Halderman J.D., Linder J. Automotive Fuel and Emissions Control Systems

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GASOLINE DIRECT-INJECTION SYSTEMS 271

UPSTREAM

OXYGEN SENSOR

DOWNSTREAM

OXYGEN SENSOR

- CATALYST

PRECATALYST

TEMPERATURE SENSOR

FIGURE 21–2 An engine equipped with a gasoline direct injection (GDI) sometimes requires a NO

catalyst to meet exhaust

emission standards.

COMMON

RAIL

INJECTOR

PRESSURE

REGULATOR

FUEL TANK

LOW-PRESSURE

FUEL PUMP

HIGH-PRESSURE

FUEL PUMP

THE HIGH-PRESSURE PUMP

DRIVE LOBE IS LOCATED

ON THE ENGINE CAMSHAFT

FIGURE 21–3 A typical direct-injection system uses two pumps—one low-pressure electric pump in the fuel tank and a

high-pressure pump driven by the camshaft. The high pressure fuel system operates at a pressure as low as 500 PSI during

light-load conditions and as high as 2,900 PSI under heavy loads.

LOW-PRESSURE SUPPLY PUMP The fuel pump in the

fuel tank supplies fuel to the high-pressure fuel pump at a pres-

sure of approximately 60 PSI. The fuel filter is located in the

DIRECT-INJECTION FUEL

DELIVERY SYSTEM

fuel tank and is part of the fuel-pump assembly. It is not usually

serviceable as a separate component. The powertrain control

module (PCM) controls the output of the high- pressure pump,

which has a range between 500 PSI (3,440 kPa) and 2,900 PSI

(15,200 kPa) during engine operation.

 SEE FIGURE 21–3.

HIGH-PRESSURE PUMP In a General Motors system,

the engine control module (ECM) controls the output of the

high-pressure pump, which has a range between 500 PSI

272 CHAPTER 21

(3,440kPa) and 2,900 PSI (15,200 kPa) during engine operation.

The high-pressure fuel pump connects to the pump in the fuel

tank through the low-pressure fuel line. The pump consists

of a single-barrel piston pump, which is driven by the engine

camshaft. The pump plunger rides on a three-lobed cam on the

camshaft. The high-pressure pump is cooled and lubricated by

the fuel itself.

 SEE FIGURE 21–4 .

FUEL RAIL The fuel rail stores fuel from the high pressure

pump for use by each injector. All injectors receive the same

pressure as what is present in the fuel rail.

FUEL-PRESSURE REGULATOR An electric pressure-

control valve is installed between the pump inlet and outlet valves.

The fuel rail pressure sensor connects to the PCM with three wires:



5-volt reference



Ground



Signal

The sensor signal provides an analog signal to the PCM

that varies in voltage as fuel rail pressure changes. Low pres-

sure results in a low-voltage signal, and high pressure results in

a high-voltage signal.

The PCM uses internal drivers to control the power feed

and ground for the pressure control valve. When both PCM driv-

ers are deactivated, the inlet valve is held open by spring pres-

sure. This causes the high-pressure fuel pump to default to low-

pressure mode. The fuel from the high-pressure fuel pump flows

through a line to the fuel rail and injectors. The actual operating

pressure can vary from as low as 900 PSI (6,200 kPa) at idle to

over 2,000 PSI (13,800 kPa) during high-speed or heavy-load

conditions.

 SEE FIGURE 21–5 .

HIGH-PRESSURE

FUEL PUMP

FUEL-PRESSURE

SENSOR

ELECTRIC PRESSURE

CONTROL VALVE

FUEL

INJECTOR

FUEL RAIL

FIGURE 21–5 A gasoline direct-injection (GDI) fuel rail

and pump assembly with the electric pressure control valve.

CAMSHAFT

PISTON

SPRING

PRESSURE

REGULATOR

CHECK

BALL

FIGURE 21–4 A typical camshaft-driven high-pressure pump

used to increase fuel pressure to 2,000 PSI or higher.

