ENGINE PERFORMANCE II

LESSON 6

AUTOMOTIVE SENSORS Part 1

WHAT IS THE FUNCTION OF A SENSOR?

To send parameter data to the computer to allow for precise control of

output functions.

WHAT INFORMATION DOES A POWER TRAIN COMPUTER NEED?

Engine temp., intake air temp., throttle position, intake manifold pressure, barometric pressure, exhaust oxygen, engine speed,

crankshaft position, camshaft position, engine load, vehicle speed…

Most sensors determine speed & position, pressure or temperature. Exceptions are the airflow sensor and exhaust oxygen sensor.

SENSOR DEVICES

Temperature sensors - a thermistor whose resistance changes as

temperature changes. 2-wire sensor

Types: PTC - Positive temp. coefficient Temp. Up >> Resistance Up

NTC - Negative temp. coefficient Temp. Up >> resistance Down

All automotive temp. sensors are NTC thermistors!

ECT - Engine Coolant Temp. IAT - Intake Air Temp.

Engine coolant and intake air temps. affect: A/F ratio, spark timing, idle speed, TCC lock-up, cooling fan, emission control devices

TEMP. SENSOR CIRCUITS

A switching temp. sensor circuit increases the sensitivity of the sensor

at temps. above 122 degrees F.

Throttle position sensor (TPS or TP) - a potentiometer that sends data about throttle angle to the PCM.

3-wire sensor

Potentiometer - a variable resistor used to produce a difference in voltage

Throttle position data affects: A/F ratio, Spark timing, Idle speed, TCC,

A/C clutch, Emissions devices

SENSORS Part 2

Pressure sensors - pressure sensors use flexible diaphragms to measure changes in pressure.

A pressure sensor has 3 wires (reference, signal and ground) similar to a TPS circuit.

Basic pressure sensor types:

· Manifold Absolute Pressure (MAP)

· Barometric Pressure (BARO)

MAP and BARO sensor output increases as sensed pressure increases.

Most pressure sensors produce an analog signal similar to a TPS.

Ford MAP sensors produce a digital frequency signal.

Most fuel injected engines do not use a BARO sensor, the MAP reads barometric pressure as the ignition is turned on before the engine is cranked. Barometric pressure data is updated during WOT.

Engine VACUUM - PRESSURE RELATIONSHIP

Engine Condition	Vacuum in in. Hg	Pressure in in. Hg	Pressure in kPa	Pressure in PSIA	MAP Output; V or Hz GM Ford
KOEO, WOT	0 in. Hg	30 in. Hg	101 kPa	~14.7 psia
	10 in. Hg	20 in. Hg	68 kPa	~10 psia
Idle	20 in. Hg	10 in. Hg	35 kPa	~ 5 psia
	30 in. Hg	0 in. Hg	0 kPa	0 psia

An engine idling has about 20” Hg of vacuum or 5 psia of pressure in its intake manifold. At WOT the vacuum is 0" Hg and the pressure is 14.7 psia.

Pressure sensors may also be used to monitor fuel, oil, transmission, A/C and exhaust pressures.

Mass Airflow Sensor (MAF) –

a sensor used to measure the mass of air entering the intake manifold in grams per second. Engine size in liters +2

The MAF is mounted between the air filter and the throttle plate.

A MAF heats an element with electrical current, as air passes around the element it cools it off. The rate of cooling of the element allows the MAF to determine how much air is entering the engine.

There are two basic element types: Hot Film and Hot Wire

GM MAF sensors produce a digital frequency signal, most other MAF’s produce an analog signal.

A vane airflow meter (VAF) is another device that is used to determine the amount of air entering the engine. Airflow through the meter opens a spring -loaded vane that turns a potentiometer wiper. The higher the airflow the higher the voltage output to the PCM.

The Karmann Vortex sensor is also a type of MAF. This sensor creates vortexes (mini tornados) in the airflow and counts them. As the airflow increases the number of vortexes increases.

SENSOR Part 3

Knock sensor (KS) or Detonation sensor - a sensor mounted to the block or cylinder head that uses a piezoelectric crystal to determine when spark knock is occurring. The PCM monitors the frequency of its signal to detect the knock

and retard the ignition timing.

Exhaust gas oxygen sensor (O2S)

Oxygen sensors are used for fuel delivery control during closed loop and to monitor catalytic converter efficiency. The O2S compares the amount of oxygen in the exhaust (0-2%) to the oxygen in the atmosphere (21%).

Types:

· Zirconium Dioxide O2S

It generates its own voltage between 0mV and 1000mV.

It must be at least 600^oF to operate. The voltage generated depends on the difference between exhaust oxygen and atmospheric oxygen. The greater the difference, the greater the voltage signal produced.

At 14.7:1 A/F ratio the O2S output voltage is 450mV.

A lean mixture leaves a lot of oxygen in the exhaust.

A lean mixture produces a voltage below 450mV.

A rich mixture leaves little oxygen in the exhaust.

A rich mixture produces a voltage above 450mV.

The greater the difference between the oxygen in the exhaust and the oxygen in the atmosphere, the higher the O2S output voltage.

O2S VOLTAGE OUTPUT CHART
A/F RATIO	EXHAUST O2 CONTENT	OUTPUT VOLTAGE
LEAN	LOTS LEFT OVER	0 – 450mV
14.7:1	JUST RIGHT	450mV
RICH	LITTLE LEFT OVER	450 -1000mV

O2S VOLTAGE OUTPUT CHART

A/F RATIO

EXHAUST O2 CONTENT

OUTPUT VOLTAGE

LEAN

LOTS LEFT OVER

0 – 450mV

14.7:1

JUST RIGHT

450mV

RICH

LITTLE LEFT OVER

450 -1000mV

An O2S that is operating properly will produce the full voltage range and switch from lean (300mV) to rich (600mV) and vice versa in less than 100ms.

An oscilloscope is the only tool that can accurately check an O2S!

· Titania O2S

It is a variable resistance sensor. It must be at least 600^oF to operate. As exhaust oxygen concentration changes, the sensor resistance changes. The sensor circuit is similar to a thermistor (temperature sensor) circuit and requires a 5-volt reference to produce an output voltage. Rich exhaust voltage is less than 2.5v, lean is above 2.5v.

· Linear, Wide Band or Air-Fuel Ratio Sensor

A wide band sensor can determine A/F ratio as lean as 23:1 in some cases. This allows the engine to run very clean and efficient. The sensor is similar to a double zirconium sensor, with atmospheric air in between. The PCM supplies a reference voltage to the sensor. The voltage “pumps” oxygen form one side of the sensor to the other. The PCM monitors the amount of current required to “pump” the oxygen. When monitoring the sensor output, the voltage on the reference wire does not change, only the current.

An O2S can be poisoned by; leaded fuel, anti-freeze, vapors from RTV sealants and long exposure to a rich mixture.

An O2S can be checked off the car by heating it with a propane torch and watching its voltage output and reaction time. Re-install with anti-seize.

Vehicle Speed Sensor - provides data about vehicle speed and distance traveled. Basic types used are;

Optical - an LED reflects light off of a spinning reflector driven by the speedo cable in the speedometer head. A buffer circuit produces a digital signal and sends it to the PCM. (GM)

PM generator - a small A.C. generator driven by the transmission output shaft produces an analog signal. The frequency of the signal represents vehicle speed. (GM , Ford)

Magnetic Reed Switch - a ground switch closed by a set of magnets spinning on a disc driven by the speedo cable produces a digital signal used by the PCM to determine vehicle speed. (Chrysler)

SENSOR DEVICES

Simple Switch Inputs