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GM Historic (OBD I) Diagnostics
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5
— DTC 12 (No RPM reference pulse) should display first since the engine
is not running. If not, problems exist in the PCM or Check Engine light
circuitry.
Code 12 will look like:
FLASH-pause-FLASH-FLASH — long pause.
Code 23 will look like:
FLASH-FLASH-pause-FLASH-FLASH-FLASH — long pause.
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 injec-
tors. 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 position 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.
Code Lookup
Refer to “
Code Lookup
” on page 4-14 of
Global OBD II Diagnostics
.
✓ Enter only the code numbers. No system designation is used for historic
DTCs, they are either History or Current codes.
✓ A “P” appears at the beginning of the DTC if the vehicle uses OBD II type
codes.
Print Data
Refer to
“Print Data
” on page 4-12 of
Global OBD II Diagnostics.