GDI FUEL INJECTORS

Each high-pressure fuel injector assembly is an electri-

cally magnetic injector mounted in the cylinder head. In the

GDI system, the PCM controls each fuel injector with 50 to

90volts (usually 60 to 70 volts), depending on the system,

which is created by a boost capacitor in the PCM. During

the high-voltage boost phase, the capacitor is discharged

through an injector, allowing for initial injector opening. The

injector is then held open with 12 volts. The high-pressure

fuel injector has a small slit or six precision-machined holes

that generate the desired spray pattern. The injector also has

an extended tip to allow for cooling from a water jacket in the

cylinder head.

 SEE CHART 21–1 for an overview of the differences

between a port fuel-injection system and a GDI system.

MODES OF OPERATION

The two basic modes of operation include the following:

1. Stratified mode . In this mode of operation, the air-fuel

mixture is richer around the spark plug than it is in the rest

of the cylinder.

2. Homogeneous mode. In this mode of operation, the air-

fuel mixture is the same throughout the cylinder.

There are variations of these modes that can be used to

fine-tune the air-fuel mixture inside the cylinder. For example,

Bosch, a supplier to many vehicle manufacturers, uses six

modes of operation, including the following:



Homogeneous mode. In this mode, the injector is

pulsed one time to create an even air-fuel mixture in the

cylinder. The injection occurs during the intake stroke.

This mode is used during high-speed and/or high-torque

conditions.

GASOLINE DIRECT-INJECTION SYSTEMS 273

As a result of this mode, the PCM does not need to

retard ignition timing as much to operate knock free.



Stratified catalyst heating mode. In this mode, there are

two injections:



The first injection is on the compression stroke just

before combustion.



The second injection occurs after combustion occurs

to heat the exhaust. This mode is used to quickly warm

the catalytic converter and to burn the sulfur from the

catalyst.



Homogeneous lean mode. Similar to the homogeneous

mode except that the overall air-fuel mixture is slightly

lean for better fuel economy. The injection occurs during

the intake stroke. This mode is used under steady,

light-load conditions.



Stratified mode. In this mode of operation, the injection

occurs just before the spark occurs, resulting in lean

combustion and reducing fuel consumption.



Homogeneous stratified mode. In this mode, there are

two injections of fuel:



The first injection is during the intake stroke.



The second injection is during the compression stroke.

As a result of these double injections, the rich air-fuel

mixture around the spark plug is ignited first. Then the

rich mixture ignites the leaner mixture. The advantages

of this mode include lower exhaust emissions than the

stratified mode and less fuel consumption than the

homogeneous lean mode.



Homogeneous knock protection mode. The purpose of

this mode is to reduce the possibility of spark knock from

occurring under heavy loads at low engine speeds. There

are two injections of fuel:



The first injection occurs on the intake stroke.



The second injection occurs during the compression

stroke with the overall mixture being stoichiometric.

INJECTOR

SPRAY-GUIDED COMBUSTION

SPARK

PLUG

FIGURE 21–6 In this design, the fuel injector is at the top

of the cylinder and sprays fuel into the cavity of the piston.

PISTON TOP DESIGNS

GDI systems use a variety of shapes of piston and injector lo-

cations depending on make and model of engine. Three of the

most commonly used designs include the following:



Spray-guided combustion. In this design, the injector

is placed in the center of the combustion chamber

and injects fuel into the dished-out portion of the

piston. The shape of the piston helps guide and direct

the mist of fuel in the combustion chamber.

 SEE

FIGURE 21–6 .



Swirl combustion. This design uses the shape of the

piston and the position of the injector at the side of the

combustion chamber to create turbulence and swirl of

the air-fuel mixture.

 SEE FIGURE 21–7 .



Tumble combustion. Depending on when the fuel

isinjected into the combustion chamber, this helps

determine how the air-fuel mixture is moved or

tumbled.

 SEE FIGURE 21–8 .

