1991 MITSUBISHI MONTERO ECU

Inhibitor Switch (Automatic Transmission Only)
Inhibitor switch senses position of transmission select
lever, indicating engine load due to automatic transmission
engagement. Based on this signal, ECU commands ISC motor to increase
throttle angle, maintaining optimum idle speed.
Intake Air Temperature Sensor
Incorporated in airflow sensor assembly, this resistor-based
sensor measures temperature of incoming air and supplies air density
information to ECU.
Motor Position Sensor (MPS)
Incorporated in ISC motor (or separate unit on some models),
senses ISC motor plunger position and sends electrical signal to ECU.
Oxygen (O2) Sensor
Located in exhaust system, generates an output voltage.
Output voltage varies with oxygen content of exhaust gas stream. ECU
adjusts air/fuel mixture based on signals from oxygen sensor.
Power Steering Oil Pressure Switch
Detects increase in power steering oil pressure. When power
steering oil pressure increases, switch contacts close, signalling
ECU. ECU commands ISC motor, raising idle speed to compensate for drop
in engine RPM due to power steering load.
TDC Sensor
See CRANKSHAFT ANGLE & TDC SENSOR ASSEMBLY.
Throttle Position Sensor (TPS)
A variable resistor mounted on throttle body. ECU uses
voltage signal received from TPS to determine throttle plate angle.
Vehicle Speed Sensor
Located in speedometer in instrument cluster, uses a reed
switch to sense speedometer gear revolutions. ECU uses gear
revolutions to determine vehicle speed.
OUTPUT SIGNALS
NOTE: Vehicles are equipped with different combinations of
computer-controlled components. Not all components listed
below are used on every vehicle. For theory and operation on
each output component, refer to the system indicated in
brackets after component.
CHECK ENGINE Light
See SELF DIAGNOSTIC SYSTEM.
EGR Control Solenoid Valve
See EXHAUST GAS RECIRCULATION (EGR) CONTROL under EMISSION
SYSTEMS.
Fuel Injectors
See FUEL CONTROL under FUEL SYSTEM.
Fuel Pressure Control Solenoid Valve (Turbo Only)
See FUEL DELIVERY under FUEL SYSTEM.
Fuel Pressure Regulator
See FUEL DELIVERY under FUEL SYSTEM.
Fuel Pump Relay (MPI Control Relay)

See FUEL DELIVERY under FUEL SYSTEM.
Idle Speed Control Servo
See IDLE SPEED under FUEL SYSTEM.
Power Transistor(s) & Ignition Coils
See IGNITION SYSTEMS.
Purge Control Solenoid Valve
See EVAPORATIVE CONTROL under EMISSION SYSTEMS.
Self-Diagnostic Connector
See SELF-DIAGNOSTIC SYSTEM.
Wastegate Control Solenoid Valve
See TURBOCHARGED ENGINES under AIR INDUCTION SYSTEM.
FUEL SYSTEM
FUEL DELIVERY
Electric fuel pump (located in gas tank) feeds fuel through
in-tank fuel filter, external fuel filter (located in engine
compartment) and fuel injector rail.
Fuel Pump
Consists of an impeller driven by a motor. Pump has an
internal check valve to maintain system pressure and a relief valve to
protect the fuel pressure circuit. Pump receives voltage supply from
Multi-Point Injection (MPI) control relay.
Fuel Pressure Control Solenoid Valve (Turbo Only)
Prevents rough idle due to fuel percolation. On engine
restart, if engine coolant or intake air temperatures reach a preset
value, ECU applies voltage to fuel pressure control solenoid valve for
2 minutes after engine re-start. Valve opens, allowing atmospheric
pressure to be applied to fuel pressure regulator diaphragm. This
allows maximum available fuel pressure at injectors, enriching fuel
mixture and maintaining stable idle at high engine temperatures.
Fuel Pressure Regulator
Located on fuel injector rail, this diaphragm-operated relief
valve adjusts fuel pressure according to engine manifold vacuum.
As engine manifold vacuum increases (closed throttle), fuel
pressure regulator diaphragm opens relief valve, allowing pressure to
bleed off through fuel return line, reducing fuel pressure.
As engine manifold vacuum decreases (open throttle), fuel
pressure regulator diaphragm closes valve, preventing pressure from
bleeding off through fuel return line, increasing fuel pressure.
FUEL CONTROL
Fuel Injectors
Fuel is supplied to engine through electronically pulsed
(timed) injector valves located on fuel rail(s). ECU controls amount\
of fuel metered through injectors based upon information received from
sensors.
IDLE SPEED
Air Conditioner Relay
When A/C is turned on with engine at idle, ECU signals ISC

