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GM Historic (OBD I) Diagnostics
5
4) When Field Service mode is on, most computer controlled relays
and solenoids will be turned on, except for the fuel pump relay
and fuel injectors. Toggle the Field Service mode on and off.
❒Use a voltmeter to measure relays and solenoids input voltage
to verify on/off conditions. An ohmmeter could be used to
check the continuity between the relay’s switch terminals.
❒The idle air control (IAC) valve is fully seated to the zero posi-
tion by the PCM.
❒The exhaust gas recirculation (EGR) solenoid is energized for
25 seconds.
KOER Procedure
✓With the engine running, the Field Service mode can be used to
measure base timing, check open loop/closed loop operation, and
determine if the engine is running rich or lean.
1)Engage parking brake and block drive wheels.
2) Verify engine is cold. If engine is hot or warm, allow it to cool.
3) On Scan Tool, place Field Service on.
4) Start engine and place transmission in Park or Neutral. The CHECK
ENGINE light will flash once.
5) Warm the engine by idling for 2 minutes at 2000 RPM.
6) Observe CHECK ENGINE light.
•If CHECK ENGINE light flashes 2.5 times a second, the
on-board computer is operating in Open Loop.
•When the engine warms up to normal operating
temperature, the on-board computer is now operating in
Closed Loop. The CHECK ENGINE light should flash once
a second. The on-board computer is now operating in Closed
Loop.
•If CHECK ENGINE light:
❒ flashes equally on/off, then the fuel system is running
normally.
❒ is mostly on, then the fuel system is running rich.
❒ is mostly off, then the fuel system is running lean.
7) On some engines, the spark advance timing is fixed during Field
Service mode. This allows the technician to measure the engine
base timing.
3. Return to Special Tests Menu.
•Press BACK.