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

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THROTTLE POSITION SENSORS 191

CLEAR FLOOD MODE If the throttle is depressed to the

floor during engine cranking, the PCM will either greatly reduce

or entirely eliminate any fuel-injector pulses to aid in cleaning

a flooded engine. If the throttle is depressed to the floor and

the engine is not flooded with excessive fuel, the engine may

not start.

TORQUE CONVERTER CLUTCH ENGAGEMENT AND

RELEASE The torque converter clutch will be released

if the PCM detects rapid acceleration to help the transmis-

sion deliver maximum torque to the drive wheels. The torque

converter clutch is also disengaged when the accelerator

pedal is released with the vehicle moving to help engine

braking.

RATIONALITY TESTING FOR MAP AND MAF SENSORS

As part of the rationality tests for the MAP and/or MAF sensor,

the TP sensor signal is compared to the reading from other sen-

sors to determine if they match. For example, if the throttle posi-

tion sensor is showing wide-open throttle (WOT), the MAP and/

or MAF reading should also indicate that this engine is under a

heavy load. If not, a diagnostic trouble code could be set for the

TP, as well as the MAP and/or MAF sensors.

AUTOMATIC TRANSMISSION SHIFT POINTS The

shift points are delayed if the throttle is opened wide to allow the

engine speed to increase, thereby producing more power and

aiding in the acceleration of the vehicle. If the throttle is barely

open, the shift point occurs at the minimum speed designed for

the vehicle.

TARGET IDLE SPEED (IDLE CONTROL STRATEGY)

When the TP sensor voltage is at idle, the PCM then controls

idle speed using the idle air control (IAC) and/or spark timing

variation to maintain the commanded idle speed. If the TP sen-

sor indicates that the throttle has moved off idle, fuel delivery

and spark timing are programmed for acceleration. Therefore, if

the throttle linkage is stuck or binding, the idle speed may not

be correct.

AIR-CONDITIONING COMPRESSOR OPERATION The

TP sensor is also used as an input sensor for air-conditioning

compressor operation. If the PCM detects that the throttle is

at or close to wide open, the air-conditioning compressor is

disengaged.

BACKS UP OTHER SENSORS The TP sensor is used

as a backup to the MAP sensor and/or MAF in the event the

PCM detects that one or both are not functioning correctly.

The PCM then calculates fuel needs and spark timing based on

theengine speed (RPM) and throttle position.

TP SENSOR COMPUTER

INPUT FUNCTIONS



The computer senses any change in throttle position and

changes the fuel mixture and ignition timing. The actual

change in fuel mixture and ignition timing is also partly

determined by the other sensors, such as the manifold

pressure (engine vacuum), engine RPM, the coolant tem-

perature, and oxygen sensor(s). Some throttle position

sensors are adjustable and should be set according to

the exact engine manufacturer’s specifications.



The throttle position (TP) sensor used on fuel-injected

vehicles acts as an “electronic accelerator pump.” This

means that the computer will pulse additional fuel from

the injectors when the throttle is depressed. Because the

air can quickly flow into the engine when the throttle is

opened, additional fuel must be supplied to prevent the

air-fuel mixture from going lean, causing the engine to

hesitate when the throttle is depressed. If the TP sensor

is unplugged or defective, the engine may still operate

satisfactorily but hesitate upon acceleration.



The PCM supplies the TP sensor with a regulated voltage

that ranges from 4.8 to 5.1 volts. This reference voltage is

usually referred to as a 5-volt reference or “Vref.” The TP

output signal is an input to the PCM, and the TP sensor

ground also flows through the PCM.

See the Ford throttle position (TP) sensor chart for an ex-

ample of how sensor voltage changes with throttle angle.

Ford Throttle Position (TP) Sensor Chart

Throttle Angle (Degrees) Voltage (V)

0 0.50

10 0.97

20 1.44

30 1.90

40 2.37

50 2.84

60 3.31

70 3.78

80 4.24

NOTE: Generally, any reading higher than 80% repre-

sents wide-open throttle to the computer.

