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Where Is Sinclair?• Dayton, Ohio.
– 142 miles north of Lexington, Ky.
– 116 miles east of Indianapolis, In.
– 75 miles west of Columbus, Ohio.
– 142 miles south of Toledo, Ohio.
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?
Resistor Network
• Resistances can be used to simplify inputs.
• Ford Manual Lever
Position (MLP) is an example.
Temperature Sensors
• Use a semiconductor material that becomes more conductive as the temperature increases.
• This is called a Negative Temperature Coefficient (NTC) thermistor.
PCM
• What will the PCM read if unplugged?
• What will the PCM read if the wire going to the sensor is shorted to 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.)
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.
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.
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.
Speed Density MAP
• 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.
Air Vane Sensor
• Usually contains an internal IAT sensor.
• Airflow moves the vane, which causes a switch to close to power the fuel pump.
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.
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.
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.
• To check for this condition, use a scan tool.
• 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.
MAF and Altitude Reading
Barometric pressure (BARO) is determined by the Powertrain Control Module (PCM) software at WOT. (There is no actual BARO sensor on MAF-equipped vehicles.) 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. The BARO reading is stored in Keep Alive Memory (KAM) after it is updated.
Visual Inspection
• Look for an off-brand air filter or a very dirty filter.
• Look for a K&N filter that has been over-oiled.
• Look carefully for “fuzz” on the sensing wire.
Cleaning a MAF Sensor
• Can clean the sensing wire using alcohol and a Q-tip, if you are careful.
• Brake clean = ?
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.
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.
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 connectors if a universal sensor is used.
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.
CAT Food
• In order for the catalytic converter to function correctly, it must have an alternating rich and lean exhaust.
• If the exhaust is
lean for too long,
the CAT dies due
to lack of fuel.
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.
High Authority Sensor
• The O2S is a high-authority sensor when the engine is operating in closed loop.
• 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.
Position Sensors
• Two Types:
–Analog sensors - such as magnetic or variable-magnetic sensors
–Digital sensors - such as Hall-Effect magnetic-resistive or optical sensors.
Magnetic Sensors
• Used for Crankshaft Position (CKP)
• Used for Camshaft Position (CMP)
• Used for wheel speed sensors (WSS)
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.
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.
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).
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?
That’s All Folks!
For a copy of this presentation, go to http://storage.jameshalderman.com
Jim HaldermanSinclair Community College
444 West Third StreetDayton, OH 45402
(937) [email protected]