The logic module uses the coolant temperature sensor to monitor the engine coolant temperature and the rate of change of coolant temperature. This information is used to determine when the air/fuel mixture control circuit should enter closed-loop and if the radiator fan should be turned on or off (for 1985 and later models). The 1984 style sensor uses a thermistor/fixed resistor combination, which measures 5,290 ohms to 5,610 ohms at 77^F. The fixed resistor increases sensor accuracy at high temperatures. The 1985 and later sensor design is just the thermistor, which measures 9,120 ohms to 10,880 ohms at 77^F. In this case, the logic module uses an internal fixed resistor which it will switch in at higher temperatures for accuracy. This accuracy is needed so that the logic module can switch the fan on and off at high temperatures, but can also read very cold temperatures during cold start. The voltage on both sensor designs should read about 2.5V at 77^F.
For 1984 models, if the sensor output voltage is less than 0.157V or greater than 3.76V, fault code 22 is stored, the power loss light is turned on, and the logic module will enter limp-in mode, using the charge temperature sensor as an indication of the coolant temperature. For 1985 and later models, if the sensor output voltage is less than 0.51V or greater than 4.96V, fault code 22 is stored, the power loss light is turned on, and the logic module will enter limp-in mode and turn on the radiator fan. All models except 1988 and later Turbo I engines will use the charge temperature sensor as an indication of the coolant temperature. For 1988 and later models if the sensor output falls back into the acceptable range, the power loss light is turned back off and the module will exit limp-in mode (the fault code will remain stored).
For 1986 models, if the sensor indicates a temperature of -20^F to 212^F at engine startup, a 20 minute timer is started. If the coolant temperature is not at least 160^F when the timer expires, a fault code 17 is stored. For 1987 and later models, an 8 minute timer is started, 12 minutes after engine startup. If the vehicle speed is above 28 mph, the engine temperature is below 174^F, and a fault code 22 is not stored, a fault code 17 is stored.
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Updated 12/17/2003.
Copyright © 1996-2003 Russ W. Knize.