PORT FUEL

INJECTION GDI

Fuel pressure 35 to 60 PSI Lift pump—50 to 60 PSI

High-pressure pump—

500 to 2,900 PSI

Injection pulse

width at idle

1.5 to 3.5 ms

About 0.4 ms (400 s)

Injector

resistance

12 to 16 ohms 1 to 3 ohms

Injector

voltage

6 V for low-

resistance

injectors, 12 V

for most injectors

50 to 90 V

Number

of injections

per event

One 1 to 3

Engine

compression

ratio

8:1 to 11:1 11:1 to 13:1

A comparison chart showing the major differences between a

port fuel-injection system and a gasoline direct-injection (GDI)

system.

PORT FUEL-INJECTION SYSTEM

COMPARED WITH GDI SYSTEM

CHART 21–1

274 CHAPTER 21

An engine equipped with GDI could use the system to start

the engine. This is most useful during idle stop mode when

the engine is stopped while the vehicle is at a traffic light to

save fuel. The steps used in the Mitsubishi start-stop system,

called the smart idle stop system (SISS) , allow the engine to

be started without a starter motor and include the following

steps:

STEP 1 The engine is stopped. The normal stopping position

of an engine when it stops is 70 degrees before top

dead center, plus or minus 20 degrees. This is because

the engine stops with one cylinder on the compres-

sion stroke, and the PCM can determine the cylinder

position, using the crankshaft and camshaft position

sensors.

STEP 2 When a command is made to start the engine by

the PCM, fuel is injected into the cylinder that is on

the compression stroke and ignited by the spark plug.

STEP 3 The piston on the compression stroke is forced

downward, forcing the crankshaft to rotate coun-

terclockwise or in the opposite direction to normal

operation.

INJECTOR

WALL-GUIDED (SWIRL) COMBUSTION

SPARK PLUG

FIGURE 21–7 The side injector combines with the shape

of the piston to create a swirl as the piston moves up on the

compression stroke.

INJECTOR

WALL-GUIDED (TUMBLE) COMBUSTION

SPARK PLUG

FIGURE 21–8 The piston creates a tumbling force as the

piston moves upward.

LEXUS PORT- AND

DIRECT-INJECTION

SYSTEMS

OVERVIEW Many Lexus vehicles use GDI, and in some

engines, they also use a conventional port fuel-injection sys-

tem. The Lexus D-4S system combines direct-injection injec-

tors located in the combustion chamber with port fuel injectors

in the intake manifold near the intake valve. The two injec-

tion systems work together to supply the fuel needed by the

ENGINE START SYSTEM

TORQUE

ENGINE SPEED (RPM) HIGH

HIGH

DIRECT INJECTION ONLY

DIRECT INJECTION

AND

PORT INJECTION

FIGURE 21–9 Notice that there are conditions when both

the port fuel injector located in the intake manifold and the

gasoline direct injector located in the cylinder operate to

provide the proper air-fuel mixture.

engine.

 SEE FIGURE 21–9 for how the two systems are used

throughout the various stages of engine operation.

COLD-START WARM-UP To help reduce exhaust emis-

sions after a cold start, the fuel system uses a stratified

change mode. This results in a richer air-fuel mixture near the

spark plug and allows for the spark to be retarded to increase

the temperature of the exhaust. As a result of the increased

exhaust temperature, the catalytic converter rapidly reaches

operating temperature, which reduces exhaust emissions.

GASOLINE DIRECT-INJECTION SYSTEMS 275



The backside of the intake valve. This is a common

place for fuel residue and carbon to accumulate on

engines equipped with GDI. The accumulation of carbon

on the intake valve can become so severe that the engine

will start and idle but lack power to accelerate the vehicle.

The carbon deposits restrict the airflow into the cylinder

enough to decrease engine power.

NOTE: Lexus engines that use both port and GDI injec-

tors do not show intake valve deposits. It is thought that

the fuel being sprayed onto the intake valve from the

port injector helps keep the intake valve clean.

CARBON CLEANING. Most experts recommend the use of

Techron

, a fuel system dispersant, to help keep carbon from

accumulating. The use of a dispersant every six months or every

6,000miles has proven to help prevent injector and intake valve

deposits.

If the lack of power is discovered and there are no stored

diagnostic trouble codes, a conventional carbon cleaning

procedure will likely restore power if the intake valves are

coated.

STEP 4 The rotation of the crankshaft then forces the compan-

ion cylinder toward the top of the cylinder.

