Diagnostic Circuit Check

Diagnostic Circuit Check:

NOTE: Check and correct any problems with vacuum hoses, electrical harness connections and routing before proceeding with this check.

The Diagnostic Circuit Check ensures that the on-board diagnostic system is functioning, determines the trouble codes to be displayed and guides diagnosis to other problem areas. The check is an organized approach for identifying problems originating with the electronic engine control system. The diagnostic charts are organized to check whether or not a stored code is "intermittent" or "hard." An "intermittent" code is one which does not reset itself and represents a fault not usually present when the vehicle is brought in for service. Loose and/or bad connections are the most probable cause for these problems. A "hard" code is one which sets and remains until the problem is diagnosed and repaired. The appropriate code chart is used to locate the source of the problem.

The circled numbers on the diagnostic chart refer to the steps listed below:

1. A steady "Check Engine" light with ignition "ON" (engine not running) confirms battery and ignition voltage to the ECM.

Reading Trouble Codes:

2. Ground the diagnostic terminal by installing a jumper wire between terminals "A" and "C" of the ALDL connector (located behind the passenger side kick panel). This will cause the light to flash code 12, indicating that "Diagnostic Mode" is working. Code 12 will flash three times, followed by any other codes stored in ECM memory. Each code will flash three times starting with the lowest numbered code and then repeat starting with code 12. If there are no other codes, code 12 will flash repeatedly until the jumper is removed or the engine is started.

3. Record all codes except 12. If the problem is "Engine Cranks But Will Not Run," proceed to Chart A-3.

4. If no codes were recorded, see DIAGNOSIS BY SYMPTOM. The "Field Service Mode" may also be helpful in diagnosis. To enter this mode, run the engine with the diagnostic connector jumpered. While in "Field Service Mode," no new trouble codes can be stored in the ECM and the "Closed Loop" timer is bypassed. The ECM will use the "Check Engine" light to display its response to oxygen sensor voltage as follows:
A. "Closed Loop" confirms that the oxygen sensor signal is being used by the ECM to control fuel delivery and that the system is functioning properly. Signal voltage will rapidly change from below .35 to above .55 volts. "Closed Loop" operation is indicated by the light flashing at the rate of once per second.
B. "Open Loop" indicates that oxygen sensor signal voltage is not being utilized by the ECM. Signal voltage will be at a constant value between .35 and .55 volts and the light will flash at a rate of 2.5 times per second. The system will indicate "Open Loop" from 30 seconds to two minutes after engine start or until the oxygen sensor reaches operating temperature. If the system fails to enter "Closed Loop," see Code 13 diagnostic chart.
C. Lean exhaust is indicated if the "Check Engine" light remains "OFF." Oxygen sensor voltage will be steady at less than .35 volts. See Code 44 diagnostic chart.
D. Rich exhaust is indicated if the "Check Engine" light remains steadily "ON." Oxygen sensor voltage will be steady above .55 volts. See Code 45 diagnostic chart.

5. Road tests utilizing the "Field Service Mode" should only be done at steady speeds. The following conditions may occur in this mode and should be considered normal:
a. Acceleration-light may be "ON" too long due to acceleration enrichment.
b. Deceleration-light may be "OFF" too long due to decel leaning or fuel cut.
c. Idle-light may be "ON" too long with idle below 1200 rpm.

6. To clear codes, switch ignition "OFF" and disconnect battery for at least 10 seconds.

For Diagnostic Circuit Check Charts, refer to DIAGNOSTIC CHARTS.