Making sense of sensors

sam58 · Nov 14, 2010

<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

<DIV dir="LTR" id="post_message_1575427">

<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
dependent.

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
loop.</BLOCKQUOTE>

<BLOCKQUOTE>

<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
injection.

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.

How
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.

MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR. This
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.

On
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>

<BLOCKQUOTE><A name="post_message_15754271"></A>MAP Sensor
Strategies: 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
normal.

THROTTLE POSITION SENSOR. Mounted on the
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.

MASS
AIRFLOW SENSOR (MAF). 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.

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

VANE AIRFLOW SENSOR (VAF). The
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
signal.

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

MANIFOLD AIR TEMPERATURE (MAT) SENSOR.
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.

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

CRANKSHAFT POSITION SENSOR. Used on engines
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>

<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.

Crank
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.

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

Knock
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.

BAROMETRIC
PRESSURE (BARO) SENSOR. 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.

VEHICLE
SPEED SENSOR (VSS). 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
engagement.

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.
</BLOCKQUOTE>

sam58 · Feb 12, 2011

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.

Making sense of sensors

sam58

Registered

sam58

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