1991 MITSUBISHI MONTERO oil

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).

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.

Fig. 2: Detection Switch Testing Locations (Montero)
Courtesy of Mitsubishi Motor Sales of America.
REMOVAL & INSTALLATION
Removal
1) Remove negative battery cable. Remove transfer case skid
plate (if equipped). Scribe alignment marks and remove both drive
shafts. Drain oil from transfer case. Disconnect wiring harness from
back-up light switch, all 4WD switches and any other electrical
connectors (if equipped).
2) Disconnect speedometer cable from drive. Unclip cable from
case. Place transfer case shifter in "2H" position and transmission in
Neutral. Remove 6 bolts holding control lever assembly. Remove control
lever assembly and gasket.
3) Remove select plunger bore plug at right side of case.
Remove select spring and plunger. Remove change shifter spring pin.
Remove change shifter. Remove transfer case mount. Remove 4 bolts and
2 nuts holding transfer case to adapter. Remove transfer case from
vehicle.

Installation
1) Position transmission shifter in Neutral and transfer case
lever in "2H" position. Install neutral return plungers and springs in
holes on top of adapter. Tighten plug until it is flush with adapter
surface. Cover plug threads with sealant.
2) Coat inside of change shifter with grease. Ensure change
shifter pin protrudes 1/8" above change shifter when installed. Mount
detent plunger spring and install plug (if equipped). To complete
installation, reverse removal procedure. Fill transfer case with API
GL-4 or higher 75W-90 or 75W-85W gear oil.
Fig. 3: Exploded View Of Transfer Case (Ram-50)
Courtesy of Mitsubishi Motor Sales of America.

2) Remove 2 seal plugs. Remove 2 poppet springs and steel
balls. Shift transfer case to "4WD" and pull high-low shift rail out,
in rear cover direction. Remove interlock plunger. Remove pulse
generator (if equipped), front output shaft cover and pulse rotor (if\
equipped). Remove snap ring from rear bearing on output shaft. Remove
chain cover, oil guide and side cover. Remove countershaft locking
plate. Remove countershaft.
3) Remove countergear, 2 thrust washers, 2 needle bearings
and spacer through side cover opening. Remove snap ring, spring
retainers and spring from 2WD-4WD shift rail. Remove front output
shaft, rear output shaft and chain as assembly.
4) Remove 2WD-4WD shift fork and distance piece. Drive out
2WD-4WD shift rail spring pin. Remove 2WD-4WD shift rail and lug.
Remove high-low shift fork and high-low synchronizer sleeve. Remove
needle bearing and snap ring from input gear. Remove input gear
assembly.
5) If either control shaft or input gear oil seals are to be
replaced, drive out roll pin from transmission control change shifter.
Separate transfer case from adapter. See Fig. 3.
Disassembly (Montero)
1) Remove dynamic damper, 5 detection switches and 3 steel
balls. Remove poppet plug, spring and steel ball. Remove speedometer
gear assembly. Remove output shaft rear cover, spacers, dust seal
guard and oil seals. Front output shaft cover, wave spring and spacer
(if equipped).
2) Remove high-low shift rail plug and high-low shift fork
spring pin. Shift transfer case to "4WD". Remove rear cover and high-
low shift rail. Remove interlock plunger. Remove rear output shaft
(viscous coupling) assembly and center differential assembly. Remove
2WD-4WD synchronizer assembly, chain and front output shaft from
transmission as a unit. With White paint make match marks in grooves
of 2WD-4WD synchronizer in 3 places. With White paint make match marks
on spline projections of 2WD-4WD synchronizer sleeve in 3 places.
Refer to match marks during reassembly.
3) Remove snap ring, spring seat, spring, 2WD-4WD shift fork
and 2WD-4WD synchronizer sleeve. Remove differential lock hub, oil dam
cover and bearing retainer. Remove side cover and gasket. Remove
transfer counter gear shaft, transfer counter gear, thrust washer,
needle bearings and spacer. Remove transfer drive shaft assembly.
Remove high-low shift fork assembly and clutch sleeve. Remove transfer
input gear assembly.
4) Remove 2WD-4WD shift lug spring pin, spring, spring
retainer 2WD-4WD shift lug and shift rail. Remove input gear oil seal,
baffle plate, dust seal guard and front output shaft oil seal. See
Fig. 4 .
COMPONENT DISASSEMBLY & REASSEMBLY
OUTPUT SHAFT ASSEMBLY (MONTERO)
Disassembly & Reassembly (Rear Shaft)
Remove snap ring and press ball bearing off without putting
pressure on vicious coupling. Remove viscous coupling. To install,
reverse removal procedure. When installing new snap ring, select
thickest ring that will fit into groove. Acceptable clearance is 0-.
003" (0-.08 mm).
Disassembly & Reassembly (Front Shaft)
Remove front bearing. Remove rear bearing. To install reverse
removal procedure.

Disassembly
Remove snap ring, 2WD-4WD synchronizer hub and synchronizer
spring. Remove outer synchronizer ring, synchronizer cone and inner
synchronizer ring. Remove drive sprocket and needle bearing from front
drive pinion. Inspect inner and outer synchronizer rings and cone for
excessive wear. Install inner and outer synchronizer rings and cone
onto drive sprocket. Measure distance between drive sprocket and outer
synchronizer. If distance is less than .0118" (.300 mm) replace
synchronizer parts as a set.
Reassembly
Install drive sprocket and needle bearing onto front drive
pinion. Apply transfer case gear oil to contacting surfaces of inner
and outer synchronizer rings and synchronizer cone. Install inner and
outer synchronizer rings and synchronizer cone. Install synchronizer
spring, 2WD-4WD synchronizer hub and snap ring to complete assembly.
When installing new snap ring, select thickest ring that will fit into
groove. Acceptable snap ring clearance is 0-.003" (0-.080 mm).
Fig. 8: Exploded View Of 2WD-4WD Synchronizer (Montero)
Courtesy of Mitsubishi Motor Sales of America.
TRANSFER CASE REASSEMBLY
NOTE: ALWAYS replace all gaskets, oil seals, snap rings and spring
pins with new parts. Coat both sides of gaskets and bolt
threads with sealant. Lubricate all sliding and rotating
parts with transfer case gear oil before assembling.
Reassembly (Ram-50)