Field Service Mode

Fig. 2 Trouble Code Test Lead Identification:

If the Diagnostic "test" terminal (B) is grounded with the engine running, the system will enter the Field Service Mode. In this mode, the "CHECK ENGINE" light will show whether the system is in open or closed loop. In open loop, the "CHECK ENGINE" light flashes two and on-half times per second. In closed loop, the light flashes once per second: Also, in closed loop, the light will stay OUT most of the time if the system is too lean. It will stay ON most of the time if the system is too rich. In either case, the Field Service Mode Check, which is part of the Diagnostic Circuit Check, will lead you to the proper chart.

While the system is in Field Service Mode, the Electronic Control Module (ECM) will be in the following mode:

1. The distributor will have a fixed spark advance.

2. New trouble codes cannot be stored in the ECM.

3. The closed loop timer is bypassed.