motor to increase idle speed. To prevent A/C compressor from switching
on before idle speed has increased, ECU momentarily opens A/C relay
circuit.
Idle Speed Control (ISC) Motor
Controls pintle-type air valve (DOHC engines) or throttle
plate angle (SOHC engines) to regulate volume of intake air at idle.
During start mode, ECU controls idle intake air volume
according to coolant temperature input. After starting, with idle
position switch activated (throttle closed), fast idle speed is
controlled by ISC motor and fast idle air control valve (if equipped).\
When idle switch is deactivated (throttle open), ISC motor
moves to a preset position in accordance with coolant temperature
input.
When automatic transmission (if equipped) is shifted from
Neutral to Drive, A/C is turned on or power steering pressure reaches
a preset value, ECU signals ISC motor to increase engine RPM.
Fast Idle Air Control Valve
Some models use a coolant temperature-sensitive fast idle air
control valve, located on throttle body, to admit additional intake
air volume during engine warm-up. Control valve closes as temperature
increases, restricting by-pass airflow rate. At engine warm-up, valve
closes completely.
IGNITION SYSTEMS
DIRECT IGNITION SYSTEM (DIS) - DOHC ENGINES
Ignition system is a 2-coil, distributorless ignition system.
Crankshaft angle and TDC sensor assembly, mounted in place of
distributor, are optically controlled.
Power Transistors & Ignition Coils
Based on crankshaft angle and TDC sensor inputs, ECU controls
timing and directly activates each power transistor to fire coils.
Power transistor "A" controls primary current of ignition coil "A" to
fire spark plugs on cylinders No. 1 and 4 at the same time. Power
transistor "B" controls primary current of ignition coil "B" to fire
spark plugs on cylinders No. 2 and 3 at the same time.
Although each coil fires 2 plugs at the same time, ignition
takes place in only one cylinder since the other cylinder is on its
exhaust stroke when plug fires.
ELECTRONIC IGNITION SYSTEM - SOHC ENGINES
Mitsubishi breakerless electronic ignition system uses a disc
and optical sensing unit to trigger power transistor.
Power Transistor & Ignition Coil
Power transistor is mounted inside distributor with disc and
optical sensing unit. When ignition is on, ignition coil primary
circuit is energized. As distributor shaft rotates, disc rotates,
triggering optical sensing unit. ECU receives signals from optical
sensing unit. Signals are converted and sent to power transistor,
interrupting primary current flow and inducing secondary voltage.
IGNITION TIMING CONTROL SYSTEM
Ignition timing is controlled by ECU. ECU adjusts timing
based upon various conditions, such as engine temperature, altitude
and detonation (turbo vehicles only).

EMISSION SYSTEMS
EXHAUST GAS RECIRCULATION (EGR) CONTROL
Federal (Non-Turbocharged)
To lower oxides of nitrogen (NOx) exhaust emissions, a non-
computer controlled exhaust gas recirculation system is used. EGR
operation is controlled by throttle body ported vacuum. Vacuum is
routed through thermovalve to prevent EGR operation at low engine
temperatures.
Spring pressure holds EGR valve closed during low vacuum
conditions (engine idling or wide open throttle). When vacuum pressure\
increases and overcomes EGR spring pressure, EGR valve is lifted and
allows exhaust gases to flow into intake manifold for combustion.
California & Turbocharged
ECU controls EGR operation by activating EGR control solenoid
valve according to engine load. When engine is cold, ECU signals EGR
control solenoid valve to deactivate EGR.
California models are equipped with an EGR temperature
sensor. When EGR malfunction occurs, EGR temperature decreases and ECU
illuminates CHECK ENGINE (malfunction indicator) light.
EGR Control Solenoid Valve
Denies or allows vacuum supply to EGR valve, based upon ECU
commands.
Thermovalve
Denies or allows vacuum supply to EGR valve based on coolant
temperature.
EVAPORATIVE CONTROL
Fuel evaporation system prevents fuel vapor from entering
atmosphere. System consists of a special fuel tank with vapor
separator tanks (if equipped), vacuum relief filler cap, overfill
limiter (2-way valve), fuel check valve, thermovalve (if equipped),
charcoal canister, purge control valve, purge control solenoid valve
and connecting lines and hoses.
Purge Control Solenoid Valve
When engine is off, fuel vapors are vented into charcoal
canister. When engine is warmed to normal operating temperature and
running above idle, ECU energizes purge control solenoid valve,
allowing vacuum to purge valve.
Canister vapors are then drawn through purge valve into
intake manifold for burning. Purge control solenoid valve remains
closed during idle and engine warm-up to reduce HC and CO emissions.
HIGH ALTITUDE CONTROL (HAC)
This system compensates for variations in altitude. When
atmospheric (barometric) pressure sensor determines vehicle is above
preset altitude, ECU compensates by adjusting air/fuel mixture and
ignition timing. If HAC system is inoperative, there will be an
increase in emissions.
PCV VALVE
Positive Crankcase Ventilation (PCV) valve operates in the
closed crankcase ventilation system. Closed crankcase ventilation

