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 one-step 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 pulse-width 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 low-authority 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 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• ECT –especially when

the engine starts and is warming up.

• O2S-while the engine is operating in closed loop

• MAF• TP

• Low-Authority• 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

jim@jameshalderman.com