PCM USES

FOR THE TP SENSOR

The TP sensor is used by the powertrain control module (PCM)

for the following reasons.

192 CHAPTER 14

NOTE: Consult the service information for exact

wire colors or locations.

3. With the engine still not running (but with the ignition still

on), slowly increase the throttle opening. The voltage

signal from the TP sensor should also increase. Look

for any “dead spots” or open circuit readings as the

throttle is increased to the wide-open position.

 SEE

FIGURE 14–4 for an example of how a good TP sensor

would look when tested with a digital storage oscillo-

scope (DSO).

NOTE: Use the accelerator pedal to depress the

throttle because this applies the same forces on the

TP sensor as the driver does during normal driving.

Moving the throttle by hand under the hood may not

accurately test the TP sensor.

4. With the voltmeter still connected, slowly return the

throttle down to the idle position. The voltage from the

TP sensor should also decrease evenly on the return

to idle.

The TP sensor voltage at idle should be within the accept-

able range as specified by the manufacturer. Some TP sensors

can be adjusted by loosening their retaining screws and moving

the sensor in relation to the throttle opening. This movement

changes the output voltage of the sensor.

All TP sensors should also provide a smooth transition

voltage reading from idle to WOT and back to idle. Replace the

TP sensor if erratic voltage readings are obtained or if the cor-

rect setting at idle cannot be obtained.

FIGURE 14–3 A meter lead connected to a T-pin that was

gently pushed along the signal wire of the TP sensor until the

point of the pin touched the metal terminal inside the plastic

connector.

A 1V DC 1:1 PROBE B 200mV OFF 1:1 PROBE

200ms / DIV SINGLE TRIG:A -3 DIV

SINGLE FREE CAPTURE MIN MAX TRIGGER

RECURRENT RUN 10 20 DIV ON A AT 50%

ZOOM

HOLD

FIGURE 14–4 A typical waveform of a TP sensor signal

as recorded on a DSO when the accelerator pedal was de-

pressed with the ignition switch on (engine off). Clean transi-

tions and the lack of any glitches in this waveform indicate a

good sensor. (Courtesy of Fluke Corporation)

TESTING THE THROTTLE

POSITION SENSOR

A TP sensor can be tested using one or more of the follow-

ing tools:



A digital voltmeter with three test leads connected in

series between the sensor and the wiring harness con-

nector or back probing using T-pins or other recom-

mended tool that will not cause harm to the connector

or wiring.



A scan tool or a specific tool recommended by the

vehicle manufacturer.



A breakout box that is connected in series between the

computer and the wiring harness connector(s). A typical

breakout box includes test points at which TP voltages

can be measured with a digital voltmeter.



An oscilloscope.

Use jumper wires, T-pins to back-probe the wires, or a

breakout box to gain electrical access to the wiring to the TP

sensor.

 SEE FIGURE 14–3 .

NOTE: The procedure that follows is the method used

by many manufacturers. Always refer to service in-

formation for the exact recommended procedure and

specifications for the vehicle being tested.

The procedure for testing the sensor using a digital multi-

meter is as follows:

1. Turn the ignition switch on (engine off).

2. Set the digital meter to read to DC volts and measure the

voltage between the signal wire and ground (reference low)

wire. The voltage should be about 0.5 volt.

THROTTLE POSITION SENSORS 193

FIGURE 14–5 Checking the 5-volt reference from the com-

puter being applied to the TP sensor with the ignition switch on

(engine off). The reading for this vehicle (5.02 volts DC) is within

the normal range for the reference voltage of 4.9 to 5.1 volts.

FIGURE 14–6 Checking the voltage drop between the TP

sensor ground and a good engine ground with the ignition on

(engine off). A reading of greater than 0.2 volt (200 mV) repre-

sents a bad computer ground.

Many digital multimeters are capable of recording voltage read-

ings over time and then displaying the minimum, maximum, and

average readings. To perform a MIN/MAX test of the TP sensor,

manually set the meter to read higher than 4 volts:

TESTING A TP SENSOR

USING THE MIN/MAX

FUNCTION

Check Power and Ground Before

Condemning a Bad Sensor

Most engine sensors use a 5-volt reference and

a ground. If the 5 volts to the sensor is too high

(shorted to voltage) or too low (high resistance), then

the sensor output will be skewed or out of range.

