Download Making Sense Out of Sensors

Making Sense
Out of Sensors
Jim Halderman
Dayton, Ohio
We started communicating
by writing on the cave walls.
As the years went by we
progressed. We invented tools
to make communication better.
Communication through Sensors
The PCM uses sensors to learn what is
happening in the engine.
Sensor Basics
• How does a computer “know” what is
going on under the hood?
• How does the voltage change inside
the PCM if changes occur at the other
end?
Pull Up
Resistors
• The voltage
changes at the
sense terminal
inside the
PCM after the
resistor.
Pull Down
Resistor
• The voltage
changes if the
switch is open or
closed.
Resistor Network
• Resistances
can be used
to simplify
inputs.
• Ford Manual
Lever
Position
(MLP) is an
example.
What would happen if some resistance
were added to the circuit?
If the resistance increases, what will
happen if you are driving at highway
speeds in drive?
Temperature Sensors
• Use a semiconductor material that becomes more
conductive as the temperature increases.
• This is called a Negative Temperature Coefficient
(NTC) thermistor.
Temperature Sensors
PCM
• What will the
PCM read if
unplugged?
• What will the
PCM read if
the wire going
to the sensor
is shorted to
ground?
ECT
• ECT should
read the
same at IAT
at key on
engine off
(KOEO).
Quick and Easy Metric
Temperature Conversion
• Double the Celsius degree number
• Add 25
• Should be close to the Fahrenheit
temperature
• Example: 50 degrees X 2= 100+25=125
• Actual= 122
A GM Stepped ECT
Switches at 1.0 volt (120°F).
ECT Sensor
• Is the sensor used in
a stepped ECT circuit
different from the
sensor used in a onestep circuit?
• How many wires are
used in a two-step
ECT sensor?
ECT Sensor Authority
• The ECT sensor is a high-authority sensor
especially at engine start
• Helps to determine the base pulse-width
• Can account for up to 60% of the pulsewidth determining factors
Intake Air Temperature (IAT)
• Similar to the ECT
sensor except it has
opening for the
airflow.
• Is used by the PCM to
modify the fuel and
spark timing program
based on the
temperature of the air
entering the engine
IAT Sensor Authority
• The IAT is usually considered to be a lowauthority sensor
• However it is usually capable of causing
the PCM to add up to 20% to the injector
pulse-width if the incoming air is cold
• The IAT can cause the PCM to reduce the
injector pulse-width by as much as 20% if
very hot air in entering the engine.
High-Performance
IAT Sensor Trick
• If a 10 K Ohm 1/2 watt resistor is used in
the place of the sensor, the PCM will
“assume” that the air temperature is about
freezing (32 degrees)
• This will cause the PCM to advance the
timing compared to if the air temperature
was warmer (4 to 8 degrees).
• Will increase the pulse width up to 20%.
Throttle Position Sensors
• Three-wire
potentiometer
• Five volts
from PCM
• Signal return
• Ground
Testing the TP sensor
• Great location to check for five-volt
reference (Vref).
• General Motors recommends checking
for PCM ground voltage drop at the TP
sensor. (There should be less than
0.035 volts between the TP sensor
ground terminal and the negative
terminal of the battery.)
TP Sensor Waveform
(Defective)
TP Sensor Authority
• The TP sensor can cause the PCM to
command up to 500% (5 times) the base
pulse width if the accelerator is depressed
rapidly to the floor
• Can cause the PCM to reduce the pulse
width by up to 70% if the throttle is rapidly
closed
MAP versus Vacuum
MAP Sensor
• Manifold Absolute Pressure (MAP)
• A decrease in manifold vacuum
means an increase in manifold
pressure.
• Compares manifold vacuum to a
perfect vacuum.
Silicon-Diaphragm Strain Gauge
Design MAP Sensor
• Most commonly used.
• Silicon wafer is exposed to engine vacuum.
• This results in changes in resistance due to
strain on the resistors attached to the wafer
(called Piezo-resistivity).
• Resistors are connected to a Wheatstone
bridge and then to a differential amplifier,
which creates a voltage in proportion to the
vacuum applied.
Silicon-Diaphragm MAP Sensor
MAP Voltage
• Normal engine Vacuum is 17-21 in. Hg.
• MAP sensor voltage is normally between 0.88
volts to 1.62 volts (GM).
• 17 in. Hg. is equal to about 1.62 volts.
• 21 in. Hg. is equal to about 0.88 volts.
• Therefore, a good reading should be about 1
volt.
Capacitor-Capsule MAP Sensor
• Used by Ford.
• Uses two alumina plates with an insulating
washer spacer to create a capacitor.
• The deflection due to engine vacuum
changes the capacitance.
• The electronics in the sensor translate this
into a frequency output.
Ford Frequency versus Vacuum
• KOEO…………. 156-159 Hz (0 in. Hg.)
• Idle (sea level).. 102-109 Hz (17-21 in.
Hg.)
• WOT……………. 156-159 Hz (Almost 0
in. Hg.)
Ceramic Disc MAP
• Used by DaimlerChrysler.
