The water flow through the shower head and the tap at the same time, when I pull the diverter. When I release the diverter, it drops down an the water continue to flow only through the tap.
The bathtub mixer is a Grohe PA 1X 3164. You will find the model/type at the rear of the mixer (red ring)
Grohe PA 1X 3164 front.
To remove the diverter I used a small screwdriver to remove the lid from the diverter.Remove the shower hose as well.
After removing the lid you will see a 6,5 mm nut.
Undo the 6,5 mm nut and remove the parts below in the order from right to left.
Use a 17 mm spanner/wrench to remove the diverter from the mixer.
In this case: The problem was that the O-ring (red square) had been pushed into the diverter. The O-ring was ok, so I just put it back in place again. This diverter can be disassembled into several pieces. By pulling in the direction of the arrow, you will be able to remove/replace the coil springs.