1991 MITSUBISHI MONTERO coolant temperature

input device usage on specific models, see appropriate wiring diagram
in M - WIRING DIAGRAMS.
Air Conditioner Switch
When A/C is turned on, signal is sent to ECU. With engine at
idle, ECU increases idle speed through Idle Speed Control (ISC) motor.\
Airflow Sensor
Incorporated in airflow sensor assembly, airflow sensor is a
Karmen Vortex-type sensor which measures intake airflow rate.
Intake air flows through tunnel in airflow sensor assembly.
Airflow sensor transmits radio frequency signals across direction of
incoming airflow, downstream of vortex. Intake air encounters vortex,
causing turbulence in tunnel.
Turbulence disrupts radio frequency, causing variations in
transmission. Airflow sensor converts frequency transmitted into a
proportionate electrical signal which is sent to ECU.
Airflow Sensor Assembly
Mounted inside air cleaner, incorporates airflow sensor,
atmospheric pressure sensor and intake air temperature sensor.
Atmospheric (Barometric) Pressure Sensor
Incorporated in the airflow sensor assembly, converts
atmospheric pressure to electrical signal which is sent to ECU. ECU
adjusts air/fuel ratio and ignition timing according to altitude.
Coolant Temperature Sensor
Converts coolant temperature to electrical signal for use by
ECU. ECU uses coolant temperature information for controlling fuel
enrichment when engine is cold.
Crankshaft Angle & TDC Sensor Assembly
Assembly is located in distributor on SOHC engines. On DOHC
engines, which use Direct (or Distributorless) Ignition System (DIS)\
,
assembly is separate unit mounted in place of distributor. Assembly
consists of triggering disc (mounted on shaft) and stationary optical
sensing unit. Camshaft drives shaft, triggering optical sensing unit.
ECU determines crank angle and TDC based on signals received from
optical sensing unit.
Detonation Sensor (Turbo Only)
Located in cylinder block, senses engine vibration during
detonation (knock). Sensor converts vibration into electrical signal.
ECU retards ignition timing based on this signal.
Engine Speed (Tach Signal)
ECU uses ignition coil tach signal to determine engine speed.
Idle Position Switch
On all DOHC engines and Sigma 3.0L, idle position switch is
separate switch mounted on throttle body. On all other models, idle
position switch is incorporated in ISC motor or throttle position
sensor, depending on vehicle application. When throttle valve is
closed, switch is activated. When throttle valve is at any other
position, switch is deactivated. This input from idle position switch
is used by ECU for controlling fuel delivery time during deceleration.
Ignition Timing Adjustment Terminal
Used for adjusting base ignition timing. When terminal is
grounded, ECU timing control function is by-passed, allowing base
timing to be adjusted.

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

problem symptoms. For model-specific Trouble Shooting,
refer to DIAGNOSTIC, or TESTING articles available in the
section(s) you are accessing.
BASIC HEATER SYSTEM TROUBLE SHOOTING CHART









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CONDITION POSSIBLE CAUSE








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Insufficient, Erratic,
or No Heat
Low Coolant Level

Incorrect thermostat.

Restricted coolant flow through
heater core.


Heater hoses plugged.

Misadjusted control cable.

Sticking heater control valve.

Vacuum hose leaking.

Vacuum hose blocked.

Vacuum motors inoperative.

Blocked air inlet.

Inoperative heater blower motor.

Oil residue on heater core fins.

Dirt on heater core fins.








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Too Much Heat
Improperly adjusted cables.

Sticking heater control valve.

No vacuum to heater control valve.

Temperature door stuck open.








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Air Flow Changes During
Acceleration
Vacuum system leak.

Bad check valve or reservoir.








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Air From Defroster At All
Times
Vacuum system leak.

Improperly adjusted control cables.

Inoperative vacuum motor.








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Blower Does Not Operate
Correctly
Blown fuse.

Blower motor windings open.

Resistors burned out.

Motor ground connection loose.

Wiring harness connections loose.

Blower motor switch inoperative.

Blower relay inoperative.

Fan binding or foreign object
in housing.


Fan blades broken or bent.








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BRAKES
BRAKE SYSTEM TROUBLE SHOOTING
NOTE: This is GENERAL information. This article is not intended
to be specific to any unique situation or individual vehicle
configuration. The purpose of this Trouble Shooting
information is to provide a list of common causes to
problem symptoms. For model-specific Trouble Shooting,
refer to SUBJECT, DIAGNOSTIC, or TESTING articles available
in the section(s) you are accessing.

Low fuel pressure Test pressure regul-
ator and fuel pump,
check for restricted
lines and filters
No distributor reference Repair ignition
pulses system as necessary
Open coolant temperature Test sensor and
sensor circuit wiring
Shorted W.O.T. switch in Disconnect W.O.T.
T.P.S. switch, engine
should start
Defective ECM Replace ECM
Fuel tank residual pressure Test for fuel
valve leaks pressure drop after
shut down









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Hard Starting Disconnected hot air tube Reconnect tube and
to air cleaner test control valve
Defective Idle Air Control Test valve operation
(IAC) valve and circuit
Shorted, open or misadjusted Test and adjust or
T.P.S. replace T.P.S.
EGR valve open Test EGR valve and
control circuit
Poor Oxygen sensor signal Test for shorted or
circuit
Incorrect mixture from PCV Test PCV for flow,
system check sealing of oil
filter cap









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Poor High Speed Low fuel pump volume Faulty pump or
Operation restricted fuel
lines or filters
Poor MAP sensor signal Test MAP sensor,
vacuum hose and
wiring
Poor Oxygen sensor signal Test for shorted or
open sensor or
circuit
Open coolant temperature Test sensor and
sensor circuit wiring
Faulty ignition operation Check wires for
cracks or poor con-
nections, test
secondary voltage
with oscilloscope
Contaminated fuel Test fuel for water