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Fig. 16 Distributor Pick-Up—Typical
COOLANT TEMPERATURE SENSOR
The SBEC receives i n f o r m a t i o n regarding engine
operating t e m p e r a t u r e from t h e Coolant
Tempera-
t u r e Sensor (engine coolant temperature i s the same
as engine o p e r a t i n g temperature). The sensor is lo-
cated near t h e thermostat housing (Fig. 17).
The sensor is a variable resistor w i t h a range o f 5°
to 129°C (40°F t o 265°F). As coolant temperature var-
ies
t h e coolant
temperature
sensors
resistance
changes r e s u l t i n g i n a different i n p u t voltage t o t h e
SBEC.
The SBEC contains different spark advance curves
for different engine operating conditions. There are
unique spark advance schedules for cold and w a r m
engine operation t h a t have been added to reduce en-
gine emission a n d improve d r i v e a b i l i t y . T h e SBEC
w i l l
supply s l i g h t l y richer air-fuel mixtures a n d
h i g h e r idle speeds u n t i l n o r m a l operating tempera-
tures are reached.
A l l spark advance testing should be done w i t h t h e
engine f u l l y w a r m e d .
MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR
The M A P sensor reacts to absolute pressure i n the
i n t a k e m a n i f o l d and provides a n i n p u t voltage to the
Single B o a r d Engine Controller (SBEC). A s engine
load changes m a n i f o l d pressure varies, causing t h e
M A P sensor o u t p u t voltage t o change r e s u l t i n g i n a
different i n p u t voltage t o the SBEC. The i n p u t volt-
age level supplies t h e SBEC w i t h information relat-
i n g t o a m b i e n t barometric pressure d u r i n g engine
start-up (cranking) a n d t o engine load w h i l e the en-
IGNITION SYSTEMS
8D - 7
gine 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 from other sensors t o adjust air-fuel m i x -
t u r e and i g n i t i o n t i m i n g .
The M A P sensor i s located on t h e t h r o t t l e body
(Fig. 18).
Fig. 18 MAP Sensor—3.9L
Engine
AUTO SHUTDOWN (ASD) RELAY
The A u t o Shutdown Relay (ASD) supplies b a t t e r y
voltage t o t h e fuel p u m p , fuel injector, i g n i t i o n coil,
and 0
2
sensor h e a t i n g element. T h e ground c i r c u i t
for t h e A S D relay is controlled b y t h e Single B o a r d
Engine Controller (SBEC). The SBEC controls t h e re-
lay operation b y s w i t c h i n g the ground c i r c u i t o n and
off.
The SBEC monitors t h e d i s t r i b u t o r pick-up s i g n a l
to determine engine speed and compute injector syn-
chronization. I f the SBEC does not receive a d i s t r i b -
u t o r signal w h e n the i g n i t i o n switch is i n the " R u n "
position i t w i l l n o t energize (not provide a ground)
t h e A S D relay, stopping the battery voltage supply t o
t h e fuel p u m p , fuel injectors, i g n i t i o n coil, a n d 0
2
sensor heater element.
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