This page contains various technical information on the Toyota Celica GT-Four ST185. Click on any of the below for information on that subject.


ECU:This refers to the electronic control unit, and it is the heart of an electronic fuel injected car. The ECU receives signals from various sensors in the engine, and it uses this information to control fuel delivery and ignition timing. The ECU also controls various other things such as VSV's and the ISC. The ECU is also commonly referred to as the "computer".

VSV: Stands for Vacuum Switching Valve and is basically the same a solenoid valve. 12 volts is sent to the VSV to engage it, and switched off to release it. The various VSV's are controlled by the ECU.

ISC: Stands for Idle Speed Controller. The ISC is located under the throttle body and is responsible for keeping the idle speed at a constant level. The ISC is controlled by the ECU.

T-VIS:Stands for Toyota Variable Induction System, click here for more info.

TPS:Stands for Throttle position Sensor, click here for more info.

AFM:Stands for Air Flow Meter, click here for more info.

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Diagnostics And Check Engine Light

The ST185 is equipped with "check engine light" that will illuminate should there be a problem detected by the ECU. When this happens a fault code is stored in the ECU's memory that will describe what caused the check engine light to illuminate. There is a very simple procedure for checking the fault codes, which is listed below.

Locate the Diagnostic Box in the engine bay, it will be near the left hand side of the firewall (at the back of the engine bay). Flip open the lid, and on the inside of the lid you will see a list showing the location of each terminal. Look for TE1 and E1, and then using a short piece of wire bent in a "U" shape, bridge the terminals TE1 and E1. Next switch on the ignition (you don't have to actually start the car, but you can). Now look at the check engine light and you will notice it flashing. An ECU with no fault code stored will cause the check engine light to flash at constant pace. If there is a fault code stored the light will blink with a serious of pauses, allowing you to read the code. All fault codes are 2 digit numbers. It is possible for more than one fault code to be stored at a time. Below is an example of how a check engine light would flash if codes 12, and 43 were stored. The "......" represent the length of time between flashes, and the "#" represents the flashes.


Once you take your time and study the check engine light, it is easy to read the fault codes.

To clear fault codes, you can either remove the EFI fuse, in the fuse box next to the battery, for a few seconds, or you can disconnect the negative cable from the battery.

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List Of Fault Codes

11 ECU (+B) Momentary interruption in power to ECU
RPM Signal No NE or G signal to ECU within 2 seconds after engine has been cranked.
Ignition Signal  No NE signal to ECU when engine speed is above 1,000 rpm
Ignition Signal No IGF signal to ECU 8-11 times in succession
Oxygen Sensor/Heater cct Detection of oxygen sensor deterioration or open or short in oxygen sensor heater.
Water Temp Signal Open or short in water temp. sensor signal (THW).
Intake Air Temp. Sensor Open or short in intake air temp. sensor signal (THA).
Air-fuel Ratio Lean Malfunction Oxygen sensor outputs a lean signal continuously for several seconds during air-fuel ratio correction. Open or short in oxygen sensor (OX).
Air-fuel Ratio Rich Malfunction Oxygen sensor outputs a rich signal continuously for several seconds during air-fuel ratio correction.
Air-Flow Meter Signal Open cct in VC signal or short cct between VS and E2 when idle contacts are closed.
Air-Flow Meter Signal Open cct in E2 or short cct between VC and VS.
Turbocharging Pressure Signal Fuel cut-off due to high turbocharging pressure.
Turbocharging Pressure Sensor Signal Open or short in turbocharging sensor pressure signal (PIM).
Throttle Position Sensor Signal Open or short in throttle position sensor signal (VTA).
Vehicle Speed Sensor Signal No SPD signal for 8 seconds when engine speed is between 2,500rpm and 6,000rpm and coolant temperature is below 80C (176F) except when racing the engine.
Starter Signal No STA signal to ECU until engine speed reaches 800rpm with vehicle not moving.
Knock Sensor Signal Open or short in knock sensor signal (KNK).
Knock Sensor Control Signal in ECU Knock control in ECU faulty.
Switch Signal No IDL signal or A/C signal to ECU, with check terminals TE1 and E1 shorted.