system consists of PCV valve, oil separator, breather and ventilation
hoses.
PCV valve is a one-way check valve, located in valve cover.
When engine is running, manifold vacuum pulls PCV valve open, allowing
crankcase fumes to enter intake manifold. If engine backfires through
intake manifold, PCV valve closes to prevent crankcase combustion.
SELF-DIAGNOSTIC SYSTEM
Self-diagnostic system monitors input and output signals. On
all models, codes can be read using analog voltmeter. On some models,
scan tool can be used to read codes. For additional information, see G
- TESTS W/ CODES article.
CHECK ENGINE Light
Also called Malfunction Indicator Light by manufacturer,
comes on when ignition is turned on. Light remains on for several
seconds after engine has started. If an abnormal input signal occurs,
light comes on and code is stored in memory. If an abnormal input
signal returns to normal, ECU turns light off but code remains stored
in memory until cleared. If ignition is turned on again, light will
not come on until ECU detects malfunction during system operation.
NOTE: ECU diagnostic memory is retained by direct power supply
from the battery. Memory is not erased by turning off
ignition but is erased if battery or ECU is disconnected.

1) Install input gear and front output shaft oil seals into
transfer case housing. Pack grease between lips of seals and press
seal circumference uniformly.
2) Install input gear assembly in transfer case. See Figs. 3
and 6. Input gear assembly snap ring is available in selective
thicknesses. Use thickest snap ring that will fit into input shaft
groove. Allowed snap ring clearance is 0-.0024" (0-.060 mm).
3) Insert needle bearing onto rear output shaft assembly.
Install high-low synchronizer sleeve and shift fork. Install 2WD-4WD
shift fork. Engage chain securely on front and rear output shaft
sprockets. Assemble 2WD-4WD synchronizer sleeve with 2WD-4WD shift
fork. Install assembly over 2WD-4WD shift rail. Install front and rear
output shafts with chain as an assembly.
4) Install 2WD-4WD shift lug, distance piece, 2WD-4WD shift
rail and spring pin. Ensure slit in spring pin is in line with 2WD-4WD
shift rail. Install 2 spring retainers with spring on 2WD-4WD shift
rail. Install snap ring to end of 2WD-4WD shift rail. See Fig. 3.
5) Insert 2 needle bearings and spacer into countergear.
Install one thrust washer at each end of countergear. Ensure tab on
thrust washers fits into groove of transfer case. Install countergear
shaft assembly with "O" ring.
6) Install side cover and gasket. Install oil guide. Apply
sealant to both sides of gasket and install gasket and chain cover.
Ensure oil guide end fits into chain cover opening. Fit snap ring into
groove of rear bearing on rear output shaft. Tighten bolts to
specification. See TORQUE SPECIFICATIONS .
7) Install interlock plunger. Shift 2WD-4WD shift rail to 4WD
position. Install high-low shift rail through high-low shift fork in
case. Install 2 poppet balls and springs. Install seal plugs. When
installing poppet springs, smaller end must be toward ball.
8) On models with pulse generator, install pulse rotor, wave
spring and spacer. Measure protrusion "A" of front output shaft rear
bearing and recess "B" of cover and calculate clearance. See Fig. 9.
If clearance is greater than .079" (2.0 mm), select and install spacer\
to bring clearance within specification. If clearance is less than .
079" (2.0 mm), use wave spring alone. Apply sealant to both sides of
gasket and install gasket and cover. Install pulse generator (if
equipped).
9) On all models, align high-low shift fork and shift rail
spring holes and drive in roll pin with punch. Roll pin should be
installed with slit on center line of shift rail. Install wave spring
on rear of rear output shaft bearing. Apply sealant to both sides of
rear cover gasket. Install gasket and cover.
10) Check output shaft end play. Measure protrusion "A" of
rear output shaft rear bearing and recess "B" of cover and calculate
clearance. Ensure end play is 0-.004" (0-.10 mm). Apply sealant to
both sides of gasket and install gasket and cover. See Fig. 9.
11) Install speedometer sleeve assembly in rear cover. Align
match mark on speedometer sleeve assembly in rear cover. See Fig. 10.
Align match mark on speedometer sleeve with mark on case according to
number of teeth on speedometer driven gear. Install speedometer driven
gear sleeve clamp and tighten bolt to specification. Install both 4WD
indicator light switches with steel balls. See TORQUE SPECIFICATIONS.

