DESCRIPTION
The Toyota Computer Control System (TCCS) is a computerized emission, ignition and fuel control system. The TCCS controls Electronic Fuel Injection (EFI), engine operation and lowers exhaust emissions while maintaining good fuel economy and driveability.
The Electronic Control Unit (ECU) controls the TCCS. The ECU contains preprogrammed information which has all data necessary for maintaining optimum ignition timing under all operating conditions. Input from the various data sensors allows the ECU to deliver spark at exactly the right moment. The ECU controls many engine related systems to constantly adjust engine operation. (Scheme 65)
The TCCS is primarily an emission control system, designed to maintain an ideal air/fuel ratio of 14.7:1 under all operating conditions. When an ideal air/fuel ratio is maintained, the catalytic converter can control Carbon Monoxide (CO), Hydrocarbons (HC), and Oxides of Nitrogen (NOx) emissions.
OPERATION
The TCCS consists of the following subsystems: Electronic Fuel Injection (EFI) system, Data Sensors, Electronic Control Unit (ECU), Electronic Spark Advance (ESA) system, Idle Speed Control (ISC) system, EGR Control, Electronic Controlled Transmission (ECT), Diagnostic System and catalytic converter.
ELECTRONIC FUEL INJECTION (EFI)
All models are equipped with a Bosch AFC fuel injection system. An electric fuel pump provides fuel to the fuel pressure regulator. From the regulator, fuel flows under pressure to the fuel injectors. The pressure regulator maintains uniform pressure differential at all times.
The constant pressure differential allows the ECU to control duration of injection. The ECU monitors engine running conditions and calculates injection duration for efficient engine operation.
The ECU activates all injectors at the same time. When the injectors are activated, they provide 1/2 the amount of fuel required for ideal combustion with each engine revolution. See FUEL INJECTION SYSTEM - BOSCH AFC article.
MR2 Computer Control System Schematic. Scheme 65
DATA SENSORS
Each sensor furnishes electrical impulses to ECU. The ECU computes fuel delivery and spark timing necessary to maintain desired air/fuel ratio and engine speed. Data sensors are interrelated with each other. Operation of each sensor is as follows
Airflow Sensor
This sensor is mounted within the airflow meter. The sensor measures airflow rate through the airflow meter and sends a proportionate electrical signal to ECU. The ECU uses airflow sensor information for controlling fuel injection duration and spark advance system.
Air Temperature Sensor
Sensor is mounted within the airflow meter. The sensor measures temperature of incoming air and sends an electrical signal to ECU. The ECU uses air temperature information for controlling fuel injection duration.
Throttle Position Sensor (TPS)
The throttle position sensor (TPS) is mounted on throttle body and is directly connected to throttle linkage. The sensor, a rotary potentiometer, signals ECU of changes in throttle valve position. This information is used for controlling fuel injection duration and idle speed control system.
Coolant Temperature Sensor
The coolant (water) temperature sensor is installed in intake manifold. This sensor is a thermistor which converts temperature of engine coolant to electrical signal for use by ECU. The ECU uses coolant temperature information for controlling fuel injection duration, spark advance system, idle speed control system and EGR system.
Oxygen Sensor
The oxygen sensor is installed in exhaust manifold. Output voltage of oxygen sensor varies with oxygen content of exhaust gases. The ECU uses exhaust gas oxygen content information for determining fuel injection duration.
Vehicle Speed Sensor (VSS)
This sensor is mounted in the instrument panel and is a component of the speedometer. Vehicle speed information is used by the ECU for cruise control and electronic control of automatic transmission.
Engine Speed
Engine speed signal information is received from the ignition coil. These signals are used by ECU for fuel injection duration control and spark advance system.
A/C Switch
A signal is sent to ECU when the air conditioner is activated. The ECU requires this input for controlling idle speed during A/C operation.
Neutral/Start Switch
A switch is installed on automatic transmission models to inform the ECU of transmission selection position. This information is used by the ECU to allow starter operation. This information is also relayed to the ECT computer. Manual transmission models are equipped with a clutch switch.
ELECTRONIC CONTROL UNIT (ECU)
The ECU controls all functions of the TCCS. The ECU consists of a printed circuit board enclosed within a metal box. The ECU receives signals from the data sensors and switches. These signals are processed by the ECU for controlling the EFI, ESA, ISC, EGR and ECT systems.
The TCCS has a fail-safe feature designed into the ECU. If the ECU malfunctions, a back-up circuit is activated to assist in getting the vehicle to a service facility. During this mode of operation, driveability will be minimal. The "CHECK ENGINE" light will also be lit when the computer malfunctions.
MR2 TCCS Component Locations. Scheme 66
ELECTRONIC SPARK ADVANCE (ESA) SYSTEM
The ECU contains preprogrammed information which has all the data necessary for maintaining optimum ignition timing under all operating conditions. Input from the various data sensors allows the ECU to deliver spark at exactly the right moment.