Before replacing the sensor that did not read cor-

rectly, measure both the 5-volt reference and ground.

To measure the ground, simply turn the ignition on

(engine off) and touch one test lead of a DMM set

to read DC volts to the sensor ground and the other

to the negative terminal of the battery. Any read-

ing higher than 0.2 volt (200 mV) represents a poor

ground.

 SEE FIGURES 14–5 AND 14–6 .

TECH TIP

STEP 1 Connect the red meter lead to the signal wire and the

black meter lead to a good ground or the ground return

wire at the TP sensor.

STEP 2 With the ignition on, engine off, slowly depress and

release the accelerator pedal from inside the vehicle.

STEP 3 Check the minimum and maximum voltage reading on

the meter display. Any 0-volt or 5-volt reading would

indicate a fault or short in the TP sensor.

TESTING THE TP SENSOR

USING A SCAN TOOL

A scan tool can be used to check for proper operation of the

throttle position sensor using the following steps:

STEP 1 With the key on, engine off, the TP sensor voltage dis-

play should be about 0.5 volt but can vary from as low

as 0.3 volt to as high as 1.2 volts.

STEP 2 Check the scan tool display for the percentage of throt-

tle opening. The reading should be zero and gradually

increase in percentage as the throttle is depressed.

STEP 3 The idle air control (IAC) counts should increase as

the throttle is opened and decrease as the throttle is

closed. Start the engine and observe the IAC counts as

the throttle is depressed.

STEP 4 Start the engine and observe the TP sensor reading.

Use a wedge at the throttle stop to increase the throttle

opening slightly. The throttle percentage reading should

increase. Shut off and restart the engine. If the percent-

age of throttle opening returns to 0%, the PCM deter-

mines that the increased throttle opening is now the new

minimum and resets the idle position of the TP sensor.

Remove the wedge and cycle the ignition key. The throt-

tle position sensor should again read 0%.

NOTE: Some engine computers are not capable of

resetting the throttle position sensor.

194 CHAPTER 14

TP SENSOR DIAGNOSTIC

TROUBLE CODES

The diagnostic trouble codes (DTCs) associated with the throt-

tle position sensor include the following:

Diagnostic Trouble Code Description Possible Causes

P0122 TP sensor low voltage • TP sensor internally shorted-to-ground

• TP sensor wiring shorted-to-ground

• TP sensor or wiring open

P0123 TP sensor high voltage • TP sensor internally shorted to 5-volt reference

• TP sensor ground open

• TP sensor wiring shorted-to-voltage

P0121 TP sensor signal does not agree with MAP • Defective TP sensor

• Incorrect vehicle-speed (VS) sensor signal

• MAP sensor out-of-calibration or defective

transmission shift points as well as rationality testing for

the MAP and MAF sensors.

4. The TP sensor signal voltage should be about 0.5 volt at

idle and increase to about 4.5 volts at wide-open throttle

(WOT).

5. A TP sensor can be tested using a digital multimeter, a

digital storage oscilloscope (DSO), or a scan tool.

1. A throttle position (TP) sensor is a three-wire variable resis-

tor called a potentiometer.

2. The three wires on the TP sensor include a 5-volt reference

voltage from the PCM, plus the signal wire to the PCM, and

a ground, which also goes to the PCM.

3. The TP sensor is used by the PCM for clear flood mode,

torque converter engagement and release, and automotive

SUMMARY

3. What is the procedure to follow when checking the 5-volt

reference and TP sensor ground?

4. How can a TP sensor be diagnosed using a scan tool?

1. What is the purpose of each of the three wires on a typical

TP sensor?

2. What all does the PCM do with the TP sensor signal

voltage?

