I have a 1991 G wagon, the previous owner has put the om606 engine on it. The issue i have is that since i bought it the differential lock wont lock in. If i press the button the yellow light comes on but the red light doesn't. I have changed the diaphragm in the transfer case but that didn't help. Does my g wagon have a auxiliary vacuum pump or does it take the vacuum from the engine? If it does take the vacuum from the engine then what does the switch do, what does it switch on?