IDLE SPEED CONTROL (ISC) SYSTEM
Engine idle speed is controlled by the ECU. The ECU contains preprogrammed information which contains specific engine speed values for different engine operating conditions.
The ECU receives signals from such sensors as the coolant temperature sensor and air conditioner sensor. Using this information, the ECU transmits a command signal to the ISC valve in the throttle body.
The ISC valve opens and closes the idle air by-pass to control the amount of air which by-passes throttle valve. The greater the amount of air which is by-passed around the throttle valve, the greater the engine idle speed.
EGR CONTROL
The EGR control system prevents recirculation of exhaust gases when coolant temperature is below 135°F (57°C). The coolant temperature sensor constantly monitors and informs the ECU of coolant temperature. When the temperature is below the specified value, the ECU opens the Vacuum Switching Valve (VSV) to prevent EGR flow.
When coolant temperature exceeds the specified value, the ECU closes the VSV to allow EGR flow. The vacuum modulator opens at idle speeds to prevent EGR flow when coolant temperature exceeds the specified value.
ELECTRONIC CONTROLLED TRANSMISSION (ECT)
Automatic transmission models are equipped with an electronic control unit to control transmission operation. The ECU of the TCCS provides signals on engine operation to the ECU of the ECT system. This information is used by the ECT computer to prevent shift up to 3rd gear or overdrive during cold engine operation.
Note. The ECU of the TCCS is not interchangeable with the ECU of the ECT system.
DIAGNOSTIC SYSTEM
The ECU of the TCCS is equipped with a self-diagnostic system which detects system failures or abnormalities. When a malfunction occurs, the ECU lights the "CHECK ENGINE" light located on instrument panel. If vehicle is equipped Super Monitor Display, the diagnostic codes will be shown on display screen.
By analyzing various signals, the ECU detects system malfunctions which are related to various operating parameter sensors. The ECU stores failure codes associated with the detected failure until the diagnosis system is cleared. Clear the diagnosis system by removing the "EFI" fuse with ignition switch in the "OFF" position.
The "CHECK ENGINE" warning light on instrument panel informs driver that a malfunction has been detected. The light goes out automatically when malfunction has been cleared.
CATALYTIC CONVERTER
All models are equipped with 3-way catalytic converters. This type of converter changes CO, HC, and NOx exhaust emissions into carbon dioxide, water vapor and nitrogen gas. The converter is a monolithic type.
DIAGNOSIS
- Ensure that all engine systems NOT related to TCCS are fully operational. Do not proceed with testing until all other problems have been fixed. Always check fuses, fusible links and wire connectors before condemning the ECU.
- Enter diagnostic mode and record trouble codes. Exit diagnostic mode. If no trouble codes were displayed, go to «DIAGNOSTIC CIRCUIT CHECK»(/toyota/mr2/w10-1984-1990/remont/testing-diagnostics/#tccs-system) charts. Follow instructions given there.
- If no trouble codes were displayed after performing diagnostic circuit check, perform voltage and resistance checks. If trouble codes are displayed, perform tests to confirm cause of malfunction which set the corresponding trouble code.
- After any repairs are made, clear trouble codes and perform diagnostic circuit check. Normal system operation code should be displayed if repair solved cause of malfunction.
Electronic Control Unit Electrical Connector ID. Scheme 67
Note. Diagnostic charts appearing in this article are provided courtesy of Toyota Motor Sales, U.S.A., Inc.
DIAGNOSTIC CODE OUTPUT
The ECU stores component failure information for the TCCS under a related trouble code which can be recalled for diagnosis and repair. A malfunction is indicated by the "CHECK ENGINE" warning light blinking a number of times equal to the malfunction code indication, with a 2.5 second delay between each indication.
The diagnostic code series will be repeated as long as the engine check connector terminals T - E1 are shorted. In the event of a number of trouble codes, indication will begin from the smaller value and continue to the larger in order.
Entering Diagnostic Mode With Super Monitor Display
- Prior to entering diagnostic mode, ensure that engine is at normal operating temperature, battery voltage above 11 volts, transmission in "P" or Neutral, throttle valve fully closed and A/C off.
- Turn ignition switch to the "ON" position, but DO NOT start engine. Simultaneously push and hold in "SELECT" key and "INPUT M" key for at least 3 seconds. (Scheme 68) The letters "DIAG" will appear on screen.
- After a short pause, hold in "SET" key for at least 3 seconds. If system is normal, "ENG -OK" will appear on screen. If there is a malfunction, the code number for the malfunction will appear on the screen. In the event of 2 or more numbers, there will be a 3 seconds pause between each number.
Super Monitor Display. Scheme 68
Entering Diagnostic Mode Without Super Monitor Display
- Ensure battery voltage is above 11 volts, throttle valve is fully closed, transmission in "P" or Neutral position, all accessory off, and engine is at normal operating temperature.