REVIEW QUESTIONS

3. A TP sensor is what type of sensor?

a. Rheostat

b. Voltage generating

c. Potentiometer

d. Piezoelectric

4. Most TP sensors have how many wires?

a. One c. Three

b. Two d. Four

1. Which sensor is generally considered to be the electronic

accelerator pump of a fuel-injected engine?

a. O2S c. Engine MAP sensor

b. ECT sensor d. TP sensor

2. Typical TP sensor voltage at idle is about ______________ .

a. 2.5 to 2.8 volts

b. 0.5 volt or 10% of WOT TP sensor voltage

c. 1.5 to 2.8 volts

d. 13.5 to 15 volts

CHAPTER QUIZ

THROTTLE POSITION SENSORS 195

9. A technician attaches one lead of a digital voltmeter to the

ground terminal of the TP sensor and the other meter

lead to the negative terminal of the battery. The ignition

is switched to on, engine off, and the meter displays

37.3 mV. Technician A says that this is the signal voltage

and is a little low. Technician B says that the TP sensor

ground circuit has excessive resistance. Which techni-

cian is correct?

a. Technician A only

b. Technician B only

c. Both Technicians A and B

d. Neither Technician A nor B

10. A P0122 DTC is retrieved using a scan tool. This DTC

means ______________ .

a. The TP sensor voltage is low

b. The TP sensor could be shorted-to-ground

c. The TP sensor signal circuit could be shorted-to-ground

d. All of the above

5. Which sensor does the TP sensor back up if the PCM de-

termines that a failure has occurred?

a. Oxygen sensor c. MAP sensor

b. MAF sensor d. Either b or c

6. Which wire on a TP sensor should be back-probed to

check the voltage signal to the PCM?

a. 5-volt reference (Vref)

b. Signal

c. Ground

d. Meter should be connected between the 5-volt refer-

ence and the ground

7. After a TP sensor has been tested using the MIN/MAX

function on a DMM, a reading of 0 volts is displayed. What

does this reading indicate?

a. The TP sensor is open at one point during the test.

b. The TP sensor is shorted.

c. The TP sensor signal is shorted to 5-volt reference.

d. Both b and c are possible.

8. After a TP sensor has been tested using the MIN/MAX

function on a DMM, a reading of 5 volts is displayed. What

does this reading indicate?

a. The TP sensor is open at one point during the test.

b. The TP sensor is shorted.

c. The TP sensor signal is shorted to 5-volt reference.

d. Both b and c are possible.

196 CHAPTER 15

chapter

MAP/BARO SENSORS

OBJECTIVES: After studying Chapter 15 , the reader should be able to: • Prepare for ASE Engine Performance (A8) certification

test content area “E” (Computerized Engine Controls Diagnosis and Repair). • Discuss how MAP sensors work. • List the methods

that can be used to test MAP sensors. • Describe the symptoms of a failed MAP sensor. • List how the operation

of the MAP sensor affects vehicle operation. • Discuss MAP sensor rationality tests. • Describe how the BARO sensor is used

to determine altitude.

KEY TERMS: Barometric manifold absolute pressure (BMAP) sensor 201 • Barometric pressure (BARO) sensor 201

• Manifold absolute pressure (MAP) sensor 196 • Piezoresistivity 198 • Pressure differential 196 • Speed density 199

• Vacuum 196

AIR PRESSURE—HIGH

AND LOW

Think of an internal combustion engine as a big air pump. As the

pistons move up and down in the cylinders, they pump in air and

fuel for combustion and pump out exhaust gases. They do this

by creating a difference in air pressure. The air outside an engine

has weight and exerts pressure, as does the air inside an engine.

As a piston moves down on an intake stroke with the intake

valve open, it creates a larger area inside the cylinder for the air to fill.

This lowers the air pressure within the engine. Because the pressure

inside the engine is lower than the pressure outside, air flows into

the engine to fill the low-pressure area and equalize the pressure.

The low pressure within the engine is called vacuum.

Vacuum causes the higher-pressure air on the outside to flow

into the low-pressure area inside the cylinder. The difference in

pressure between the two areas is called a pressure differential.

 SEE FIGURE 15–1 .

