Description of On-Board Diagnostics

CIRCUIT OPERATION
The ECU will substitute the input signals for the engine temperature sensor and the throttle sensor circuits if they fail. It will also substitute the voltage signal to the fuel pump circuit. This will allow the vehicle to driven until repairs can be made.
NOTE: The engine will operate at reduced capacity.

Input / Output Signal Line:

C.P.U. MALFUNCTION OF E.C.U. AND CRANK ANGLE SENSOR MALFUNCTION
The fail-safe system makes it possible to start the engine when a malfunction is detected in the E.C.U.'s C.P.U. circuit. In previous model years, starting was difficult under the above conditions. The fail-safe system now used makes starting is possible. Crank angle sensor fail-safe operation is implemented when, all of the following conditions are met for several seconds.
1. No 120° pulse signal (reference signal) detected, or 1° signal (position signal) is equivalent to 0 rpm.
2. Ignition in start position.
3. System voltage greater than 10 volts with ignition on.
4. Neutral switch on or inhibitor switch in "P" or "N" position.
5. With ignition switch in the start position, battery voltage is at least 1 volt lower than with switch in the on position.
When a malfunction is detected, the "Check Engine Light" comes on to warn the driver that a malfunction has occurred.

Fig. 69 Fail-Safe Operation:

AIR FLOW METER MALFUNCTION
If the ECU senses that the air flow meter output voltage is out of the normal range it will substitute the throttle sensor signal for the correct signal. Engine rpm will be limited to 2000 in order to inform the driver that the ECU is in the "Fail-Safe Mode".

Engine Temperature Sensor Fail-Safe Operation:

ENGINE TEMPERATURE SENSOR MALFUNCTION
When the ECU senses that the engine temperature output voltage is out of the normal range, it will substitute voltage at preset values. Refer to image for operating conditions.

NOTE: The radiator fan will operate on HIGH speed if a faulty temperature sensor condition exists.

FUEL PUMP CIRCUIT MALFUNCTION
When the ECU senses that the output circuit to the fuel pump is open or shorted it will activate the fuel pump relay. This will supply battery voltage directly to the fuel pump.

DETONATION SENSOR MALFUNCTION
When an abnormal output signal from the detonation sensor is detected by the ECU, it is judged to be malfunctioning. The ignition timing will then be retarded according to the operating conditions.