- Turn ignition switch to "ON" position, but DO NOT start engine. Using a jumper wire, short terminals T - E1 of engine check connector. (Scheme 69)
- Read code(s) as indicated by number of flashes of "CHECK ENGINE" warning light. (Scheme 70) See «TROUBLE CODE IDENTIFICATION»(/toyota/mr2/w10-1984-1990/remont/testing-diagnostics/#tccs-system) chart in this article.
- The "CHECK ENGINE" warning light will blink a number of times equal to the malfunction code indication with a 2.5 second delay between each indication. The diagnostic code series will be repeated as long as engine check connector terminals T - E1 are shorted.
Entering Diagnostic Mode at Engine Check Connector (MR2). Scheme 69
Clearing Trouble Codes
After any repairs are performed, clear ECU memory of all stored trouble codes. To clear memory, turn ignition off and remove the "EFI" ("STOP" fuse on Corolla) fuse from fuse block for 30 seconds or more. The lower the ambient (outside) temperature, the longer the fuse must be left out. Replace fuse and exit diagnostic mode.
Exiting Diagnostic Mode
To exit diagnostic mode with Super Monitor Display, turn off ignition or push any button (except "SET") of Super Monitor Display to display time.
For diagnostic mode without Super Monitor Display, remove jumper wire from engine check connector. Turn ignition off. After cancellation, perform a road test to confirm that a normal code is now displayed.
| Code No. | Circuit Affected |
|---|---|
| 1 | System Normal |
| 2 | Airflow Meter Signal |
| 3 | Ignition Signal |
| 4 | Coolant Temp. Sensor Signal |
| 5 | Oxygen Sensor Signal |
| 6 | RPM Signal |
| 7 | Throttle Position Sensor Signal |
| 8 | Intake Air Temp. Sensor Signal |
| 10 | Starter Signal |
| 11 | Switch Signal |
TROUBLE CODE IDENTIFICATION
| Code No. | Probable Cause |
|---|---|
| 1 | System Normal |
| 2 | Airflow Meter or Circuit, ECU |
| 3 | Ignitor or Circuit, ECU |
| 4 | Coolant Temp. Sensor or Circuit, ECU |
| 5 | Oxygen Sensor or Circuit, ECU |
| 6 | Ignitor or Circuit, Distributor, Starter Signal Circuit, ECU |
| 7 | Throttle Position Sensor or Circuit, ECU |
| 8 | Intake Air Temp. Sensor or Circuit, ECU |
| 10 | Starter Relay or Circuit, Ignition Switch or Circuit, ECU |
| 11 | A/C Switch, Neutral/Start Switch, Idle Switch, ECU |
TROUBLE CODES & PROBABLE CAUSE
Example of Trouble Code Output for MR2. Scheme 70
TCCS Voltage Tests
The TCCS can be checked using a voltmeter. Before making voltage tests, make sure battery voltage is at least 11 volts. Perform voltage tests with harness connectors connected to ECU and ignition switch in "ON" position. Connect probes to each wire cavity of ECU connectors. If voltage values are not as specified, perform resistance tests.
TCCS Resistance Tests
The TCCS can be checked using an ohmmeter. Before making resistance tests, turn ignition off and disconnect wiring connectors at ECU. Insert ohmmeter probes into wiring connectors from wire side. Measure resistance at connector terminals, NOT at ECU terminals.
Airflow Meter
Turn ignition off. Disconnect wiring connector from airflow meter. Using an ohmmeter, measure resistance between each terminal. If resistance values do not meet specifications shown in table, replace airflow meter.
Measuring Airflow Meter Resistance. Scheme 71
| Terminals | Condition | Ohms |
|---|---|---|
| E1 - Fc | Flap Closed | Infinity |
| E2 - THA | 4°F (-20°C) | 10,000-20,000 |
| E2 - THA | 32°F (0°C) | 4000-7000 |
| E2 - THA | 68°F (20°C) | 2000-3000 |
| E2 - THA | 104°F (40°C) | 900-1300 |
| E2 - THA | 140°F (60°C) | 400-700 |
| E2 - Vb | N/A | 200-400 |
| E2 - Vc | N/A | 100-300 |
| E2 - Vs | Flap Open | 200-1200 |
AIRFLOW METER RESISTANCE SPECIFICATIONS
Turn ignition off and disconnect electrical connector at TPS. Insert a thickness gauge between throttle stop screw and throttle lever. Using an ohmmeter, check resistance values. (Scheme 72) If values are not as specified in table, adjust or replace TPS.
Checking Throttle Position Sensor (MR2). Scheme 72
| Throttle Clearance | Terminals | Ohmmeter Reading |
|---|---|---|
| 0" (0 mm) | VTA - E2 | 200-800 |
| .014" (.35 mm) | IDL - E2 | 2300 or Less |
| .023" (.59 mm) | IDL - E2 | Infinity |
| Fully Open | VTA - E2 | 3300-10,000 |
| Fully Open | Vcc - E2 | 3000-7000 |
TPS RESISTANCE SPECIFICATIONS
