This section will
give you a basic understanding of the TCCS (Toyota Computer-Controlled
System) functions.
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TCCS
The
TCCS uses an ECU (Electronic Control Unit) with a built in microprocessor.
The ECU stores such data as the optimum fuel injection duration (the duration
is the length of time the injectors are held open, the longer the duration
the more fuel that is injected), injection timing (when the injectors are
opened), ignition timing, and the correct idle speed for various running
conditions. The ECU is feed input from various sensors and then chooses
the data stored that is suitable for the current engine conditions, and
then sends the most suitable signals to the actuators. The TCCS can be
divided into three main sections, the sensors, the ECU and the actuators.
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TCCS
Block Diagram
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EFI
In
an Electronic Fuel Injection system fuel is delivered via fuel injectors.
The system works by monitoring various sensors which inform the ECU how
much fuel needs to be delivered. The EFI system in the ST185 uses a high
pressure fuel pump and a fuel pressure regulator mounted in the fuel rail
to keep the fuel pressure in the rail at a constant pressure. On problem
with turbocharged cars is that the boost has a negative effect on the pressure
when the injectors open. This problem is addressed by the use of fuel pressure
regulator that increases fuel pressure when the boost pressure increases.
A vacuum line runs form the intake manifold to the fuel pressure regulator,
when there is boost pressure in the intake manifold, the fuel pressure
regulator will increase fuel pressure by 1 psi for every 1 psi increase
in boost.
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Main
Components of EFI And Their Functions
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Air
Flow Meter
The air
flow meter detects intake air volume so the ECU can determine the basic
injection duration, and the basic ignition advance angle. The AFM uses
a potentiometer to detect air volume, the potentiometer is attached to
a flap in the AFM that opens more as air volume increases. The AFM operates
on a 5 volt supply from the ECU.
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Throttle
Position Sensor
The
TPS is mounted on on the throttle body. It converts the throttle opening
angle into a voltage, and the ECU uses this information to detect whether
the engine is idling or under heavy load. It also detects engine acceleration
by detecting the speed at which the throttle opening angle changes. The
TPS uses an idle contact to in inform the ECU when the throttle is completely
closed (idle) and a variable resistor, which linearly converts the throttle
opening angle into a voltage signal. The TPS operates on a 5 volt supply
from the ECU.
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Water
Temperature Sensor
The
Water Temperature Sensor informs the ECU of the coolant temperature, which
plays a role in ECU's adjustment of injector duration and ignition timing.
The sensor itself varies it's resistance with temperature, and as such
the ECU knows the coolant temperature at all times based on the voltage
returned to it by the Water Temperature Sensor.
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Intake
Air Temperature Sensor
The Intake
Air Temperature Sensor works in much the same way as the Water Temperature
Sensor, varying it's resistance with air temperature, which informs the
ECU of the incoming air temperature. This sensor is located within the
AFM.
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Vehicle
Speed Sensor
The Speed
Sensor informs the ECU of the vehicle's speed.
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Distributor
The Distributor
consists of three signals, that can be seperated into two categories.
First
G1 and G2
signals, which sense the standard crankshaft angle, allowing the ECU to
determine the position of each piston and thus determining injection timing,
which cylinder is to be fired, and ignition timing.
The
Ne signal informs the ECU of engine speed and crankshaft angle.
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Fuel
Injectors
Fuel
is injected into the intake port of each cylinder before the intake stroke
in accordance with injection signals from the ECU, via the Fuel Injectors.
A fuel injector is mainly made up of a needle valve, a plunger and a solenoid.
By applying voltage to the injector the solenoid engages and opens the
injector allowing fuel to be delivered.
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IGNITION
SWITCH
The
Ignition Switch (Starting Signal) determines whether or not the engine
is starting.
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Turbo
Pressure Sensor
This sensor
is responsible for informing the ECU of the turbocharging pressure in the
intake manifold, and also feeds the signal for the stock turbo pressure
gauge in the instrument cluster. The ECU uses this signal to determine
if an over boost situation occurs. As of date, I have not been able to
find out if the ECU uses the signal from Turbo Pressure Sensor to aid in
the determination of other parameters, such as fuel injection duration
and ignition advance.
