What component controls dwell time or period?

The component that controls dwell time or period is primarily the ignition system's primary circuit, along with the engine control module (ECM) in modern vehicles.

In traditional ignition systems, the primary circuit consists of the ignition coil and associated components that manage the current flow to the coil. When the ignition system activates, the dwell time is the period during which the coil is charged before the ignition event occurs. This charging time must be carefully controlled to ensure that the coil develops sufficient energy to create a strong spark at the spark plug.

In modern vehicles, the ECM plays a crucial role in adjusting the dwell time to optimize engine performance, emissions, and fuel efficiency. The ECM takes various inputs like engine speed and load to dynamically adjust the dwell time to meet the specific needs of the engine during different operating conditions.

Since both the primary circuit and the ECM are involved in controlling the dwell time, the correct association in this case is that both the primary circuit and the ECM work together to provide the appropriate dwell period for efficient ignition system performance.