What does the 3-4 synchronizer assembly do when shifting from third to fourth speed?

The function of the 3-4 synchronizer assembly during a shift from third to fourth gear is primarily to facilitate the engagement of the fourth gear by matching the speed of the mainshaft to that of the countershaft gear. When shifting, the synchronizer assembly helps to align the speeds of the components involved so that the gear engagement can occur smoothly and without grinding.

Specifically, in the context of a shift from third to fourth, the 3-4 synchronizer engages to speed up the countershaft gear, ensuring it is rotating at a compatible speed with the mainshaft. This synchronization is crucial because if speeds are not adequately matched, it can lead to difficulty in shifting and increased wear on the transmission components.

Consequently, while the mainshaft does continue to rotate, it is the role of the synchronizer assembly to speed up the countershaft gear for a successful shift to fourth gear. The correct answer highlights this essential function of the synchronizer, emphasizing its role in enabling the shift without causing damage or wear to the transmission.