EMISSIONS CONTROL PARTS & FUNCTIONS
 

Oxygen Sensor
Mounted in the exhaust manifold, where it can monitor the contend of the exhaust gas stream.  By monitoring the voltage
output of this sensor, the ECM will know what fuel mixture command to give to the fuel system.  The sensor produces no
voltagewhen it's below its normal operating tempurature of 600-degrees F.  During this initial period before warm-up, the
ECM operated in open-loop mode.  In normal operation, when the sensor has been warmed up by exhaust gases, the
sensor will produce a signal voltage that varies between 1.0 (rich limit) and 0.1 volt (lean limit).  A bad sensor will produce a code 13.

To check: Use a high-impedance Digital Voltmeter.  WArm the engine to normal temerature, then turn the engine off.  Unplug the sensor's electrical connector. If three wires, is a heated-type sensor - connect the volt meter to the wire routed to the ECM, (usually a yellow wire, with a blue tracer).  Do not let the sensor wire or voltmeter leads touch the exaust manifold.  Ground the NEGATIVE probe of the meter, turn the meter to Millivolt, and start the engine.  The readings should fluctuate between 100, and 1000 milivolts (0.1, and 1.0).  If the meter reading isn't within these specs, the sensor is probably bad (although a fuel system problem could also cause it).


Coolant Temporature Sensor
This is a thermistor (a resistor which varies in value in accordance with tempurature changes) and is located on the side
of the intake manifold.  A failure in this sensor or circuit shouls set a code 14 and 15.

Check:  Unplug the electrical connector and use an Ohmmeter to measure the resistance across the sensor terminals with the engine cold.  If the difference in resistance is not approx. 500ohms, the sensor is probably bad.


Manifold Air Tempurature  (MAT) Sensor
Used on Fule Injected models only, is a thermister which constantly measures temperature of the air entering
the intake manifold (TBI Models), or air cleaner housing (MPFI Models).  As the ar temperature varies, the ECM,
while monitoring the MAT, adjusts the amount of fuel accordingly.  A failure in the MAT sensor or circuit will often set a diagnostic code of 23.

Manifold Absolute Pressure (MAP) Sensor
This is also used on TBI models, and monitors the the intake manifold oressure changes resluting from engine load, and
speed, and converts the information into a voltage output.  The ECM uses this to control fuel delivery and ignition timing.
Other than checking the vacuum hose and electrical connector for proper connection and damage, inspection of the MAP sensor should be left to a dealer.  The sensor could be damaged if not checked properly.

If this sensor is damaged or bad, the engine may stall, or run very rough. If you wish to test for this, remove the
vacuum line while the engine is running, to dertermine symptoms.  Replace the vacuum line, and unplug the elcetrical connector to see what it does this way.  Most of the time, the engine will stall right away.


Mass Airflow (MAF) Sensor
Used on MPFI systems,t he MAF sensor measures the volume of airflow into the engine.  This information is used by the ECM
to help calculate the correct air-fuel ratio for the Multi-Port Injection system.  A MAF sensor failure will often give a code
of 33 or 34.

These sensors are usually locates in the Air-box, or tube leading to the Plenum, or other intake system.


Throttle Position (TPS) Sensor
This is located on the throttle shaft at the TBI unit.  By monitoring the input voltage from the TPS, the ECM can
determine fuel delivery based on the throttle valve angle (driver demand).  A broken or loose TPS can cause intermittent
bursts of fuel from the enjector becuse the ECM thinks the throttle is moving.   A problem in the TPS will generate the
error code 21 or 22.  Once the codes have been set, the ECM will use an artificial default value for TPS, and some vehicle performance will return.

Vehicle Speed Sensor (VSS)
This sensor consists of the lead switch and magnet that is built into the speedometer.  A the magnet turns with the speed-o cable. Its magnetic force causes the lead switch to turn on and off.  This pulsating voltage signal is sent to the ECM and is
coverted into miles per hour.  A failure will result in a code 24.

The information was duplicated as above for it's purpose, so I don't have that information.  Suffice to say, it is a bad idea to
disconnect the speed-o cable.


Crank Angle Sensor
The crank angle sensor is located in the distributor, and serves a dual purpose by also being the pickup assembly for
the ignition system.  It consists of a signal generator and signal rotor.  As the rotor turns, pulsing AC voltage is generated in the pickup assembly.  This signal is sent to the ECM, where it is used to calculate the engine speed, and also as one of the
signals to control various devices.  If a failure occurs, code 42 will be set.
Crankshaft Position Sensor (CKP) (1996 models)
This sensor is located behind the crankshaft pully, and mounted on the oil pan.  As the crank rotates, and the toothed
timing belt pully passes the sensor tip, and AC voltage pulse is sent to the ECM.  The ECM evaluates the crank's rotation
speed from this signal, using the information to determine misfires.

Emissions Maint. Reminder Light (Check Engine light)
On 1989 and latermodels, a sensor light will illuminate every 50,000 - 80,000 - and 100,000 miles. This will occur with a warm engine at 1500 - 2000 RPM.  This indicates the ECM is in good condition, and the exygen sensor needs serviced.
Once checked by an authorized service center, locate the red, white, and blue 3-wire "sensor" switch attached to the steering
column bracket.  Slide the switch to the opposite to reset the light.  Test drive the vehicle to be sure the light does not flash.