PRINCIPLES

OF PRESSURE SENSORS

Intake manifold pressure changes with changing throttle posi-

tions. At wide-open throttle, manifold pressure is almost the

same as atmospheric pressure. On deceleration or at idle, mani-

fold pressure is below atmospheric pressure, thus creating a

vacuum. In cases where turbo- or supercharging is used, under

part- or full-load condition, intake manifold pressure rises above

atmospheric pressure. Also, oxygen content and barometric

pressure change with differences in altitude, and the computer

must be able to compensate by making changes in the flow

of fuel entering the engine. To provide the computer with

changing airflow information, a fuel-injection system may use

the following:



Manifold absolute pressure (MAP) sensor



Manifold absolute pressure (MAP) sensor plus barometric

absolute pressure (BARO) sensor



Barometric and manifold absolute pressure sensors

combined (BMAP)

The manifold absolute pressure (MAP) sensor may

be a ceramic capacitor diaphragm, an aneroid bellows, or a

piezoresistive crystal. It has a sealed vacuum reference input on

one side; the other side is connected (vented) to the intake man-

ifold. This sensor housing also contains signal conditioning cir-

cuitry.

 SEE FIGURE 15–2 . Pressure changes in the manifold

cause the sensor to deflect, varying its analog or digital return

signal to the computer. As the air pressure increases, the MAP

sensor generates a higher voltage or frequency return signal to

the computer.

CONSTRUCTION OF

MAP SENSORS

The MAP sensor is used by the engine computer to sense

engine load. The typical MAP sensor consists of a ceramic or

silicon wafer sealed on one side with a perfect vacuum and

exposed to intake manifold vacuum on the other side. As the

MAP/BARO SENSORS 197

20 PSI

MEASURED IN

UNITS OF POUNDS

PER SQUARE INCH

PSI (KPa)

ATMOSPHERIC PRESSURE

MEASURED IN

UNITS OF INCHES

OF MERCURY

(in. Hg) (mmHg)

PERFECT VACUUM

ENGINE VACUUM AT IDLE

ENGINE VACUUM AT

WIDE-OPEN THROTTLE

(WOT)

0 PSIG (PSI GAUGE)

20 in. Hg

10 in. Hg

10 PSI

29.4 psia

ATMOSPHERIC PRESSURE

(Barometric pressure)

MAP = 4.8 V

KOEO or WOT

MAP = 2.94 V

Cruise/Light Load

MAP = 1.10 V

Idle

PERFECT VACUUM

29.92 in. Hg

14.7 psia 0 psig / 0 in. Hg

9.8 psia 10 in. Hg

4.9 psia 20 in. Hg

0 psia 29.92 in. Hg

PSIA

(Absolute pressure)

PSIG

(Gauge pressure)

(a)

(

)

FIGURE 15–1 (a) As an engine is accelerated under a load, the engine vacuum drops. This drop in vacuum is actually an

increase in absolute pressure in the intake manifold. A MAP sensor senses all pressures greater than that of a perfect vacuum.

(b) The relationship between absolute pressure, vacuum, and gauge pressure.

FIGURE 15–2 A clear plastic MAP sensor used for training

purposes showing the electronic circuit board and electrical

connections.

engine vacuum changes, the pressure difference on the wafer

changes the output voltage or frequency of the MAP sensor.

A MAP sensor is used on many engines for the PCM to

determine the load on the engine. The relationship among

barometer pressure, engine vacuum, and MAP sensor voltage

includes the following:



Absolute pressure is equal to barometric pressure minus

intake manifold vacuum.



A decrease in manifold vacuum means an increase in

manifold pressure.



The MAP sensor compares manifold vacuum to a perfect

vacuum.



Barometric pressure minus MAP sensor reading equals intake

manifold vacuum. Normal engine vacuum is 17 to 21 in. Hg.



Supercharged and turbocharged engines require a MAP

sensor that is calibrated for pressures above atmospheric

as well as for vacuum.

198 CHAPTER 15

SILICON-DIAPHRAGM STRAIN GAUGE MAP SENSOR

This is the most commonly used design for a MAP sensor, and

the output is a DC analog (variable) voltage. One side of a sili-

con wafer is exposed to engine vacuum, and the other side is

exposed to a perfect vacuum.

