Bogdan, your English is fine. However, please turn off the Caps Lock on your computer. On the internet it looks like you are shouting.

As to the reserve light, I would look at the point where the wires plug into the sending unit. The light comes on when the float is at the bottom at closes the electric circuit. if the gauge shows the right amount, it cannot be the float, unless there is a loose piece of wire shorting the circuit.

As my problem-thanx, guys. the sending unit is the original. the gauge is not jumpy, but I notice that if I drive for a long didtance, the gauge is "less wrong" until a sharp turn or breaking. Then after the needle "dips", it does not come back. That seems to indicate a problem with the float. If it's an empty sealed plastic box, a pinhole could be a problem (simply siiting too low). However, the reserve light comes on when it should and NOT before, as it would with the low float. as you see, mixed symptoms- very frustrating!
BTW, I DID clean the contacts on the float.

__________________
1985 190D 2.2l Sold-to Brother-in-law
1996 Mustang 3.8l -"thinks it's a sports car"
1988 Grand Wagoneer - Sold (good home)
1995 Grand Cherokee Ltd -"What was I thinking??!!"