On-Board Diagnostics Operation

The SMEC is equipped with a self-diagnostic system that detects system failures or abnormalities. The self-diagnostic system is divided into 2 categories: faults requiring immediate attention and faults which do not require immediate attention.

Faults requiring immediate attention cause the SMEC to substitute the failed value with a calculated value, to store a trouble code in memory, and to turn on the "CHECK ENGINE" light to alert the driver that service is required. This is the "LIMP-IN" mode of operation. In this mode, driveability characteristics are greatly affected. The following faulty circuits will cause the system to enter the "LIMP-IN" mode:

• MAP Sensor  - If MAP sensor signal is too low (below .02 volt), too high (above 4.9 volts), or not changing (no change from start to full throttle condition). The SMEC then uses the TPS signal and engine speed signal to create a substitute MAP signal. Under this condition, the AIS motor is commanded to its minimum position by the SMEC.
• TPS  - If throttle position signal is too low (below .16 volt), or too high (above 4.7 volts). The SMEC then uses MAP sensor signal to crate a substitute throttle position signal.
• CTS  - If coolant temperature signal exceeds 4.96 volts when engine is cold, or below 5.1 volts when engine is warm. The SMEC will set a fixed value of 150°F (66°C). The cooling fan will run continuously in this mode.
• Battery Voltage  - If battery voltage drops below 4 volts, or to 7 1/2-8 1/2 volts after engine has been running for at least one minute. The SMEC will operate the charging system at a fixed rate in this mode.
• Battery Voltage Too High  - If battery voltage exceeds the predetermined control voltage by one volt.

If these faults correct themselves during the engine operating cycle in which they were detached, the "CHECK ENGINE" light will be turned off and the system will return to normal operation. The fault code will remain in memory for diagnostic evaluation. If the fault does not occur after 20-40 engine starts, the code will be cleared from memory.

Faults that do not require immediate attention will cause a trouble code to be stored in memory. During diagnostics, these codes can be evaluated, and appropriate action can be taken. Fault codes will be retained in memory as long as the monitored circuit remains faulty. The code will be erased from memory only if the fault does not appear again within 20-40 engine starts.

As a bulb and system test, the "CHECK ENGINE" light will glow when the ignition switch is turned on and will remain on for 3 seconds.