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14 - 20
FUEL SYSTEM
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magnet) t h r o u g h w h i c h the shutter blades rotate. A s
the s h u t t e r blades pass t h r o u g h the pick-up, t h e y i n -
t e r r u p t t h e magnetic field. The H a l l effect device i n
the p i c k - u p senses t h e change i n t h e magnetic
field
and switches on and off (creating pulses), generating
the i n p u t signal t o t h e SBEC. The SBEC calculates
engine speed t h r o u g h t h e number of pulses gener-
ated.
CAP
SWITCH PLATE
SHUTTER
BLADE
HOUSING
ROTOR
HALL
EFFECT
SWITCH
SHUTTER
BLADE
SHAFT
J9014-142
Fig. 4 Distributor Pick-Up—Typical
IDLE CONTACT SWITCH-SBEC INPUT
The idle contact switch is i n t e g r a l w i t h t h e idle
speed (ISC) actuator and provides a n i n p u t signal to
the SBEC ( F i g . 5). T h i s i n p u t enables t h e SBEC to
increase or decrease t h e throttle stop angle (by ex-
t e n d i n g or r e t r a c t i n g the ISC actuator) i n response to
engine operating conditions.
ISC
MOTOR
IDLE
CONTACT
SWITCH
GROUND
J9014-159
MANIFOLD ABSOLUTE PRESSURE (MAP)
SENSOR-SBEC INPUT
The M A P sensor reacts t o absolute pressure i n t h e
i n t a k e m a n i f o l d and provides a n i n p u t voltage t o t h e
Single B o a r d Engine Controller (SBEC). A s engine
load changes manifold pressure varies, causing t h e
M A P sensors resistance t o change. T h e change i n
M A P sensor resistance results i n a different i n p u t
voltage t o t h e SBEC. T h e i n p u t voltage level supplies
the SBEC w i t h i n f o r m a t i o n r e l a t i n g t o a m b i e n t baro-
metric pressure d u r i n g engine start-up (cranking)
and t o engine load w h i l e the engine is r u n n i n g . T h e
SBEC uses t h i s i n p u t along w i t h i n p u t s f r o m other
sensors t o adjust air-fuel m i x t u r e .
The M A P sensor is mounted o n t h e t h r o t t l e body
(Fig. 6) a n d is connected t o the t h r o t t l e body w i t h a
v a c u u m hose and to the SBEC electrically.
MAP
SENSOR
3-WAY ELECTRICAL
CONNECTOR
TO VACUUM
SOURCE
Fig. 5 Idle Contact Switch
8914-96
Fig. 6 Manifold Absolute Pressure (MAP) Sensor
Location
OXYGEN SENSOR (0
2
SENSOR)-SBEC INPUT
The 0
2
sensor is located i n t h e exhaust down pipe
on 3.9L engines (Fig. 7) and o n 5.2L a n d 5.9L e n -
gines above the left exhaust m a n i f o l d outlet (Fig. 8).
The 0
2
sensor provides a n i n p u t voltage t o the Single
Board Engine Controller (SBEC) r e l a t i n g the oxygen
content of the exhaust gas. The SBEC uses t h i s i n -
f o r m a t i o n t o fine t u n e the air-fuel r a t i o b y a d j u s t i n g
injector pulse w i d t h .
The 0
2
sensor produces voltages f r o m 0 to 1 v o l t ,
depending upon t h e oxygen content of t h e exhaust
gas i n the exhaust manifold. W h e n a large a m o u n t o f
oxygen is present (caused by a l e a n air-fuel m i x t u r e ) ,
the sensor produces a l o w voltage. W h e n there is a
lesser a m o u n t present (rich air-fuel m i x t u r e ) i t pro-
duces a higher voltage. B y m o n i t o r i n g t h e oxygen
content a n d converting i t to electrical voltage, t h e
sensor acts as a rich-lean switch.
The oxygen sensor is equipped w i t h a h e a t i n g ele-
m e n t t h a t keeps the sensor a t proper operating tern-
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