART A-6 in that case.


3. If the "Scan" tool is not operating, try it on another vehicle. If it operates on another vehicle, the cigar lighter socket should be checked for battery voltage and a good ground. If the "Scan" tool displays "No data" or "No ALDL", with the ignition "ON," see CHART A-6.

4. "No codes" at this point, indicates that the fault is intermittent and the "System Performance Check" should be performed. If a code or codes is displayed, use the appropriate chart for that code.