Which unit contains both a hold-in winding and a pull-in winding?

The starter solenoid is the unit that contains both a hold-in winding and a pull-in winding.

In the context of automotive electronics, the starter solenoid serves a critical function in starting the engine. It acts as a relay to engage the starter motor. When the ignition is turned on, current flows to the pull-in winding, creating an electromagnetic force that pulls the solenoid's plunger in, which connects the battery to the starter motor and initiates the cranking process.

Once the plunger has moved to its fully engaged position, the hold-in winding takes over to maintain the solenoid’s position while the engine is cranking. This design ensures that the starter motor receives continuous power while cranking the engine until the ignition switch is released. The distinction between the two windings is crucial; the pull-in winding operates initially for engagement, and the hold-in winding sustains that engagement during operation.

This functionality is not found in the other options. For instance, the field coil typically generates a magnetic field in brushless motors but does not contain both types of windings. The armature and ignition switch serve different roles entirely in the vehicle's electrical system and do not incorporate the same winding design as the starter solenoid.