• Ceramic disc converts manifold
pressure into a capacitive discharge.
• The discharge controls the amount of
voltage drop delivered by the sensor to
the PCM.
• The output is the same as the
previously used strain
gauge/Wheatstone bridge design.
Ceramic Disc MAP
MAP versus BARO
• KOEO MAP should equal BARO.
• Will vary with altitude and weather
conditions.
• The BARO reading is set at key on and
updated if the throttle is detected to be
at WOT and will update the BARO
reading.
Testing a MAP Sensor
• Key on – engine off (KOEO).
Voltage should be 4.6 to 4.8
volts at sea level.
• Check for vacuum to the
sensor.
• Check the hose.
• Replace the MAP sensor if
anything comes out of the
sensor.
MAP Sensor Authority
• The MAP sensor is a high-authority
sensor on an engine that uses the
Speed-Density method of fuel control.
• If the exhaust is rich, try disconnecting
the MAP sensor.
• If the engine now runs OK, then the
MAP sensor is skewed or giving the
PCM wrong information.
High-Performance MAP Sensor
Trick
• Insert a plastic vacuum fitting into the
vacuum line to the MAP sensor
• Use a hot straight pin and burn a small
hole in the plastic fitting creating a small
vacuum leak
• Do not exceed 0.020 inch hole
• PCM “assumes” a higher engine load and
increases the injector pulse-width
Air Vane Sensor
• Usually contains an internal IAT sensor
• Works similar to a TP sensor where the air
vane is used to move a potentiometer
• Airflow moves the vane, which causes a
switch to close to power the fuel pump.
Air Vane Sensor
This is not a mass air flow sensor.
Karman-Vortex
• Named for Theodore Van Karman, a
Hungarian scientist (1881 – 1963).
• He observed the vortex phenomenon in
1912.
• This type of sensor has proven to be
very reliable and not subject to dirt.
Karman-Vortex
This is not a mass air flow sensor.
Ultrasonic Karman Vortex
• Used by Mitsubishi in many vehicles.
• Very reliable.
• Early versions used LEDs and
phototransistors, which were subject
to dirt.
Pressure-Type Karman Vortex
• DaimlerChrysler uses a Karman Vortex
sensor that uses a pressure sensor to
detect the vortexes.
• As the flow increases, so do the
number of pressure variations.
• The electronic circuitry in the sensor
converts these pressure variations to a
square wave signal that is proportional
to the airflow through the sensor.
Mass Air Flow
• A hot wire is used to measure the mass of the
air entering the engine.
• The electronics, in the sensor itself, try to
keep the wire 70° C above the temperature of
the incoming air.
• The more current (amperes) needed to heat
the wire, the greater the mass of air.
• The current is converted to a frequency.
MAF Sensor
Normal MAF Readings
• Use a scan tool to look at the grams per
second.
• Warm the engine at idle speed with all
accessories off. Should read 3 to 7 grams per
second.
• GM 3800 V-6 should read 2.37 to 2.52 KHz.
• If not within this range, check for false air or
contamination of the sensor wire.
MAF Sensor Diagnosis
• If the MAF sensor wire were to become coated,
it cannot measure all of the incoming air.
• A normal warm engine at idle should be 3 to 7
grams per second.
• Rapidly depress the accelerator pedal to WOT.
It should read over:
• 100 grams per second or
• higher than 7 kHz or
• 4 volts
MAF and Altitude Reading
Barometric pressure (BARO) is
determined by the Powertrain Control
Module (PCM) software at WOT.
At high airflows, a contaminated MAF
sensor will under estimate airflow
coming into the engine, and therefore,
the PCM determines that the vehicle is
operating at a higher altitude.
Visual Inspection
• Look for a very dirty
filter.
• Look for a K&N filter
that has been overoiled.
• Look for fuzz on the
sensing wire from
fibers coming off of the
filter paper.
Fuzzy MAF
Cleaning a MAF Sensor
• Can clean the
sensing wire using
alcohol and a Q-tip,
if you are careful.
• Brake clean ?
• Contact Cleaner ?
• Cable Tie ?
False Air
Mass Air Flow (MAF)-False Air
Usually affects operation in drive; may run OK if driving in reverse.
MAF Sensor Authority
• High authority sensor
• If in Doubt-Take it Out
• If the MAF sensor is disconnected, the PCM
substitutes a backup value.
• If the engine runs OK with the MAF
disconnected, then the MAF has been
supplying incorrect information.
Oxygen Sensors
• Oxygen sensors react to the presence or
absence of oxygen in the exhaust.
• The voltage signal is used by the PCM
for fuel control.
• The mixture must switch from rich to
lean for the three-way catalytic (TWC)
converter to work.
O2 Sensor
Oxygen Sensors
• An absence of
oxygen results
in a voltage of
higher than 450
mV.
• The presence of
oxygen results
in a voltage
lower than 450
mV.
Mud-Coated O2S
Do Not Solder O2S Wires
• Universal oxygen sensors are often
used when replacement becomes
necessary.
• Many O2S “breathe” through the wiring
itself.
