1993 CHEVROLET PLYMOUTH ACCLAIM temperature

(Fig. 9). This short circuits the electrodes. Spark
plugs with electrode gap bridging can be cleaned us-
ing standard procedures.
SCAVENGER DEPOSITS Fuel scavenger deposits may be either white or yel-
low (Fig. 10). They may appear to be harmful, but
are a normal condition caused by chemical additives
in certain fuels. These additives are designed to
change the chemical nature of deposits and decrease
spark plug misfire tendencies. Accumulation on the
ground electrode and shell area may be heavy but
the deposits are easily removed. Spark plugs with
scavenger deposits can be considered normal in con-
dition and be cleaned using standard procedures.
CHIPPED ELECTRODE INSULATOR A chipped electrode insulator usually results from
bending the center electrode while adjusting the
spark plug electrode gap. Under certain conditions,
severe detonation also can separate the insulator
from the center electrode (Fig. 11). Replace spark
plugs with chipped electrode insulators. PREIGNITION DAMAGE
Excessive combustion chamber temperature can
cause preignition damage. The center electrode dis-
solves first and the ground electrode dissolves some-
what later (Fig. 12). Insulators appear relatively
deposit free. Determine if the spark plug has the cor-
rect heat range rating for the engine, if ignition tim-
ing is over advanced or if other operating conditions
are causing engine overheating. The heat range rat-
ing refers to the operating temperature of a particu-
lar type spark plug. Spark plugs are designed to
operate within specific temperature ranges depend-
ing upon the thickness and length of the center elec-
trode and porcelain insulator.
SPARK PLUG OVERHEATING Overheating is indicated by a white or gray center
electrode insulator that also appears blistered (Fig.
13). The increase in electrode gap will be consider-
ably in excess of 0.001 in per 1000 miles of operation.
This suggests that a plug with a cooler heat range
rating should be used. Over advanced ignition tim-
Fig. 9 Electrode Gap Bridging
Fig. 10 Scavenger Deposits
Fig. 11 Chipped Electrode Insulator
Fig. 12 Preignition Damage
Ä IGNITION SYSTEMS 8D - 5

ing, detonation and cooling system malfunctions also
can cause spark plug overheating.
SPARK PLUG SERVICE
When replacing the spark plug and coil cables,
route the cables correctly and secure them in the ap-
propriate retainers. Failure to route the cables prop-
erly can cause the radio to reproduce ignition noise,
cross ignition of the spark plugs or short circuit the
cables to ground.
SPARK PLUG REMOVAL
Always remove the spark plug cable by grasping at
the spark plug boot turning, the boot 1/2 turn and
pulling straight back in a steady motion. (1) Prior to removing the spark plug spray com-
pressed air around the spark plug hole and the area
around the spark plug. (2) Remove the spark plug using a quality socket
with a rubber or foam insert. (3) Inspect the spark plug condition. Refer to
Spark Plug Condition in this section.
SPARK PLUG GAP ADJUSTMENT Check the spark plug gap with a gap gauge. If the
gap is not correct, adjust it by bending the ground
electrode (Fig. 6).
SPARK PLUG INSTALLATION (1) To avoid cross threading, start the spark plug
into the cylinder head by hand. (2) Tighten spark plugs to 28 N Im (20 ft. lbs.)
torque. (3) Install spark plug cables over spark plugs.
POWERTRAIN CONTROL MODULE (PCM)
The ignition system is regulated by the powertrain
control module (PCM) (Fig. 14). The PCM supplies
battery voltage to the ignition coil through the Auto
Shutdown (ASD) Relay. The PCM also controls the
ground circuit for the ignition coil. By switching the ground path for the coil on and off, the PCM adjusts
ignition timing to meet changing engine operating
conditions.
During the crank-start period the PCM advances
ignition timing a set amount. During engine opera-
tion, the amount of spark advance provided by the
PCM is determined by these input factors:
² coolant temperature
² engine RPM
² available manifold vacuum
The PCM also regulates the fuel injection system.
Refer to the Fuel Injection sections of Group 14.
