"Non-Scan" Diagnostic Circuit Check
The diagnostic circuit check is an organized approach for identifying a problem caused by the Fuel Injection System. Driver complaints fall into 3 categories: steady "SERVICE ENGINE SOON" light, driveability problems, and engine "CRANKS BUT, WON'T RUN".
NOTE:
The following step numbers refer to the numbers in the accompanying flow chart(s).
- 1) A steady "SERVICE ENGINE SOON" light, with ignition on and engine not running, confirms battery and ignition voltage to ECM.
- 2) Ground diagnosis terminal by jumpering terminal A to B in ALCL connector located below instrument panel. The ECM will cause "SERVICE ENGINE SOON" light to flash Code 12, indicating that ECM diagnostics are working. Code 12 will flash 3 times, followed by any other trouble codes stored in memory. Each additional code will flash 3 times starting with lowest code, and then start over again with Code 12. If there are no other codes, Code 12 will flash until diagnostic terminal jumper is disconnected or engine is started.
- 3) Record all stored codes except Code 12.
- 4) With engine running and diagnostic terminal grounded, ECM will respond to O2 sensor signal voltage and use the "SERVICE ENGINE SOON" light to display this information as follows:
- A) Closed loop confirms that O2 sensor voltage is being used to control fuel delivery. Signal voltage will vary from .35-.55 volt.
- B) Open loop confirms that O2 sensor voltage to ECM is unusable. Signal voltage is a fixed value between .35 and .55 volt. System will flash open loop for 30 seconds to 2 minutes or until O2 sensor reaches operating temperature.
- C) O2 sensor signal voltage will be less than .35 volt. Refer to Code 44.
- D) O2 sensor signal voltage will be more than .55 volt. Refer to Code 45.
- 5) Road test of system in field service mode must be done at steady speeds. In this mode the following conditions may be observed and should be considered normal: light on too long under acceleration (due to acceleration enrichment); light off too long under deceleration (due to decel leaning of fuel mixture or fuel cut-off); light on too long at idle with idle below 1200 RPM.
- 6) Clearing codes. Ignition off. Disconnect battery pigtail or ECM B fuse for 10 seconds.