• If the wires are soldered, then the
sensor cannot detect outside oxygen.
• Use crimp and seal connectors if a
universal sensor is used.
Crimp and Seal Connectors
Oxygen Sensor Diagnosis
• If the fuel system is functioning correctly, the
oxygen sensor voltage should fluctuate from
above 800 mV to below 200 mV.
• As the oxygen sensor degrades, the range
narrows.
• A sensor should at least be capable of
reading above 600 mV and go below 300 mV.
O2 Sensor Diagnosis (Continued)
• Use a digital multimeter (DMM) set to read
DC volts and use the min/max feature.
• Back probe the signal wire and operate the
engine normally while recording the readings
using min/max.
• The maximum reading should be above 800
mV and the minimum reading below 200 mV.
O2 Sensor Diagnosis (Continued)
• Check the average of the O2 sensor
readings:
– If the average is above 450 mV, then the
engine is operating rich or the sensor is
skewed high.
– If the average is below 450 mV, then the
engine is operating lean or the sensor is
skewed low.
Rich to Lean
Should switch in less than 100 ms.
False Lean Readings
False Lean Readings
• A cracked exhaust manifold can cause
oxygen to be drawn into the exhaust
upstream from the oxygen sensor.
• An ignition misfire can also cause a
false low-oxygen sensor reading.
• Remember that the oxygen sensor
looks at the oxygen in the exhaust, not
the unburned fuel!
Antifreeze Contamination
• If the engine has had a blown head
gasket, be sure to check or replace the
oxygen sensor.
• The silicates can coat the sensor.
• Dexcool and other organic acid
technology (OAT) coolants do not
cause this problem.
Antifreeze on an O2 Sensor
Oxygen Sensor Authority
• The O2S is a high-authority sensor when the
engine is operating in closed loop.
• Can add or subtract up to 25% from the base
pulse width (Some vehicles even more)
• If the sensor is skewed, it can create a
driveability problem.
• If in doubt, take it out.
• If the sensor voltage is not connected, the
PCM will go into open loop.
High/Low Authority Sensors
• High-Authority
• Low-Authority
• ECT –especially when
the engine starts and is
warming up.
• O2S-while the engine is
operating in closed loop
• MAF
• TP
•
•
•
•
IAT
TFT
PRNDL
KS
Position Sensors
• Two Types:
–Analog sensors - such as magnetic or
variable-magnetic sensors
–Digital sensors - such as Hall-Effect
magnetic-resistive or optical sensors.
The first magnetic sensors were called
pulse generators (pickup coils).
Magnetic
Sensor
Operation
Magnetic Sensors
• Used for Crankshaft
Position (CKP)
• Used for Camshaft
Position (CMP)
• Used for wheel
speed sensors
(WSS)
Magnetic Sensors
• First,
be sure
they are
magnetic.
• A cracked
magnet
becomes
two weak
magnets.
Magnetic Sensor
• The
sensor
housing
should
not be
cracked
or melted.
Ford Probe Story (2.5 liter V-6)
• Problem occurred at highway speed.
• Stopped running.
• Was towed to shop.
• Runs, but at idle only.
• Dies as soon as accelerator is depressed.
• Everything checks out OK.
Magnetic Sensor
Found a
loose
CKP
sensor.
Magnetic Sensors
• Produce
an AC
voltage
signal
when the
magnetic
field
strength
changes.
Crankshaft Position (CKP) Sensor
Camshaft Position (CMP) Sensor
• Can be magnetic
• Look at the sequence
to determine
application
• Notice that this is a
2,3,2 sequence
sensor wheel.
Wheel Speed Sensor
The Soldering Gun Trick
• Hold a soldering gun near a magnetic
sensor and the changing magnetic field
around the soldering gun will induce a
voltage into the windings of the
magnetic sensor,
• The frequency will be 60 Hz.
• If used on a VSS and the ignition is on,
the speedometer will read 54 mph (GM).
Hall-Effect Sensors
• Effect discovered by Edwin H. Hall in 1879.
• He discovered that a voltage is created if a
magnetic field is exposed to an element.
• The voltage goes to zero if the magnetic field
is shunted or blocked.
• Very accurate.
Hall-Effect
• Three-wire sensor
– Power
– Ground
– Signal
• Output is a square wave.
• Very accurate and will work at lower
speeds than a magnetic sensor.
Hall-Effect
Hall-Effect
Optical Sensor
Optical Sensor
• Produces a square wave signal.
• Very accurate – can be used to show every
1 degree of crankshaft rotation.
• Optical sensors do not like light; must be
shielded (sparks inside the distributor can
cause problems if the sensor is not
shielded).
Optical Sensor
Magnetic-Resistive Sensors
• Generates a
digital signal
using two
magnets.
• Electronics in
the sensor
generate a
square-wave
output signal.
CKP and CMP Waveforms
What type of sensor is used for the CKP?
What type of sensor is used for the CMP?
Toyota Wheel Speed Sensor
• Can
detect
forward
or reverse
motion.
Questions?
Jim Halderman
[email protected]