Carter BBD Stepper Motor

BBD With Stepper Motor

The variable air bleeds consist of tapered metering pins positioned in orifices by the stepper motor. This drive mechanism moves the pins in defined steps in response to signals from the oxygen sensor located in the exhaust and processed by the electronic control unit. The stepper motor moves the pins until the exhaust sensor indicates that the desired air-fuel ratio has been reached. Thus the pin movement adjusts the air-fuel ratio to compensate for changes detected in the exhaust gases.

Stepper Motor & Air Metering Pins
The digital linear actuator or stepper motor moves the metering pins, .400 inches from full lean to full rich. Full movement requires 100 steps at .004 inches per step.
Fast speed is 100 steps per second, slow speed is 12 steps per second.
During the initial power-up of the stepper motor, the metering pins must be sent to an end stop to give the electronic control unit a stable reference. The metering pins must then be backed off to the desired position.
This initialization of the metering pins occurs in open-loop mode. When the ignition is turned on, and again when the starter is engaged, the metering pins move inward (rich position) 127 steps and outward (lean position) 35 steps. This locates the pins near the position to give the average air-fuel ratio for complete fuel combustion (stoichiometric ratio).

Coil Windings

The stepper motor incorporates a unipolar winding which has 2 coils wound on the same bobbin, per stator half, for a total of 4 coils.
A threaded shaft provides linear movement which is bidirectional. Movement and direction is controlled by motor phasing sequence.

Checking Stepper Motor

To check stepper operation, remove the air cleaner from the carburetor. The air metering pin for the high-speed circuit is visible looking into the air horn of the carburetor.
Turning the ignition switch on should cause the initialization of the air metering pins. If no movement is observed, check electrical connections and windings.

Checking Coils

Check each winding of the stepper motor by disconnecting the wiring harness from the stepper motor. With an ohmmeter, check each coil by connecting to the “C” terminal and to the ground terminal of each coil. There should be 78 ohms, plus or minus 25 ohms at room temperature.
Movement of the air metering pins can be accomplished by applying 12 volts to the “C” terminal and grounding the coils as per the phasing sequence.