DISTRIBUTOR PICK-UPÐPCM INPUT
The engine speed input is supplied to the power-
train control module (PCM) by the distributor pick-
up. The distributor pick-up is a Hall Effect device
(Fig. 15 or Fig. 16).
A shutter (sometimes referred to as an interrupter)
is attached to the distributor shaft. The shutter con-
tains four blades, one per engine cylinder. A switch
plate is mounted to the distributor housing above the
shutter. The switch plate contains the distributor
Fig. 14 Powertrain control module (PCM)
Fig. 15 DistributorÐ2.2L and 2.5L TBI Engines
Fig. 13 Spark Plug Overheating
8D - 6 IGNITION SYSTEMS Ä

pick-up (a Hall Effect device and magnet) through
which the shutter blades rotate. As the shutter
blades pass through the pick-up, they interrupt the
magnetic field. The Hall effect device in the pick-up
senses the change in the magnetic field and switches
on and off (which creates pulses), generating the in-
put signal to the PCM. The PCM calculates engine
speed through the number of pulses generated. On 2.5L MPI (flexible fuel AA-Body) engines, one
of the shutter blades has a window cut into it. The
PCM determines injector synchronization from the
window. Also, the PCM uses the input for detonation
control.
DISTRIBUTOR PICK-UPÐ3.0L ENGINE
The distributor pick-up provides two inputs to the
powertrain control module (PCM). From one input
the PCM determines RPM (engine speed). From the
other input it derives crankshaft position. The PCM
regulates injector synchronization and adjusts igni-
tion timing and engine speed based on these inputs. The distributor pick-up contains two signal gener-
ators. The pick-up unit consists of 2 light emitting
diodes (LED), 2 photo diodes, and a separate timing
disk. The timing disk contains two sets of slots. Each
set of slots rotates between a light emitting diode
and a photo diode (Fig. 17). The inner set contains 6
large slots, one for each cylinder. The outer set con-
tains several smaller slots. The outer set of slots on the rotating disk repre-
sents 2 degrees of crankshaft rotation. Up to 1200
engine RPM, the PCM uses the input from the outer
set of slots to increase ignition timing accuracy. The outer set of slots contains a 10 degree flat spot.
This area is not slotted (Fig. 17). The flat spot tells
the PCM that the next piston at TDC will be number
6. Each piston's position is referenced by one of the
six inner slots (Fig. 18). As each slot on the timing disk passes between the
diodes, they interrupt the beam from the light emit-
ting diode. This creates an alternating voltage in
each photo diode which is converted into on-off
pulses. The pulses are the input to the PCM. During cranking, the PCM cannot determine which
cylinder will be at TDC until the 10 degree flat spot
on the outer set of slots rotates through the optical
unit. Once the flat spot is detected, the PCM knows
piston number 6 will be the next piston at TDC. Since the disk rotates at half crankshaft speed, it
may take up to 2 engine revolutions during cranking
before the PCM determines the position of piston
number 6. For this reason the PCM energizes all six
injectors at the same time until it senses the position
of piston number 6.
COOLANT TEMPERATURE SENSOR
On 2.2L TBI, 2.5L TBI and 2.5L MPI engines, the
coolant temperature sensor is installed behind the
thermostat housing and ignition coil in the hot box
(Fig. 19). On 3.0L engines the sensor is located next
Fig. 16 DistributorÐ2.5L MPI (Flexible Fuel AA-Body)Fig. 17 Distributor Pick-upÐ3.0L Engine
Fig. 18 Inner and Outer Slots of Rotating DiskÐ3.0L Engine
Ä IGNITION SYSTEMS 8D - 7

to the thermostat housing (Fig. 20). The sensor pro-
vides an input voltage to the powertrain control mod-
ule (PCM). The sensor is a variable resistance
(thermistor) with a range of -40ÉF to 265ÉF. As cool-
ant temperature varies, the sensors resistance
changes, resulting in a different input voltage to the
PCM. The PCM contains different spark advance sched-
ules for cold and warm engine operation. The sched-
ules reduce engine emissions and improve
driveability. Because spark advance changes at dif-
ferent engine operating temperatures during warm-
up, all spark advance testing should be done with the
engine fully warmed. The PCM demands slightly richer air-fuel mixtures
and higher idle speeds until the engine reaches nor-
mal operating temperature. The coolant sensor input is also used for radiator
fan control.
MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR
The MAP sensor reacts to absolute pressure in the
intake manifold and provides an input voltage to the
powertrain control module (PCM). As engine load
changes, manifold pressure varies. The changes in
engine load causes the MAP sensors output voltage
to change. The change in MAP sensor output voltage
results in a different input voltage to the PCM. The input voltage level supplies the PCM with in-
formation relating to ambient barometric pressure
during engine start-up (cranking) and engine load
while its operating. The PCM uses this input along
with inputs from other sensors to adjust air-fuel mix-
ture. On 2.2L TBI, 2.5L TBI and 2.5L MPI (flexible fuel
AA-body) engines, the MAP sensor is mounted to the
dash panel (Fig. 21 or Fig. 22). On 3.0L engines, the
sensor is mounted to a bracket across from the dis-
tributor (Fig. 23). The sensor is connected to the
throttle body or intake manifold with a vacuum hose
and to the PCM electrically.
AUTO SHUTDOWN (ASD) RELAY AND FUEL PUMP
RELAY
The powertrain control module (PCM) operates the
auto shutdown (ASD) relay and fuel pump relay
through one ground path. The PCM operates the re-
lays by switching the ground path on and off. Both
relays turn on and off at the same time. The ASD relay connects battery voltage to the fuel
injector and ignition coil. The fuel pump relay con-
nects battery voltage to the fuel pump and oxygen
sensor heating element. The PCM turns the ground path off when the igni-
tion switch is in the Off position. Both relays are off.
When the ignition switch is in the On or Crank po-
sition, the PCM monitors the distributor pick-up sig-
Fig. 19 Coolant Temperature SensorÐ2.2L TBI, 2.5L TBI and 2.5L MPI Engines
Fig. 20 Coolant Temperature SensorÐ3.0L Engines
Fig. 21 MAP SensorÐ2.2L and 2.5L TBI Engines
8D - 8 IGNITION SYSTEMS Ä

2.2L TBI, 2.5L TBI, 2.5L MPI AND 3.0L IGNITION SYSTEMSÐDIAGNOSTIC PROCEDURES
INDEX
page page
Coolant Temperature Sensor Test ............ 13
Failure to Start TestÐ2.5L TBI and 3.0L Engines . 12
General Information ....................... 11
Ignition Coil ............................. 11 Manifold Absolute Pressure (MAP) Sensor Test . 13
Poor Performance Test .................... 13
Spark Plugs ............................ 11
Testing for Spark at Coil ................... 11
GENERAL INFORMATION
For additional information, refer to On Board Di-
agnostics in the Fuel Injection General Diagnosis
sections of Group 14. Also, refer to the DRBII scan
tool and appropriate Powertrain Diagnostic Proce-
dures Manual.
SPARK PLUGS
Faulty or fouled spark plugs may perform well at
idle speed, but frequently fail at higher engine
speeds. Faulty plugs can be identified in a number of
ways: poor fuel economy, power loss, decrease in en-
gine speed, hard starting and, in general, poor en-
gine performance. Spark plugs also malfunction because of carbon
fouling, excessive electrode air gap, or a broken insu-
lator. Refer to the General Information Section of
this group for spark plug diagnosis.
IGNITION COIL
The ignition coil is designed to operate without an
external ballast resistor. Inspect the coil for arcing. Test the coil according
to coil tester manufacturer's instructions. Test coil
primary and secondary resistance. Replace any coil
that does not meet specifications. Refer to the Coil
Resistance chart. If the ignition coil is replaced due to a burned
tower, carbon tracking, arcing at the tower, or dam-
age to the terminal or boot on the coil end of the sec-
ondary cable, the cable must be replaced. Arcing at the tower will carbonize the nipple which, if it is con-
nected to a new coil, will cause the coil to fail. If a secondary cable shows any signs of damage,
the cable should be replaced with a new cable and
new terminal. Carbon tracking on the old cable can
cause arcing and the failure of a new coil.
