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Discussion Starter · #1 ·
<H2 ="western">This is a guide to help you reach a logical conclusion on whether a very expensive sensor is indeed defective, rather than changing them willy nilly in the hope of fixing your problems.</H2>This information is freely available on the net, and was compiled by a very generous and knowledgable gentleman. I do hope this is useful for someone out there.<DIV dir="LTR" id="postlist">
All you need to know about sensors

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  1. <BLOCKQUOTE>COOLANT SENSOR. Usually located on the cylinder
    head or intake manifold, this sensor is used to monitor the
    temperature of the engine's coolant. Its resistance changes in
    proportion to coolant temperature. Input from the coolant sensor
    tells the computer when the engine is warm so the PCM can go into
    closed loop feedback fuel control and handle other emission
    functions (EGR, canister purge, etc.) that may be temperature

    Coolant Sensor Strategies: The coolant
    sensor is a pretty reliable sensor, but if it fails it can prevent
    the engine control system from going into closed loop. This will
    result in a rich fuel mixture, excessive fuel consumption and
    elevated carbon monoxide (CO) emissions - which may cause the
    vehicle to fail an emissions test.

    A bad sensor can be
    diagnosed by measuring its resistance and watching for a change as
    the engine warms up. No change, or an open or closed reading would
    indicate a bad sensor.

    OXYGEN (O2) SENSOR. Used on
    both carbureted and fuel injected engines since 1981, the oxygen
    (O2) sensor is the key sensor in the fuel mixture feedback control

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    <A name="post_message_1575427"></A>Mounted in the
    exhaust manifold, the O2 sensor monitors the amount of unburned
    oxygen in the exhaust. On many V6 and V8 engines, there are two
    such sensors (one for each bank of cylinders).

    The O2
    sensor generates a voltage signal that is proportional to the
    amount of unburned oxygen in the exhaust. When the fuel mixture is
    rich, most of the oxygen is consumed during combustion so there is
    little unburned oxygen in the exhaust. The difference in oxygen
    levels between the exhaust inside the manifold and the air outside
    creates an electrical potential across the sensor's platinum and
    zirconium tip. This causes the sensor to generate a voltage
    signal. The sensor's output is high (up to 0.9v) when the fuel
    mixture is rich (low oxygen), and low (down to 0.1v) when the
    mixture is lean (high oxygen).

    The sensor's output is
    monitored by the computer and is used to rebalance the fuel
    mixture for lowest emissions. When the sensor reads "lean"
    the PCM increases the on-time of the injectors to make the fuel
    mixture go rich. Conversely, when the sensor reads "rich"
    the PCM shortens the on-time of the injectors to make the fuel
    mixture go lean. This causes a rapid back-and-forth switching from
    rich to lean and back again as the engine is running. These even
    waves result in an "average" mixture that is almost
    perfectly balanced for clean combustion. The switching rate is
    slowest in older feedback carburetors, faster is throttle body
    injection systems and fastest in multiport sequential fuel

    If the O2 sensor's output is monitored on an
    oscilloscope, it will produce a zigzagging line that dances back
    and forth from rich to lean. Take a look at the waveform on the
    opposite page - that's what a technician wants to see when he
    checks the O2 - think of it as a kind of heart monitor for the
    engine's air/fuel mixture.

    O2 Sensor Strategies:
    Unheated one- or two-wire O2 sensors on 1976 through early 1990s
    applications should be replaced every 30,000 to 50,000 miles to
    assure reliable performance. Heated 3 and 4-wire O2 sensors on
    mid-1980s through mid-1990s applications should be changed every
    60,000 miles. On OBD II equipped vehicles, the recommended
    replacement interval is 100,000 miles. The O2 sensor's

    responsiveness and voltage output can diminish with age and
    exposure to certain contaminants in the exhaust such as lead,
    sulfur, silicone (coolant leaks) and phosphorus (oil burning). If
    the sensor becomes contaminated, it may not respond very quickly
    to changes in the air/fuel mixture causing a lag in the PCM's
    ability to control the air/fuel mixture.

    The sensor's
    voltage output may decline giving a lower than normal reading.
    This may cause the PCM to react as if the fuel mixture were leaner
    than it really is resulting in an overly rich fuel mixture.

    common is this problem? One EPA study found that 70 percent of the
    vehicles that failed an I/M 240 emissions test needed a new O2

    sensor is mounted on or connected to the intake manifold to
    monitor intake vacuum. It changes voltage or frequency as manifold
    pressure changes. The computer uses this information to measure
    engine load so ignition timing can be advanced and retarded as
    needed. It performs essentially the same job as the vacuum advance
    diaphragm on an old fashioned mechanical distributor.

    engines with a "speed density" type of fuel injection,
    the MAP sensor also helps the PCM estimate airflow. Problems here
    may cause an intermittent check engine light (light comes on when
    accelerating or when the engine is under load), hesitation when
    accelerating, elevated emissions and poor engine performance. The
    engine will run with a bad MAP sensor, but it will run poorly.
    Some PCMs can substitute "estimated data" for a missing
    or out of range MAP signal, but engine performance will be
    drastically reduced.</BLOCKQUOTE>

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  1. <BLOCKQUOTE><A name="post_message_15754271"></A>MAP Sensor
    Some MAP sensor problems are not the fault of the
    sensor itself. If the vacuum hose that connects the MAP sensor to
    the intake manifold is loose, leaking or plugged, the sensor can't
    produce an accurate signal. Also, if there is a problem within the
    engine itself that causes intake vacuum to be lower than normal
    (such as a vacuum leak, EGR valve that's stuck open or leaky PCV
    hose), the MAP sensor's readings may be lower than

    throttle shaft of the carburetor or throttle body, the throttle
    position sensor (TPS) changes resistance as the throttle opens and
    closes. The computer uses this information to monitor engine load,
    acceleration, deceleration and when the engine is at idle or wide
    open throttle. The sensor's signal is used by the PCM to enrich
    the fuel mixture during acceleration, and to retard and advance
    ignition timing.