There are four resistors attached to the silicon wafer, which

changes in resistance when strain is applied to the wafer. This

change in resistance due to strain is called piezoresistivity. The

resistors are electrically connected to a Wheatstone bridge cir-

cuit and then to a differential amplifier, which creates a voltage

in proportion to the vacuum applied.

A typical General Motors MAP sensor voltage varies from

0.88 to 1.62 at engine idle:



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 from the

MAP sensor on a sound engine at idle speed. See the following

chart that shows engine load, engine vacuum, and MAP.

Engine

Load

Manifold

Vacuum

Manifold

Absolute

Pressure

MAP Sensor

Volt Signal

Heavy

(WOT)

Low (almost

0 in. Hg)

High (almost

atmospheric)

High (4.6–4.8 V)

Light (idle) High

(17–21 in. Hg)

Low (lower than

atmospheric)

Low (0.8–1.6 V)

HARNESS

CONNECTOR

JUMPER

WIRES

BP/MAP

SENSOR

COM

mA A

DIGITAL MULTIMETER

RECORD

MAX MIN

3 4 5

MAXMIN

1500

RED

BLACK

FIGURE 15–3 MAP sensors use three wires: 1. 5-volt reference from the PCM 2. Sensor signal (output signal) 3. Ground. A DMM set to

test a MAP sensor. (1) Connect the red meter lead to the V meter terminal and the black meter lead to the COM meter terminal. (2) Select

DC volts. (3) Connect the test leads to the sensor signal wire and the ground wire. (4) Select hertz (Hz) if testing a MAP sensor whose

output is a varying frequency; otherwise, keep it on DC volts. (5) Read the change of voltage (frequency) as the vacuum is applied to the

sensor. Compare the vacuum reading and the frequency (or voltage) reading to the specifications. (Courtesy of Fluke Corporation)

CAPACITOR-CAPSULE MAP SENSOR A capacitor-

capsule is a type of MAP sensor used by Ford which uses two

ceramic (alumina) plates with an insulating washer spacer in the

center to create a capacitor. Changes in engine vacuum cause

the plates to deflect, which changes the capacitance. The elec-

tronics in the sensor then generate a varying digital frequency

output signal, which is proportional to the engine vacuum.

 SEE

FIGURE 15–3 .

 SEE FIGURE 15–4 for a scope waveform of a

digital MAP sensor. Also see the Ford MAP sensor chart.

MAP/BARO SENSORS 199

CERAMIC DISC MAP SENSOR The ceramic disc MAP

sensor is used by Chrysler and it converts manifold pressure into

a capacitance discharge. The discharge controls the amount of

voltage delivered by the sensor to the PCM. The output is the

same as the previously used strain gauge/Wheatstone bridge

design and is interchangeable.

 SEE FIGURE 15–5 . See the

Chrysler MAP sensor chart.

Ford MAP Sensor Chart

MAP Sensor

Output

Engine Operating

Conditions

Intake Manifold

Vacuum

156–159 Hz Key on, engine off 0 in. Hg

102–109 Hz Engine at idle (sea level) 17–21 in. Hg

156–159 Hz Engine at wide-open

throttle (WOT)

About 0 in. Hg

FIGURE 15–5 Shown is the electronic circuit inside a

ceramic disc MAP sensor used on many Chrysler engines.

The black areas are carbon resistors that are applied to the

ceramic, and lasers are used to cut lines into these resistors

during testing to achieve the proper operating calibration.

5.4

2.4

4.8

1.8

1.2

0.6

0.0

0 5 10 20

CH A: FREQUENCY (Hz)

MAP SENSOR (DIGITAL)

30 40 50

4515 25 35

109.2

4.2

3.6

3.0

FIGURE 15–4 A waveform of a typical digital MAP sensor.

If It’s Green, It’s a Signal Wire

Ford-built vehicles often use a green wire as the signal

wire back to the computer from the sensors. It may not

be a solid green, but if there is green somewhere on

the wire, then it is the signal wire. The other wires are

the power and ground wires to the sensor.

