BMW V8 No-Reverse – 5HP24 (A5S-440Z) Automatic Transmission, 540i, 740i, 840i, X5
This answer is applicable for many BMWs in addition to the one listed below.
The car has a very delayed shift into reverse, maybe about 30 seconds to engage.
I will have no ability to reverse until it engages and until then it seems the car is in neutral. Reverse will work on flat ground, but not from a dead stop uphill. If I start the car and immediately shift into reverse and give gas, the motor just spins and no go. If I put the car into reverse and wait a short while (maybe 30 seconds to a minute and when the car is warm it would be closer to the 30 seconds mark), I can feel a slight movement when the transmission actually engages and then I can give gas and the car will move backwards if only minimal force is required. If I were on a hill, or even a slight incline, the car would not go in reverse.
Loss of reverse in this ZF manufactured transmission is, unfortunately, not an uncommon occurrence. The typical fault is failure of the F-Clutch actuating piston. More specifically, the inner and outer sealing rings that are molded to the inside and the outside edges of the piston. As the piston’s seals fail, the clutch pack is not compressed fully and starts to slip. This slippage causes wear to the clutch material. Eventually, the clutch pack does not have enough clamping force and friction to move the vehicle. This shows as a partial or full loss of reverse.
The “fix” is to perform a full rebuild using a master overhaul kit that includes not only the clutch plates, but the intermediate steel plates that go between each clutch plate. The reason the kit with the steel plates is recommended is due to the F-clutch being fully worn out. The original steel plates will have more wear than typical for a normal rebuild. Obviously, the F-clutch piston must be replaced as well, due to the worn-out and/or damaged seals.