problem … when I sometimes turn on the machine and enter the D or R gear, nothing happens. then I turn it back on and it works … or try a game with the gears and it works. what can it be? 
Would help to know what year your XJ40 is. Early cars (at least in North America) 1988-89 had fully hydraulic ZF gearboxes and beginning in 1990 they went to an electronically controlled unit in which internal solenoids operated the valves. Hydraulically controlled unit was a ZF HP22, electronically controlled ZF HP24. Both were four speed type with torque converter lock up feature.
1987… thanks for your attention
OK, I’m still a little confused. If the car is a 1987 in the North American market that would be considered a Series III XJ6, a completely different animal than a 1988 model year which was launched here in May 1987. To definitively determine what you have: is your shift quadrant laid out in a straight line or is it in the form of a “J” with 2nd and 3rd gears on the left side of the J?
It’s a xj40…in Europe begin in 1987 the first year of IV series
Max … you have the ZF 4HP 22 transmission which is purely hydro mechanical, no electronics involved.
Check under the car where the control cable is bolted to the transmission just before the control lever. There is a barrel at the bracket that the cable runs through. The rubber inside the barrel breaks down and allows the cable to slide back and forth instead of being held tight.
Very easy to check while you have someone move the gear shifter in the car.