Description of On-Board Diagnostics

***UPDATED BY TSB #EF90004 DATED MAY 1990
CIRCUIT OPERATION
The ECU will substitute the input signals for the air flow meter, engine and air temperature sensors throttle sensor, vehicle speed sensor and the neutral/inhibitor switch if the circuits fail. This will allow the vehicle to driven until repairs can be made.
The CHECK ENGINE lamp will illuminate (except when the vehicle speed sensor or neutral/inhibitor switch) when the ECU goes into the "Limp Home" (fail-safe) mode. It will remain on until the key is turned off.
NOTE: The engine will operate at reduced capacity.

Air Flow Meter Malfunction:

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 2400 in order to inform the driver that the ECU is in the "Fail-Safe Mode". Refer to the figure above for operating conditions.

Engine Temperature Sensor Malfunction:

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

AIR TEMPERATURE SENSOR MALFUNCTION
When the ECU senses that the air temperature sensor output voltage is out of the normal range, it will substitute voltage at preset values.

THROTTLE SENSOR MALFUNCTION
When the ECU senses that the throttle sensor voltage is out of the normal range it will substitute a voltage signal at a preset value.

VEHICLE SPEED SENSOR MALFUNCTION
When the ECU senses that the vehicle speed sensor or circuit is malfunctioning, it will reduce the engine rpm to 2000 rpm. After that it will limit engine rpm to 2500 rpm to inform the driver that the ECU is in the fail-safe mode.

NEUTRAL/INHIBITOR SWITCH MALFUNCTION
When the neutral or inhibitor switch is shorted and vehicle speed exceeds 37 mph (60 km/h) the ECU will limit engine rpm to 2500 rpm to inform the driver that the ECU is in the fail-safe mode.

***The original OE service manual did not include descriptions for the VSS and neutral/inhibitor switch fail-safe operation.