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Description For ECU Connections

The diagram below shows the three connectors for the ECU and the details for locating the various input and output wires.

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Basic Information About A Turbocharger

A turbo is a mechanical devise that utilizes exhaust gases to create forced induction (boost). It is made up of a few main parts the  turbine, the impeller (compressor), and the wastegate. The turbine is where it all starts, fast moving hot exhaust gases spin the turbine, which is in turn connected to the impeller by means of a shaft. The impeller in turn spins, and compresses the air to produce more air in a given volume of space. By doing this a greater amount of air can enter the combustion chamber, increasing the volumetric efficiency of the engine. In simpler terms, more air means more fuel, which means a bigger bang. However if this processes were to go unchecked, too much boost may be produced for the engine to handle, to stop this from happening a device  called a wastegate is used. The wastegate operates by allowing exhaust gases to bypass the turbine at a set pressure. In other words if the wastegate was rated at 12psi, when the boost reaches this level, the wastegate would open, and as the boost starts to fall below 12 psi it would close. The wastegate uses an actuator connected to a diaphragm which is feed boost from the impeller side of the turbo, when the boost reaches a certain level the actuator will open the wastegate. Another component which is not a mechanical part of a turbocharger, but is often used with such a system, is an intercooler. An intercooler is a device used to reduce the temperature of the charge air that the turbo has produced during the compression process, as such it is placed between the turbo and the intake manifold. Reducing the temperature of the charge air increases the density of the air and also helps to reduce
the chances of detonation.

Check out Kip Anderson's Turbo Basics for some good info on turbos, and if you're interested in learning a lot more check out Garrett Turbochargers' About Charge Air Systems.

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How The Turbocharger Pressure Is Controlled

As mentioned in the Basic Information About Turbocharger section, the turbo utilizes a wastegate to control the boost, but on many turbo cars, a solenoid valve is also used. On the ST185 the solenoid valve is called the Turbo VSV, and the computer uses this to allow a boost increase above the wastegate rating. When conditions are favorable the ECU opens the VSV allowing some boost pressure to bleed away from the wastegate diaphragm, thus allowing higher boost pressures to be achieved. The stock boost pressure for the ST185 is about 8.5 psi (approx. 0.60 bar), although in the higher gears boost pressure will get as high 10 psi (approx. 0.70 bar). The wastegate is rated at approx. 7 psi (approx. 0.50 bar).

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Toyota Variable Induction System

Referred to as the T-VIS, this system is used to make air speed in the intake runners optimal for both low rpm and high rpm speeds.

If the diameter of the intake manifold is made larger, the intake resistance is smaller, so high output can be obtained in the high engine speed range. However, in the low engine speed range, since the speed of the intake air becomes low, the intake efficiency drops and high output can't be obtained. However if the diameter of the intake manifold is made smaller, high output can be obtained in the low engine speed range, but in the high engine speed range, the intake resistance will become high, and high output will not be available.

The T-VIS solves this problem by varying the area of the intake air path, when running at low and high engine speeds. Instead of the usual four intake runners, there are eight (four pairs), and in one of the runners in each pair there is a control valve. At engine speeds below 4,300rpm, the control valves remain closed, and at speeds of 4,300rpm and above, the control valves open.

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Knock Control

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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Checking Ignition Timing

The base ignition timing for the ST185 is 10o BTDC. A mistake that can be made when checking ignition timing is to not bridge out the ECU, using the TE1 and E1 terminals, see the Diagnostics and Check Engine Light section for information on how to do this. Doing this will stop the ECU from advancing timing and the timing light will be able to read the true timing of the car. Setting the timing is the same as with most cars, slacken the distributor bolts, and slowly turn the distributor until the timing mark on the crankshaft pulley lines up with the 10o BTDC mark.

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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

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.