STEP 5 Fuel is injected, and the spark plug is fired, forcing

the piston down, causing the crankshaft to rotate in

the normal (clockwise) direction. Normal combustion

events continue, allowing the engine to keep running.

INTAKE VALVES

FUEL INJECTOR

EXHAUST VALVES

SPARK PLUG

FIGURE 21–10 There may become a driveability issue

because the gasoline direct-injection (GDI) injector is exposed

to combustion carbon and fuel residue.

GDI SERVICE

NOISE ISSUES GDI systems operate at high pressure, and

the injectors can often be heard with the engine running and the

hood open. This noise can be a customer concern because the

clicking sound is similar to noisy valves. If a noise issue is the

customer concern, check the following:



Check a similar vehicle to determine if the sound is louder

or more noticeable than normal.



Check that nothing under the hood is touching the fuel

rail. If another line or hose is in contact with the fuel

rail, the sound of the injectors clicking can be transmit-

ted throughout the engine, making the sound more

noticeable.



Check for any technical service bulletins (TSBs) that may

include new clips or sound insulators to help reduce the

noise.

CARBON ISSUES Carbon is often an issue in engines

equipped with GDI systems. Carbon can affect engine opera-

tion by accumulating in two places:



On the injector itself. Because the injector tip is in the

combustion chamber, fuel residue can accumulate on

the injector, reducing its ability to provide the proper

spray pattern and amount of fuel. Some injector de-

signs are more likely to be affected by carbon than oth-

ers. For example, if the injector uses small holes, these

tend to become clogged more often than an injector

that uses a single slit opening, where the fuel being

sprayed out tends to blast away any carbon.

 SEE

FIGURE 21–10 .

SUMMARY

1. A GDI system uses a fuel injector that delivers a short

squirt of fuel directly into the combustion chamber rather

than in the intake manifold, near the intake valve on a port

fuel injection system.

2. The advantages of using GDI instead of port fuel injection

include the following:

•

Improved fuel economy

•

Reduced exhaust emissions

•

Greater engine power

3. Some of the disadvantages of GDI systems compared with

a port fuel-injection system include the following:

•

Higher cost

•

The need for NO

storage catalyst in some applications

•

More components

4. The operating pressure can vary from as low as 500 PSI dur-

ing some low-demand conditions to as high as 2,900 PSI.

5. The fuel injectors are open for a very short period of time and

are pulsed using a 50- to 90-V pulse from a capacitor circuit.

276 CHAPTER 21

3. How is the fuel delivery system different from a port

fuel-injection system?

4. What are the basic modes of operation of a GDI system?

REVIEW QUESTIONS

1. What are two advantages of GDI compared with port fuel

injection?

2. What are two disadvantages of GDI compared with port

fuel injection?

CHAPTER QUIZ

1. Where is the fuel injected in an engine equipped with

GDI?

a. Into the intake manifold near the intake valve

b. Directly into the combustion chamber

c. Above the intake port

d. In the exhaust port

2. The fuel pump inside the fuel tank on a vehicle equipped

with GDI produces about what fuel pressure?

a. 5 to 10 PSI

b. 10 to 20 PSI

c. 20 to 40 PSI

d. 50 to 60 PSI

3. The high-pressure fuel pumps used in GDI systems are

a. Electricity (DC motor)

b. Electricity (AC motor)

c. The camshaft

d. The crankshaft

4. The high-pressure fuel pressure is regulated by using

________ .

a. An electric pressure-control valve

b. A vacuum-biased regulator

c. A mechanical regulator at the inlet to the fuel rail

d. A non-vacuum-biased regulator

5. The fuel injectors operate under a fuel pressure of about

________ .

a. 35 to 45 PSI

90 to 150 PSI

c. 500 to 2,900 PSI

d. 2,000 to 5,000 PSI

6. The fuel injectors used on a GDI system are pulsed on

using what voltage?

a. 12 to 14 V

b. 50 to 90 V

c. 100 to 110 V

d. 200 to 220 V

7. Which mode of operation results in a richer air-fuel mixture

near the spark plug?

a. Stoichiometric

b. Homogeneous

c. Stratified

d. Knock protection

8. Some engines that use a GDI system also have port

injection.

a. True

b. False

9. A GDI system can be used to start an engine without the

need for a starter.

a. True

b. False

10. A lack of power from an engine equipped with GDI could

be due to ________ .

a. Noisy injectors

b. Carbon on the injectors

c. Carbon on the intake valves

d. Both b and c

6. GDI systems can operate in many modes, which are sepa-

rated into the two basic modes:

•

Stratified mode

•

Homogeneous mode

7. GDI can be used to start an engine without the use of a

starter motor for idle-stop functions.