    Throttle Position Sensor Strategies:
    Many TPS sensors require an initial voltage adjustment when
    installed. This adjustment is critical for accurate operation. On
    some engines, a separate idle switch and/or wide open throttle
    (WOT) switch may also be used. Driveability symptoms due to a bad
    TPS can be similar to those caused by a bad MAP sensor: The engine
    will run without this input, but it will run poorly.

    Mounted ahead of the throttle body on
    multiport fuel injected engines, this sensor monitors the volume
    of air entering the engine. The sensor uses either a hot wire or
    heated filament to measure both airflow and air density.

    Sensor Strategies:
    The sensing element in MAF sensors can be
    easily contaminated causing hard starting, rough idle, hesitation
    and stalling problems.

    VAF has a mechanical flap-style sensor that is used on Bosch and
    other import multiport fuel injected engines. The function is the
    same as a mass airflow sensor, but air pushing against a
    spring-loaded flap moves a rheostat to generate an electronic

    VAF Sensor Strategies: The drivability symptoms
    for the VAF are the same as those of a mass airflow sensor if the
    sensor fails.

    Mounted on the intake manifold, this sensor changes resistance
    to monitor incoming air temperature. The sensor's input is used
    to adjust the fuel mixture for changes in air density.

    Sensor Strategies: Problems with the manifold air temp sensor can
    affect the air/fuel mixture, causing the engine to run rich or

    with distributorless ignition systems, the crankshaft position
    sensor serves essentially the same purpose as the ignition pickup
    and trigger wheel in an electronic distributor. It generates a
    signal that the PCM needs to determine the position of the
    crankshaft and the number-one cylinder. This information is
    necessary to control ignition timing and the operation of the fuel
    injectors. The signal from the crank sensor also tells the PCM how
    fast the engine is running (engine rpm) so ignition timing can be
    advanced or retarded as needed. On some engines, a separate
    camshaft position sensor is also used to help the PCM determine
    the correct firing order. The engine will not run without this
    sensor's input.

    There are two basic types of crankshaft
    position sensors: magnetic and Hall effect. The magnetic type uses
    a magnet to sense notches in the crankshaft or harmonic balancer.
    As the notch passes underneath, it causes a change in the magnetic
    field that produces an alternating current signal.</BLOCKQUOTE>

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  1. <BLOCKQUOTE><A name="post_message_15754272"></A>The frequency of
    the signal gives the PCM the information it needs to control
    timing. The Hall effect type of crank sensor uses notches or
    shutter blades on the crank, cam gear or balancer to disrupt a
    magnetic field in the Hall effect sensor window. This causes the
    sensor to switch on and off, producing a digital signal that the
    PCM reads to determine crank position and speed.

    Position Sensor Strategies:
    If a crank position sensor fails,
    the engine will die. The engine may, however, still crank but it
    won't start. Most problems can be traced to faults in the
    sensor's wiring harness. A disruption of the sensor supply
    voltage (Hall effect types), ground or return circuits can cause a
    loss of the all-important timing signal.

    The knock sensor detects engine vibrations that indicate
    detonation is occurring so the computer can momentarily retard
    timing. Some engines have two knock sensors.

    Sensor Strategies:
    A failure with the knock sensor can cause
    spark knock and engine damaging detonation because the PCM won't
    know to retard ignition timing if knock is occurring.

    The baro sensor measures barometric
    pressure so the computer can compensate for changes in altitude
    and/or barometric pressure that would affect the fuel mixture or
    timing. Some MAP sensors also perform this function.

    The vehicle speed sensor, or VSS, monitors
    vehicle speed so the computer can regulate torque converter clutch
    lockup, shifting, etc. The sensor may be located on the
    transmission, differential, transaxle or speedometer head.

    Vehicle Speed Sensor Strategies: A problem with the
    vehicle speed sensor can disable the cruise-control system as well
    as affect transmission shifting and converter

    Replacing a sensor won't solve a drivability
    or emissions problem if the problem isn't the sensor. Common
    conditions such as fouled spark plugs, bad plug wires, a weak
    ignition coil, a leaky EGR valve, vacuum leaks, low compression,
    dirty injectors, low fuel pressure or even low charging voltage
    can all cause driveability symptoms that may be blamed on a bad
    sensor. If there's are no sensor-specific fault codes, these
    kinds of possibilities should be ruled out before much time is
    spent on electronic diagnosis.

61 Posts
Discussion Starter · #2 ·
Read the above if you are:
In distress about sensors

At your wits end

Your car is

You are

Or you are
to ask a question.

Just thought I should bring this list up the queue a bit for new members to find this info. a bit easier than groping around the murky depths of November last year.
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