Problems with handling GPS PPS interrupts on MEGA 2560

Dear All,

I have a GPS (ublox Neo 6M) and I made a wire from the LED to get the PPS. If I connect that wire to my Leonardo PIN 7 (INT4) then I can handle that interrupt (with RISING) but if I do the same on my MEGA 2560 PIN 19 (INT4) then it wouldn't work. I tried all interrupt pin's but none is working with my GPS's PPS on the MEGA 2560. If I connect y RTC's (DS3231) 1Hz output then it works so I know my code is ok. I know that the level is lower for the PPS (~2.8V) but it works on the Leonardo without shifting but on the MEGA 2560 not. Could anybody give me some advice how I can solve this issue?

Do you have a ground connection between the GPS and the Mega?