User Manual

GM Historic Diagnostics • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •
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4
3) The Check Engine light will begin to flash codes. Each DTC is displayed
three (3) times. The DTCs are displayed starting with the lowest numbered
one. After all DTCs are displayed, the sequence keeps repeating until the
ignition key or Field Service mode is turned OFF.
• Count CHECK ENGINE Light Flashes to obtain trouble codes. The first digit
is sent first. A short pause separates digits while a long pause separates each
DTC. All codes contain 2 digits.
• 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 circuit.
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 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 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.