8. GDI does create a louder clicking noise from the fuel injec-

tors than port fuel-injection injectors.

9. Carbon deposits on the injector and the backside of the

intake valve are a common problem with engines equipped

with GDI systems.

ELECTRONIC THROTTLE CONTROL SYSTEM 277

chapter

ELECTRONIC THROTTLE

CONTROL SYSTEM

OBJECTIVES: After studying Chapter 22 , the reader should be able to: • Prepare for ASE Engine Performance (A8) test

content area “E” (Computerized Engine Controls Diagnosis and Repair). • Describe the purpose and function of an electronic

throttle control (ETC) system. • Explain how an electronic throttle control system works. • List the parts of a typical electronic

throttle control system. • Describe how to diagnose faults in an electronic throttle control system.

KEY TERMS: Accelerator pedal position (APP) sensor 277 • Coast-down stall 283 • Default position 279

• Drive-by-wire 277 • Electronic throttle control (ETC) 277 • Fail-safe position 279 • Neutral position 279

• Servomotor 279 • Throttle position (TP) sensor 277

ADVANTAGES OF ETC The absence of any mechani-

cal linkage between the throttle pedal and the throttle body

requires the use of an electric actuator motor. The electronic

throttle system has the following advantages over the conven-

tional cable:



Eliminates the mechanical throttle cable, thereby

reducing the number of moving parts.



Eliminates the need for cruise control actuators and

controllers.



Helps reduce engine power for traction control (TC) and

electronic stability control (ESC) systems.



Used to delay rapid applications of torque to the

transmission/transaxle to help improve driveability

and to smooth shifts.



Helps reduce pumping losses by using the electronic

throttle to open at highway speeds with greater fuel

economy. The ETC opens the throttle to maintain engine

and vehicle speed as the powertrain control module

(PCM) leans the air-fuel ratio, retards ignition timing, and

introduces additional exhaust gas recirculation (EGR) to

reducing pumping losses.



Used to provide smooth engine operation, especially

during rapid acceleration.



Eliminates the need for an idle air control valve.

The electronic throttle can be called drive-by-wire, but

most vehicle manufacturers use the term electronic throttle

ELECTRONIC THROTTLE

CONTROL (ETC) SYSTEM

control (ETC) to describe the system that opens the throttle

valve electrically.

PARTS INVOLVED The typical ETC system includes the

following components:

1. Accelerator pedal position (APP) sensor, also called ac-

celerator pedal sensor (APS)

2. Electronic throttle actuator (servomotor), which is part of

the electronic throttle body

3. T hrottle position (TP) sensor

4. Electronic control unit, which is usually the powertrain

control module (PCM)

 SEE FIGURE 22–1 .

Driving a vehicle equipped with an ETC system is about the

same as driving a vehicle with a conventional mechanical

throttle cable and throttle valve. However, the driver may notice

some differences that are to be considered normal. These nor-

mal conditions include the following:



The engine may not increase above idle speed when

depressing the accelerator pedal when the gear

selector is in Park.



If the engine speed does increase when the accelerator is

depressed with the transmission in Park or Neutral, the en-

gine speed will likely be limited to less than 2,000RPM.



While accelerating rapidly, there is often a slight delay

before the engine responds.

 SEE FIGURE 22–2 .

NORMAL OPERATION

OF THE ETC SYSTEM

278 CHAPTER 22

ACCELERATOR

PEDAL POSITION

THROTTLE

ACTUATOR

POSITION

MILLISECONDS

FIGURE 22–2 The opening of the throttle plate can be

delayed as long as 30 milliseconds (0.03 sec.) to allow time for

the amount of fuel needed to catch up to the opening of the

throttle plate.

