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IAT TEMP (°C) (Intake Air Temperature - PCM input)
IAT TEMP (°F)
Display: Numeric temperature value: -40 to 419 (°F) or -40 to 215 (°C)
This is a measure of intake air temperature to determine correct air/fuel ratios and
spark timing operations. The voltage from a thermistor in the intake manifold is sent
to the PCM. The PCM converts this voltage signal to a numerical temperature value.
IGN ADV (DEG) (Ignition Advance - PCM output)
Display:-64 to +63.5
This is a signal from the PCM to the Ignition Control Module (ICM) telling it how
much spark advance to add to base engine timing (expressed in crankshaft degrees).
The ICM sends a base timing signal taken from a sensor (either in the distributor
or on the crankshaft) and sends it to the PCM. The PCM decides how much advance
to add to the signal based on operating conditions, then sends the signal back to the
ignition to fire the spark plugs. Depending on the vehicle, this signal may mean one
of two things. In some systems, the display shows exact timing (base timing plus
PCM-directed advance). In other systems, the display will show only the amount of
advance the PCM is adding to the base timing supplied by the ICM.
LT FL TRM 1(%) (Long-Term Fuel Trim Bank 1/2 - PCM calculated value)
LT FL TRM 2(%)
Display: Positive (rich fuel trim) or Negative (lean fuel trim) Percentage
This value represents the long-term correction to the fuel control calculations. The
Long Term Fuel Trim is an indication of the PCMs commanded fuel mixture adjust-
ments. The number can range from -100% to +100%, with the midpoint being 0. A
positive reading indicates that the PCM has commanded a long-term rich mixture
correction in response to a lean operating condition. A negative reading indicates that
the PCM has commanded a long-term lean mixture in response to a rich operating
condition. Fuel Trim is used to adjust the fuel injector pulse width calculations.
Long-Term Fuel Trim corrections usually operate in closed loop mode only. In
open loop mode, the number typically defaults to a fixed value of 0. Depending on the
vehicle, the Long Term Fuel Trim values may be reset to the default of 0 every time
the engine is shut off, or corrected values may be retained in computer memory.
Retained values are reapplied when the vehicle is restarted.
Some PCMs are able to control the left and right cylinder bank Fuel Trim individu-
ally, shown by the LT FL TRM 1 & 2 parameters. LT FL TRM 1 is use in injection Bank
1 fuel correction and LT FL TRM 2 for Bank 2.
MAF (LB/M) (Mass-Air Flow rate - PCM calculated value)
MAF (GM/S)
Display: 0 - 86.5 lb/min or 0 - 655.35 gm/s
A signal sent from the Mass Air Flow Sensor to the PCM indicating the mass of the
air entering the engine, as a voltage reading. This voltage reading may be used with
other engine information obtained to calculate the Mass Air Flow Rate. This informa-
tion is used to determine the correct injector pulsewidths to maintain proper air/fuel
ratio. MAF readings should be low at idle and increase as the throttle opens.
NOTE: Some engines may display an MAF reading even with the engine off.
Most MAF Sensor operates on a hot film or wire in the air intake between the air
filter and the throttle body. The wire is heated to a preset temperature. As incoming air
cools the wire or film, the PCM measures the voltage required to maintain the preset
temperature. The PCM translates this voltage reading into airflow measurements
based on internal lookup tables.