All sensors are important. The computer is the brains of a
computerized engine control system and sensors are its link to
what's happening under the hood.
Some sensors have more influence on engine performance than others.
These include the coolant temperature sensor, oxygen sensor,
throttle position sensor, and manifold absolute pressure sensor.
The coolant sensor is often called the master sensor because the
computer uses its input to regulate many other functions, including:
- Activating and deactivating the Early Fuel Evaporation (EFE)
system such as the electric heating grid under carburetor or the
thermactor air system.
- Open/closed loop feedback control of the air/fuel mixture. The
system won't go into closed loop until the engine is warm.
- Start up fuel enrichment on fuel-injected engines, which the
computer varies according to whether the engine is warm or cold.
- Spark advance and retard. Spark advance is often limited until the
engine reaches normal operating temperature.
- EGR flow, which is blocked while the engine is cold to improve
driveability.
- Canister purge, which does not occur until the engine is warm.
- Throttle kicker or idle speed.
- Transmission torque converter clutch lockup.
The coolant sensor is usually located on the head or intake manifold
where it screws into the water jacket. Sensors come in two basic
varieties: variable resistor sensors called thermistors because
their electrical resistance changes with temperature, and on/off
switches, which work like a conventional temperature sending unit or
electric cooling fan thermostat by closing or opening at a preset
temperature.
Variable resistor coolant sensors provide the computer with a more
accurate indication of actual engine temperature than a simple
temperature switch. The computer feeds the sensor a fixed reference
voltage of about five volts when the key is on.
The resistance in the sensor is high when cold and drops about 300
ohms for every degree Fahrenheit as the sensor warms up. This alters
the return voltage signal back to the computer which the computer
then reads to determine engine temperature.
The switch-type sensor may be designed to remain closed within a
certain temperature range, or to open only when the engine is warm.
Switch-type coolant sensors can be found on GM "T" car minimum
function systems, Ford MCU, and Chrysler Lean Burn systems.
Because of the coolant sensor's central role in triggering many
engine functions, a faulty sensor (or sensor circuit) can cause a
variety of cold performance problems. The most common symptom is
failure of the system to go into closed loop once the engine is
warm. Other symptoms include poor cold idle, stalling, cold
hesitation or stumble, and/or poor fuel mileage.
The oxygen sensor (O2) measures how much unburned oxygen is in the
exhaust. The computer uses this as an indication of how rich or lean
the fuel mixture is so adjustments can be made to keep it properly
balanced.
A problem with the O2 sensor will prevent the computer from keeping
the fuel mixture balanced under changing driving conditions,
allowing the mixture to run rich or lean.
The throttle position sensor (TPS) is used with feedback carburetion
and electronic fuel injection (EFI) to inform the computer about the
rate of throttle opening and relative throttle position. A separate
idle switch and/or wide open throttle (WOT) switch may also be used
to signal the computer when these throttle positions exist.
The throttle position sensor may be mounted externally on the
throttle shaft (the case on most fuel injection throttle bodies), or
internally in the carburetor (as in Rochester Varajet, Dualjet and
Quadrajet).
The TPS is essentially a variable resistor that changes resistance
as the throttle opens. It is the electronic equivalent of a
mechanical accelerator pump. By signaling the computer when the
throttle opens, the computer enriches the fuel mixture to maintain
proper air/fuel ratio.
Initial TPS setting is critical because the voltage signal the
computer receives tells it the exact position of the throttle.
Initial adjustment must be set as close as possible to factory
specs. Most specs are given to the nearest hundredth of a volt.
The classic symptom of a defective or misadjusted TPS is hesitation
or stumble during acceleration. The fuel mixture leans out because
the computer doesn't receive the right signal telling it to add fuel
as the throttle opens. The oxygen sensor feedback circuit will
eventually provide the necessary information, but not quickly enough
to prevent the engine from stumbling.
When the sensor is replaced, it must be adjusted to the specified
reference voltage. The TPS on most remanufactured carburetors is
preset at the factory to an average setting for the majority of
applications the carburetor fits. Even so, the TPS should be reset
to the specific application upon which it is installed.
MAP sensor function is to sense air pressure or vacuum in the intake
manifold. The computer uses this input as an indication of engine
load when adjusting air/fuel mixture and spark timing. Computerized
engine control systems that do not use a MAP sensor rely on throttle
position and air sensor input to determine engine load.
Under low-load, high-vacuum conditions, the computer leans the fuel
mixture and advances spark timing for better fuel economy. Under
high-load, low-vacuum conditions (turbo boost, for example), the
computer enriches the fuel mixture and retards timing to prevent
detonation.
The MAP sensor serves as the electronic equivalent of both a
distributor vacuum advance diaphragm and a carburetor power valve.
The MAP sensor reads vacuum and pressure through a hose connected to
the intake manifold. A pressure sensitive ceramic or silicon element
and electronic circuit in the sensor generates a voltage signal that
changes in direct proportion to pressure.
MAP sensors should not be confused with VAC (Vacuum) sensors, DPS
(Differential Pressure sensors), or BARO or BP (Barometric Pressure)
sensors. A vacuum sensor (same as a differential pressure sensor)
reads the difference between manifold vacuum and atmospheric
pressure (the difference in air pressure above and below the
throttle plate). A VAC sensor is sometimes used instead of a MAP
sensor to sense engine load.
A MAP sensor measures manifold air pressure against a precalibrated
absolute (reference) pressure. What's the difference? A vacuum
sensor only reads the difference in pressure, not absolute pressure,
so it doesn't take into account changes in barometric (atmospheric)
pressure.
A separate BARO sensor is usually needed with a vacuum sensor to
compensate for changes in altitude and barometric pressure. Some
early Ford EEC-III and EEC-IV systems have a combination barometric
pressure/MAP sensor called a BMAP sensor, combining both functions.
Anything interfering with accurate sensor input can upset both fuel
mixture and ignition timing. Problems with the MAP sensor itself,
grounds or opens in the sensor wiring circuit, and/or vacuum leaks
in the intake manifold.
Typical driveability symptoms include detonation due to too much
spark advance and a lean fuel ratio, and loss of power and/or fuel
economy due to retarded timing and an excessively rich fuel ratio.
A vacuum leak can cause a MAP sensor to indicate low manifold
vacuum, causing the computer to think the engine is under more load
than it really is. Consequently, timing is retarded and the fuel
mixture is enriched.
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