TAC

MOTOR

THROTTLE

BLADE

THROTTLE

POSITION

SENSOR

ACCELERATOR PEDAL

POSITION (APP) SENSOR

PEDAL

ASSEMBLY

TAC

MODULE

PCM

FIGURE 22–1 The throttle pedal is connected to the

accelerator pedal position (APP) sensor. The electronic throttle

body includes a throttle position (TP) sensor to provide throttle

angle feedback to the vehicle computer. Some systems use a

throttle actuator control (TAC) module to operate the throttle

blade (plate).

0.00 V

5 V

DEGREES OPENING

FIGURE 22–3 A typical accelerator pedal position (APP) sen-

sor, showing two different output voltage signals that are used

by the PCM to determine accelerator pedal position. Two

(or three in some applications) are used as a double check

because this is a safety-related sensor.



While at cruise speed, the accelerator pedal may or may

not cause the engine speed to increase if the accelerator

pedal is moved slightly.

CABLE-OPERATED SYSTEM Honda Accords until

2008 model year used a cable attached to the accelerator pedal

to operate the APP sensor located under the hood. A similar

arrangement was used in Dodge RAM trucks in 2003. In both of

these applications, the throttle cable was simply moving the APP

sensor and not moving the throttle plate. The throttle plate is con-

trolled by the PCM and moved by the ETC motor.

TWO SENSORS The accelerator pedal position sensor uses

two and sometimes three separate sensors, which act together

to give accurate accelerator pedal position information to the

controller but also are used to check that the sensor is working

properly. They function just like a throttle position (TP) sensor,

and two are needed for proper system function. One APP sensor

output signal increases as the pedal is depressed, and the other

signal decreases. The controller compares the signals with a

lookup table to determine the pedal position. Using two or three

signals improves redundancy should one sensor fail and allows

the PCM to quickly detect a malfunction. When three sensors

ACCELERATOR PEDAL

POSITION SENSOR

are used, the third signal can either decrease or increase with

pedal position, but its voltage range will still be different from the

other two.

 SEE FIGURE 22–3 .

The throttle body assembly contains the following components:



Throttle plate



Electric actuator DC motor



Dual TP sensors



Gears used to multiply the torque of the DC motor



Springs used to hold the throttle plate in the default location

THROTTLE BODY

ASSEMBLY

ELECTRONIC THROTTLE CONTROL SYSTEM 279

THROTTLE PLATE AND SPRING The throttle plate is

held slightly open by a concentric clock spring. The spring

applies a force that will close the throttle plate if power is lost to

the actuator motor. The spring is also used to open the throttle

plate slightly from the fully closed position.

ELECTRONIC THROTTLE BODY MOTOR The actuator

is a DC electric motor and is often called a servomotor. The

throttle plate is held in a default position by a spring inside

the throttle body assembly. This partially open position, also

called the neutral position or the fail-safe position, is about

16% to 20% open. This default position varies depending on

the vehicle and usually results in an engine speed of 1,200 to

1,500 RPM:



The throttle plate is driven closed to achieve speeds

lower than the default position, such as idle speed.



The throttle plate is driven open to achieve speeds higher

than the default position, such as during acceleration.

 SEE FIGURE 22–4 .

The throttle plate motor is driven by a bidirectional pulse-

width-modulated (PWM) signal from the PCM or ETC module

using an H-bridge circuit.

 SEE FIGURE 22–5a , b .

The H-bridge circuit is controlled by the powertrain control

module (PCM) by the following:



Reversing the polarity of power and ground brushes to

the DC motor



Pulse-width modulating the current through the motor

The PCM monitors the position of the throttle from the two

TP sensors. The PCM then commands the throttle plate to the

desired position.

 SEE FIGURE 22–6 .