REMOVAL
1) Disconnect negative battery cable. Remove front exhaust
pipe. On Montero, remove transfer case shift lever knob, dust boot and
retainer plate or console. Remove transfer case gearshift assembly.
2) On all models, raise and support vehicle. Remove
undercarriage cover and/or skid plate(s). Drain transmission and
transfer case (if applicable). Place reference mark on drive shaft(s)\
and remove. Disconnect all external solenoid and switch connections.
3) Disconnect speedometer cable and control cables at
transmission. Remove starter and bellhousing cover. Place reference
mark on torque converter and drive plate for reassembly reference.
Remove torque converter bolts.
4) Disconnect transmission cooler lines. Remove oil filler
tube. Secure transmission on a jack. Raise transmission slightly to
take weight off mount. Remove crossmember-to-mount bolts and
crossmember.
5) Remove transfer case mounting bracket and mount (if
equipped). Remove transmission-to-engine mounting bolts. Carefully
lower transmission from vehicle.
CAUTION: Ensure torque converter is fully seated in transmission
before installation.
INSTALLATION
1) To install, reverse removal procedure. Tighten
transmission-to-engine bolts and torque converter-to-drive plate bolts
to specification. See TORQUE SPECIFICATIONS table at end of article.
Tighten mount bolts with weight of engine and transmission on mounts.
Ensure reference marks on drive shaft(s) and torque converter-to-drive\
plate align.
2) Apply sealant to transfer case gearshift assembly gasket
before installation. Coat transmission oil filler tube "O" ring with
transmission fluid before installation. Refill transmission fluid to
specified level. Adjust all control cables.
TORQUE SPECIFICATIONS
TORQUE SPECIFICATIONS








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Applications Ft. Lbs. (N.m)
FWD Models
Torque Converter-To-Drive Plate Bolt
Precis ....................................... 53-55 (72-76)
All Others ................................... 34-38 (46-52)
Transaxle-To-Engine Block Bolt
Mirage
8-mm Bolt ............................................ (1)
10-mm Bolt ................................. 22-25 (30-34)
12-mm Bolt ................................. 31-40 (42-54)
Eclipse & Galant
8-mm Bolt ............................................ (1)
10-mm Bolt ................................. 22-25 (30-34)
Precis
8-mm Bolt .................................. 22-25 (30-34)
10-mm Bolt ................................. 31-40 (42-54)
3000GT
Upper Coupling Bolts ............................. 54 (73)
Lower Coupling Bolts ............................. 65 (88)

3800 engines were suffering from exactly this. The point is that a
lack of detail could cause misdiagnosis.
As you might have guessed, a lab scope would not miss this.
RELATIONSHIP BETWEEN DWELL & DUTY CYCLE READINGS TABLE (1)









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Dwell Meter (2) Duty Cycle Meter
1
.................................................... 1%
15 .................................................. 25%
30 .................................................. 50%
45 .................................................. 75%
60 ................................................. 100%
( 1) - These are just some examples for your understanding.
It is okay to fill in the gaps.
( 2) - Dwell meter on the six-cylinder scale.









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THE TWO TYPES OF INJECTOR DRIVERS
OVERVIEW
There are two types of transistor driver circuits used to
operate electric fuel injectors: voltage controlled and current
controlled. The voltage controlled type is sometimes called a
"saturated switch" driver, while the current controlled type is
sometimes known as a "peak and hold" driver.
The basic difference between the two is the total resistance
of the injector circuit. Roughly speaking, if a particular leg in an
injector circuit has total resistance of 12 or more ohms, a voltage
control driver is used. If less than 12 ohms, a current control driver
is used.
It is a question of what is going to do the job of limiting
the current flow in the injector circuit; the inherent "high"
resistance in the injector circuit, or the transistor driver. Without
some form of control, the current flow through the injector would
cause the solenoid coil to overheat and result in a damaged injector.
VOLTAGE CONTROLLED CIRCUIT ("SATURATED SWITCH")
The voltage controlled driver inside the computer operates
much like a simple switch because it does not need to worry about
limiting current flow. Recall, this driver typically requires injector
circuits with a total leg resistance of 12 or more ohms.
The driver is either ON, closing/completing the circuit
(eliminating the voltage-drop), or OFF, opening the circuit (causing \
a
total voltage drop).
Some manufacturers call it a "saturated switch" driver. This
is because when switched ON, the driver allows the magnetic field in
the injector to build to saturation. This is the same "saturation"
property that you are familiar with for an ignition coil.
There are two ways "high" resistance can be built into an
injector circuit to limit current flow. One method uses an external
solenoid resistor and a low resistance injector, while the other uses
a high resistance injector without the solenoid resistor. See the left
side of Fig. 1.
In terms of injection opening time, the external resistor
voltage controlled circuit is somewhat faster than the voltage
controlled high resistance injector circuit. The trend, however, seems
to be moving toward use of this latter type of circuit due to its
lower cost and reliability. The ECU can compensate for slower opening