TECH TIP

Chrysler MAP Sensor Chart

Vacuum (in. Hg) MAP Sensor Signal Voltage (V)

0.5 4.8

1.0 4.6

3.0 4.1

5.0 3.8

7.0 3.5

10.0 2.9

15.0 2.1

20.0 1.2

25.0 0.5

PCM USES OF THE

MAP SENSOR

The PCM uses the MAP sensor to determine the following:



Load on the engine. The MAP sensor is used on a

speed density -type fuel-injection system to determine

engine load, and therefore the amount of fuel needed. On

engines equipped with a mass airflow (MAF) sensor, the

200 CHAPTER 15

: SEA LEVEL

NAL VOLTA

: 6,500 ALTITUDE

LOWER HIGHER

VACUUM (INCHES OF MERCURY)

20 16 12 8 4 0

3.0 V

ENGINE LOAD

FIGURE 15–6 Altitude affects the MAP sensor voltage.

Use the MAP Sensor as a Vacuum Gauge

A MAP sensor measures the pressure inside the in-

take manifold compared with absolute zero (perfect

vacuum). For example, an idling engine that has 20 in.

Hg of vacuum has a lower pressure inside the intake

manifold than when the engine is under a load and the

vacuum is at 10 in. Hg. A decrease in engine vacuum

results in an increase in manifold pressure. A normal

engine should produce between 17 and 21 in. Hg at

idle. Comparing the vacuum reading with the voltage

reading output of the MAP sensor indicates that the

reading should be between 1.62 and 0.88 volt or 109

to 102 Hz or lower on Ford MAP sensors. Therefore, a

digital multimeter (DMM), scan tool, or scope can be

used to measure the MAP sensor voltage and be used

instead of a vacuum gauge.

NOTE: This chart was developed by testing a

MAP sensor at a location about 600 feet above

sea level. For best results, a chart based on

your altitude should be made by applying a

known vacuum, and reading the voltage of

a known-good MAP sensor. Vacuum usually

drops about 1 inch per 1,000 feet of altitude.

TECH TIP

Vacuum (in. Hg) GM (DC volts) Ford (Hz)

0 4.80 156–159

1 4.52

2 4.46

3 4.26

4 4.06

5 3.88 141–143

6 3.66

7 3.50

8 3.30

9 3.10

10 2.94 127–130

11 2.76

12 2.54

13 2.36

14 2.20

15 2.00 114–117

16 1.80

17 1.62

18 1.42 108–109

19 1.20

20 1.10 102–104

21 0.88

22 0.66

MAP is used as a backup to the MAF, for diagnosis of

other sensors, and systems such as the EGR system.



Altitude, fuel, and spark control calculations. At key

on, the MAP sensor determines the altitude (acts as a

BARO sensor) and adjusts the fuel delivery and spark

timing accordingly:



If the altitude is high, generally over 5,000 feet

(1,500 m), the PCM will reduce fuel delivery and advance

the ignition timing.



The altitude is also reset when the engine is acceler-

ated to wide-open throttle and the MAP sensor is used

to reset the altitude reading.

 SEE FIGURE 15–6 .



EGR system operation. As part of the OBD-II stan-

dards, the exhaust gas recirculation (EGR) system must

be checked for proper operation. One method used by

many vehicle manufacturers is to command the EGR

valve on and then watch the MAP sensor signal. The

opening of the EGR pintle should decrease engine

vacuum. If the MAP sensor does not react with the

specified drop in manifold vacuum (increase in manifold

pressure), an EGR flow rate problem diagnostic trouble

code is set.



Detect deceleration (vacuum increases). The en-

gine vacuum rises when the accelerator is released,

which changes the MAP sensor voltage. When

deceleration is detected by the PCM, fuel is either

stopped or greatly reduced to improve exhaust

emissions.



Monitor engine condition. As an engine wears, the

intake manifold vacuum usually decreases. The PCM is

programmed to detect the gradual change in vacuum and

is able to keep the air-fuel mixture in the correct range.

If the PCM were not capable of making adjustments for

engine wear, the lower vacuum could be interpreted

as increased load on the engine, resulting in too much