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Making Sense Out of Sensors Jim Halderman Sinclair Community College Dayton, Ohio

Making Sense Out of Sensors Jim Halderman Sinclair Community College Dayton, Ohio

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Making Sense Out of Sensors

Jim Halderman

Sinclair Community College

Dayton, Ohio

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.

Virtual Tour of the Automotive Technology Center

Main Lobby

Offices and Classrooms on the Second Floor

Student Lounge Area

Automotive Department Computer Lab

Seminar Room

GM Training Center

Main Vehicle Lab

Engine Lab

High-Performance Engine Course

Donated Engines and Transmissions

Automatic Transmission Lab

Manual Transmission Lab

Fuel and Emissions Lab

Engine Dyno

Chassis Dyno

Race Vehicle Fabrication

We started communicating by writing on the cave walls.

As the years went by we progressed. We invented tools to make communication better.

The chalk board was an effective tool for many years for communications.

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).

A GM Stepped ECT

Switches at 1.0 volt (120°F).

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)

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.

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.

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.

• 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 = ?

Mass Air Flow (MAF)-False Air

Usually affects operation in drive; may run OK if driving in reverse.

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 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.

Rich to Lean

Should switch in less than 100 ms.

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.

Rich Mixture

• A rich exhaust all the time will also kill the CAT.

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

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.

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.

Magnetic Sensor (Continued)

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?

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]

[email protected]