TESTING FOR SPARK AT COIL
WARNING: APPLY PARKING BRAKE AND/OR
BLOCK THE WHEELS BEFORE PERFORMING ANY
TEST WITH THE ENGINE RUNNING.
CAUTION: Spark plug cables may be damaged if
this test is performed with more than 1/4 inch clear-
ance between the cable and engine ground.
Remove the coil secondary cable from the distribu-
tor cap. Hold the end of cable about 6 mm (1/4-inch)
away from a good engine ground (Fig. 1). Crank the
engine and inspect for spark at the coil secondary ca-
ble. There must be a constant spark at the coil second-
ary cable. If the spark is constant, have a helper con-
tinue to crank engine and, while slowly moving coil
secondary cable away from ground, look for arcing at
the coil tower. If arcing occurs at the tower, replace
the coil. If spark is not constant or there is no spark,
proceed to the failure to start test. If a constant spark is present and no arcing occurs
at the coil tower, the ignition system is producing
the necessary high secondary voltage. However,
COIL RESISTANCE
Ä IGNITION SYSTEMS 8D - 11

(10) If no spark is produced, replace the ignition coil.
POOR PERFORMANCE TEST
To prevent unnecessary diagnostic time and
possible incorrect results, the Testing For Spark
At Coil procedure should be performed before
this test.
WARNING: APPLY PARKING BRAKE AND/OR BLOCK
THE WHEELS BEFORE PERFORMING ANY ENGINE
RUNNING TESTS.
Check and adjust basic timing (refer to the specifica-
tion section of this group and see service procedures).
COOLANT TEMPERATURE SENSOR TEST
(1) With key off, disconnect wire connector from
coolant temperature sensor (Fig. 6). (2) Connect one lead of ohmmeter to one terminal of
coolant temperature sensor. (3) Connect the other lead of ohmmeter to remaining
terminal of coolant temperature sensor. The ohmmeter
should read as follows;
² Engine/Sensor at normal operating temperature
around 200ÉF should read approximately 700 to 1,000
ohms.
² Engine/Sensor at room temperature around 70ÉF,
ohmmeter should read approximately 7,000 to 13,000
ohms. Refer to On Board Diagnostics in the General
Diagnosis section of Group 14. Also, refer to the
DRBII scan tool and the appropriate Powertrain
Diagnostic Procedures manual for additional
test procedures.
MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR
TEST
Refer to the DRB II scan tool and appropriate Pow-
ertrain Diagnostic Procedures manual for further test
procedures.
Fig. 5 60-Way Electrical Connector, Powertrain con- trol module
Fig. 6 Coolant Temperature Sensor Test
Ä IGNITION SYSTEMS 8D - 13

2.2L TBI, 2.5L TBI, 2.5L MPI AND 3.0L IGNITION SYSTEMSÐSERVICE PROCEDURES
INDEX
page page
Coolant Temperature Sensor ................ 14
Distributor Pick-UpÐ2.2L TBI, 2.5L TBI and 2.5L MPI Engines ....................... 18
Distributor ServiceÐ3.0L Engine ............. 18
DistributorÐ2.2L TBI, 2.5L TBI and 2.5L MPI Engines .............................. 17
Idle RPM TestÐ2.5L and 3.0L Engines ........ 16
Ignition CoilÐ2.2L TBI, 2.5L TBI and 2.5L MPI Engines ....................... 14 Ignition CoilÐ3.0L Engines
................. 15
Ignition Timing ProcedureÐ2.2L TBI, 2.5L TBI, 2.5L MPI, and 3.0L Engines ............... 16
Manifold Absolute Pressure (MAP) Sensor ServiceÐ2.5L TBI and 3.0L Engines ........ 22
Powertrain Control Module (PCM) ............ 14
Spark Plug Service ....................... 15
POWERTRAIN CONTROL MODULE (PCM)
The powertrain control module (PCM) is located
next to the battery (Fig. 1).
