Are you aligning the marks on the belt with the marks on the pulleys? You're supposed to take the tensioner off and put it in a vise and push the end back in and the put a pin in the tensioner to hold it. Then you bolt the tensioner back in, put the belt on, and release the pin. That's when you get your tension. You should be replacing your tensioner as that's the weak spot in your timing belt system, the new one will already be pushed back and have a pin in place.

1997 Trooper
2002 Rodeo