AIRFLOW

THROTTLE POSITION

SPRING-LOADED TO

DEFAULT POSITION

AIRFLOW

ACCELERATION

MOTOR DRIVES THROTTLE

SHAFT OPEN FROM THE

DEFAULT POSITION

AIRFLOW

IDLE POSITION

MOTOR DRIVES THROTTLE

SHAFT CLOSED FROM THE

DEFAULT POSITION

FIGURE 22–4 The default position for the throttle plate is in

slightly open position. The servomotor then is used to close it

for idle and open it during acceleration.

What Is the “Spring Test”?

The spring test is a self-test performed by the PCM

whenever the engine is started. The PCM operates

the throttle to check if it can react to the command

and return to the default (home) position. This self-

test is used by the PCM to determine that the spring

and motor are working correctly and may be noticed

by some vehicle owners by the following factors:

• A slight delay in the operation of the starter motor.

The PCM performs this test when the ignition

switch is turned to the “on” position. While it takes

just a short time to perform the test, it can be

sensed by the driver that there could be a fault in

the ignition switch or starter motor circuits.

• A slight “clicking” sound may also be heard

coming from under the hood when the ignition is

turned on. This is normal and is related to the

self-test on the throttle as it opens and closes.

FREQUENTLY ASKED QUESTION

Why Not Use a Stepper Motor for ETC?

A stepper motor is a type of motor that has multiple

windings and is pulsed by a computer to rotate a

certain number of degrees when pulsed. The disad-

vantage is that a stepper motor is too slow to react

compared with a conventional DC electric motor

and is the reason a stepper motor is not used in ETC

systems.

FREQUENTLY ASKED QUESTION

280 CHAPTER 22

12 V

MOTOR

DC MOTOR H-BRIDGE CIRCUIT

(a)

FIGURE 22–5 (a) An H-bridge circuit is used to control the direction of the DC electric motor of the electronic throttle control

(ETC) unit. (b) To reverse the direction of operation, the polarity of the current through the motor is reversed.

12 V

MOTOR

DC MOTOR H-BRIDGE CIRCUIT

(b)

PCM

TP 1

TP 2

TP 1 SIGNAL

TP 2 SIGNAL

5V SOURCE

GROUND

ETC

(PWM) 12V OR GROUND

CONSTANT 12V OR GROUND

FIGURE 22–6 Schematic of a typical electronic throttle con-

trol (ETC) system. Note that terminal #5 is always pulse-width

modulated and that terminal #3 is always constant, but both

power and ground are switched to change the direction of

the motor.

Two TP sensors are used in the throttle body assembly to pro-

vide TP signals to the PCM. Two sensors are used as a fail-safe

measure and for diagnosis. There are two types of TP sensors

used in ETC systems: potentiometers and Hall effect.

THREE-WIRE POTENTIOMETER SENSORS These sen-

sors use a 5-volt reference from the PCM and produce an

analog (variable) voltage signal that is proportional to the throttle

plate position. The two sensors produce opposite signals as the

throttle plate opens:



One sensor starts at low voltage (about 0.5 V) and in-

creases as the throttle plate is opened.



The second sensor starts at a higher voltage (about 4.5V)

and produces a lower voltage as the throttle plate is

opened.

 SEE FIGURE 22–7 .

TP SENSOR

0% 50% 100%

SENSOR 1

TP SIGNAL VOLTAGE

SENSOR 2

FIGURE 22–7 The two TP sensors used on the throttle body

of an electronic throttle body assembly produce opposite volt-

age signals as the throttle is opened. The total voltage of both

combined at any throttle plate position is 5 volts.

How Do You Calibrate a New APP Sensor?

Whenever an accelerator pedal position (APP) sensor

is replaced, it should be calibrated before it will work

correctly. Always check service information for the

exact procedure to follow after APP sensor replace-

ment. Here is a typical example of the procedure:

STEP 1 Make sure the accelerator pedal is fully

released.

STEP 2 Turn the ignition switch on (engine off)

and wait at least 2 seconds.

STEP 3 Turn the ignition switch off and wait at

least 10 seconds.

STEP 4 Turn the ignition switch on (engine on)

and wait at least 2 seconds.

STEP 5 Turn the ignition switch off and wait at

least 10 seconds.

FREQUENTLY ASKED QUESTION