REMOVAL
(1) Remove air cleaner duct or air cleaner assem-
bly. (2) Remove battery.
(3) Remove PCM mounting screws.
(4) Remove 60-way wiring connector from the
PCM. (5) Remove PCM.
INSTALLATION
(1) Connect 60-Way electrical connector to PCM
(Fig. 1). (2) Install PCM. Tighten mounting screws.
(3) Install battery.
(4) Install air cleaner duct or air cleaner assembly.
COOLANT TEMPERATURE SENSOR
On 2.2L TBI, 2.5L TBI and 2.5L MPI (flexible fuel
AA-Body) engines, the coolant temperature sensor is
located behind the ignition coil (Fig. 2). On 3.0L en-
gines the sensor is located next to the thermostat
housing (Fig. 3).
REMOVAL
(1) Drain cooling system until coolant level is below
coolant sensor. Refer to Group 7, Cooling System. (2) Disconnect electrical connector from sensor.
(3) Remove sensor from engine.
INSTALLATION
(1) Install coolant sensor. Tighten 2.2L TBI, 2.5L
TBI or 2.5L MPI engine coolant sensor to 28 N Im (20
ft. lbs.) torque. Tighten the 3.0L engine coolant sensor
to7N Im (60 in. lbs.) torque.
(2) Connect electrical connector to sensor.
(3) Fill cooling system. Refer to Group 7, Cooling
System.
IGNITION COILÐ2.2L TBI, 2.5L TBI AND 2.5L MPI
ENGINES
The ignition coil mounts to the thermostat housing
(Fig. 4).
Fig. 1 Powertrain control module (PCM)
Fig. 2 Coolant Temperature SensorÐ2.2 TBI, 2.5L TBI and 2.5L MPI Engines
8D - 14 IGNITION SYSTEMS Ä

REMOVAL
(1) Disconnect the coil to distributor ignition cable
(Fig. 4). (2) Disconnect the wiring harness connector from
the coil. (3) Remove ignition coil mounting screws.
INSTALLATION
(1) Install ignition coil onto the bracket. Tighten
the screws to 9.5 N Im (85 in. lbs.) torque. (2) Connect the wiring harness connector.
(3) Connect the coil to distributor ignition cable.
IGNITION COILÐ3.0L ENGINES
The ignition coil is located at the back of the intake
manifold (Fig. 5).
REMOVAL
(1) Remove air cleaner assembly.
(2) Disconnect ignition cable from coil.
(3) Disconnect wiring harness connector from coil.
(4) Remove coil mounting screws.
INSTALLATION
(1) Loosely install ignition coil on intake manifold.
Tighten the intake manifold fastener to 13 N Im(115
in. lbs.) torque. Tighten ignition coil bracket fasteners
to 10 N Im (96 in. lbs.) torque.
(2) Connect the wiring harness connector.
(3) Connect the coil to distributor ignition cable.
(4) Install the air cleaner assembly. Tighten the air
cleaner fasteners to 25 N Im (225 in. lbs.) torque.
SPARK PLUG SERVICE
When replacing the spark plug and coil cables, route
the cables correctly and secure them in the appropriate
retainers. Incorrectly routed cables can cause the radio
to reproduce ignition noise. It can also cause cross
ignition of the spark plugs or short circuit the cables to
ground.
SPARK PLUG REMOVAL
Always remove cables by grasping at boot, rotating
the boot 1/2 turn, and pulling straight back in a steady
motion. (1) Prior to removing the spark plug spray com-
pressed air around the spark plug hole and the area
around the spark plug. (2) Remove the spark plug using a quality socket
with a rubber or foam insert.
Fig. 3 Coolant Temperature SensorÐ3.0L Engine
Fig. 4 Ignition CoilÐ2.2L TBI, 2.5L TBI and 2.5L MPI Engines
Fig. 5 Ignition CoilÐ3.0L Engine
Ä IGNITION SYSTEMS 8D - 15