Pontiac Fiero 1988 Service Manual page 520

DRIVEABILITY AND EMISSIONS - FUEL INJECTION (TBI) 6E2-C1-3
Figure C1-4
- MAT
Sensor
MAT Sensor
(Figure C1-4)
The manifold air temperature (MAT) sensor is a
thermistor, a resistor which changes value based on
temperature, mounted in the intake manifold. Low
manifold air temp, produces a high resistance
(100,000 ohms at -40°C), while high temp, causes low
resistance (70 ohms at 130°C/266°C).
The ECM supplies a 5 volt signal to the MAT
Sensor, through a resistor in the ECM, and monitors
the voltage. The voltage will be high when the
manifold air is cold and low when the air is hot. By
monitoring the voltage, the ECM calculates the air
temp, and adjusts fuel and spark advance.
A failure in the MAT circuit should set either a
Code 23 or Code 25. Proper use of the code charts
should lead to either repairing a wiring problem or
replacing the sensor.
Oxygen (O2) Sensor
(Figure C1-5)
The exhaust oxygen sensor (O2) is mounted in the
exhaust system where it can monitor the oxygen
content of the exhaust gas stream. The oxygen content
in the exhaust reacts with the sensor to produce a
voltage output. This voltage ranges from
approximately .1 volt (high O2 - lean mixture) to .9
volts (low O2 - rich mixture). This voltage can be
measured with a digital voltmeter having at least 10
megohms input impedance. Use of standard shop type
voltmeters will result in very inaccurate readings.
By monitoring the voltage output of the O2 sensor,
the ECM will know what fuel mixture command to
give to the Injector (lean mixture - low O2 voltage =
rich command; rich mixture - high O2 voltage = lean
command).
The O2 sensor circuit, if open, should set a Code
13. A constant low voltage in the sensor circuit should
EXHAUST O X Y G E N (0
2
)
SENSOR
10-2-86
* 4S 0078-6E
Figure C1-5 - Exhaust Oxygen (O2)
Sensor
set a Code 44, while a constant high voltage in the
circuit should set a Code 45. Codes 44 and 45 could
also be set as a result of fuel system problems. See
Code Charts.
Throttle Position Sensor (TPS)
(Figure C1-6)
The throttle position sensor (TPS) is a
potentiometer, connected to the throttle shaft on the
throttle body. The TPS electrical circuit consists of a
5V supply line and a ground path line, both provided
by the ECM. A third wire is used as a signal line to
the ECM. By monitoring the voltage on this signal
line, the ECM calculates throttle position. As the
throttle valve angle is changed (accelerator pedal
moved), the signal voltage of the TPS also changes. At
a closed throttle position, the signal of the TPS is
below 1.25 volts. As the throttle valve opens, the
signal voltage increases so that, at wide-open throttle,
it should be approximately 5 volts.
The ECM can determine fuel delivery based on
throttle valve angle (driver demand). A broken or
loose TPS can cause intermittent bursts of fuel from
the injector and an unstable idle, because the ECM
thinks the throttle is moving. A problem, in any of the
TPS circuits, will set either a Code 21 or 22. Once a
trouble code is set, the ECM will use an artificial
default value for TPS, and some vehicle performance
will return.
The TPS is not adjustable. The ECM uses the
reading at closed throttle for the zero reading, so no
adjustment is necessary.
Park/Neutral Switch (Auto Only)
The park/neutral (P/N) switch indicates to the
ECM when the transmission is in Park or Neutral.
This information is used for the TCC, and the IAC
valve operation.