My praire 2002/650 does not lock into 4 wheel drive when I hit the switch. This is what I have done so far. I changed the oil in the front diff three times and it had a green tint to it, not like I have seen before, usually a little milky but not green tint, got that taken care of and then checked the (switch) with my multi-meter switch works fine.Still no 4x4. I saw no shavings in oil. Please help.