The logic module uses this motor to control the idle speed of the engine during idle and deceleration. 1984 to 1987 Turbo I engines (pull-through throttle body) use a standard, two-terminal DC motor and a planetary gear system to open and close the idle speed control valve. For 1986 and up TBI engines, 1988 and up Turbo I engines, and 1987 and up Turbo II engines, a four-terminal brushless stepper motor is used to move the idle speed control valve pintle. The logic module uses two drivers (or sets of drivers for 1984) to drive the two-terminal motor. 12V is supplied to the motor armature at short intervals and the polarity is switched to turn the motor the other way. Four drivers are used to drive the two field magnets on the four-terminal stepper motor. The fields are energized in sequence at switching polarities to rotate the shaft 1/4 turn at a time.
When the logic module is turning on or off any of the motor drivers, it momentarily checks the output of the drivers to see if they are responding the way they should. If it does not see a voltage when the driver is on (open in the driver circuit or short in the wiring) or it does see voltage when the driver is off (short in the driver circuit), a fault code 25 is stored. The logic module cannot detect an open circuit in the wiring. For 1988 and later models, a fault code 25 will be stored if the target idle speed is not reached after a number of attempts.
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Copyright © 1996-2003 Russ W. Knize.