Downshift lock (722.5) - RA27057229330X(27-9330)

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General

Transmission 722.5 features a downshift lock to protect the engine and transmission against incorrect operation of the shift lever.

A 4 → 3 or 3 → 2 selector lever downshift is inhibited in the upper engine speed range (kickdown downshift limit exceeded) by means of the downshift lock. The downshift lock is controlled hydraulically by the governor pressure and the mechanical lock is moved into place below the detent plate by a locking lever.

Function

Two additional functions are added to the previous N → R locking function of the locking piston (65) for reverse gear. In other words, the locking piston may extend in 3 instead of in only one stage. A 2-stage stepped locking lever (152), which is connected by a linkage (150) to the reverse gear locking piston (65), moves into place below the detent plate (82). This prevents a selector lever downshift being performed.

A manual downshift is only possible again once the vehicle speed has dropped below a certain level and the reverse gear locking piston moves the locking lever out of its respective locking position.

Fig 1: Identifying Downshift Lock

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

5 → 4 Downshift lock (electronically controlled)

From a certain vehicle speed, 5th gear also remains engaged if a braking shift from position "D" into "4" is performed with the selector lever. This is controlled by the 5-speed automatic transmission control unit (N15/1).

There are two versions of the control unit, one for vehicles with a mechanical speedometer and one for vehicles with an electronic speedometer. These two versions must not be mixed up other wise the 5 → 4 downshift lock will not operate.