Welcome to the forum both seroyan and mask.
Sadly I know little about the AL4 and the following is based on my knowledge of the AL4's big brother, the 4HP20, which i do know a little about and works on much the same principles...
Gear changing is acheievd by operating electrovales with modulated signals from the ECU which slowly engages and disengages brakes and clutches appropriate to the gear being changed. It does this by slowing increasing feed pressure to those to be engaged and slowly draining pressure away from those being released to ensure a smooth change. The electrovalves are not the whole story as each electrovalve works in conjunction with a pair of slide valves in the valve block. One to permit operation and one to hold the clutch/brake operated. These slide valves depend on the correct hydraulic pressure to operate correctly.
The problem you are experiencing can be caused by faulty electrovalves or it can equally be caused by a problem in the valve block and the slide valves relating to the clutch/brake in question. Often the problem with a slide valve is a broken preload spring.
Equally, it can be caused by low or improperly regulated main hydraulic pressure in the gearbox.
AL4s are known for valve block problems. It accessible under the big tin cover but the fluid must be drained to gain access.
Sadly, I cannot tell what electrovalve does what as my AL4 documentation gives no indication and is silent on all valve block details. In contrast, the 4HP20 documentation gives all this detail, including hydraulic diagrams.
In any case, the first task in diagnosing any AL4 problem should be a fluid change. Drain and refill only with Esso/Mobil LT71141 following the very precise instructions
here.
Have you had any diagnostics carried out to determine if the ECUs have any stored fault codes in them?
Sorry for the vague reply to this thread.