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Knock
Sensor
A knock
sensor is used in the ST185 to allow the ECU to detect knocking (detonation
or pre-ignition). The sensor is screwed into the block and it listens for
knocking, if knock is detected, the ECU will retard the timing to save
the engine. There are many theories as to whether the ECU does anything
else, and also as to how much the timing is retarded. My personal belief
is that when knock is detected the ECU retards timing one degree at a time
for a certain number of degrees until there is no more knock, and then
will increase it until knock is detected, and then lower again, producing
a cycle, however if knock does not cease within a certain amount of degrees
of retard, it will retard it back to base, resulting in a loss of power
feeling.
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Oxygen
Sensor
The O2
sensor is responsible for informing the ECU of the density of oxygen in
the exhaust gases. By knowing the oxygen content of the exhaust the ECU
is able to determine the air-fuel mixture. The ECU uses signals from the
O2 sensor as one of the parameters to determine injection during what is
called closed loop mode. The ECU ignores the O2 sensor's signal during
open loop mode.
Closed
loop is used to keep the engine as close to a stoichiometric (14.7:1
air-fuel) ratio as possible, and is used when the engine is in low load
situations, i.e highway cruising.
Open
loop is used at WOT (wide open throttle) when the engine is under high
load, i.e accelerating, with throttle angle above a certain percentage.
Once in open loop the O2 sensor is ignored and a fixed fuel map is used
by the ECU. This fuel map is usually around 13:1 air-fuel ratio.
The O2 sensor is also ignored when it is cold.
The
O2 sensor is easily contaminated, and as such leaded gasoline should never
be used, as well as silicone sealants/gaskets that are not O2 safe.
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AIR
CONDITIONER SWITCH
The
A/C switch senses whether or not the A/C compressor is on or off.
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COLD
START INJECTOR
The Cold
Start Injector is used to inject extra fuel into the intake air chamber
during engine cranking to improve stability. The cold start injector is
turned on when the coolant temperature is around 20oC or below, and is
controlled by the ECU until the coolant warms up to around 40oC.
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ECU
The ECU
first determines the basic injection duration based on the intake air volume
signal and the engine rpm signal. It then determines the injection duration
actually required by the engine by adjusting the basic injection duration
based upon the signals from various engine sensors, about the operating
conditions of the engine. At the same time the ECU determines the basic
ignition advance angle from engine speed and the air intake volume, and
adjusts this angle depending on signals from various other sensors.
The
ECU sends the appropriate signals, based on various sensor inputs, to the
igniter. The igniter is responsible for shutting off primary current resulting
in the spark plugs igniting and sends an ignition conformation signal back
to the ECU after ignition.
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EFI
Block Diagram
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ESA
(Electronic Spark Advance)
In order to maximize engine
power, the air-fuel mixture must be ignited when the maximum combustion
pressure occurs, which is about 10o after TDC. However, the time from ignition
of the air-fuel mixture to the development of maximum combustion pressure
varies depending on engine rpm and the manifold pressure, ignition must
occur earlier when engine speed is higher and later when engine speed is
lower. For the ST185, boost pressure also influences the point at which
ignition should occur. The ESA system provides the engine with nearly ideal
ignition timing. The ECU determines ignition timing from its internal memory,
which contains optimum ignition timing data for each engine condition,
and then sends the appropriate signals to the igniter.
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Main
Components of ESA And Their Functions
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ESA
Block Diagram
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This page
was created by Dennis Heath.
If you should
wish to ask a question about the GT-Four/All-Trac, you can join the GT-Four
Mailing List, where I, and many others with GT-Four's, might have your
answer. For information on joining go to http://www.onelist.com
Please note that
I am not a mechanic by trade, and that any information offered on this
web page is free and without guarantee. Should you choose to perform any
of the procedures listed on this site, you will be doing so of your own
free will, and I will not be held responsible or liable for any damages
that might occur from using information obtained here.