When you say the engine tries to turn over but wont, do you mean that it spins (cranks) but will not start?
What engine is in the van?
Other than the starter, what have you done or checked trying to fix this problem?
Ok there are 3 main things that have to happen to make it start.
1. It must have fuel
2. It must have spark
3. It must be turning
You can visibly verify that the engine is turning, so we are down to fuel and spark.
When you turn the key to "on" can you hear the fuel pump run for a couple seconds? If you cant hear it, take off the gas cap and listen down the filler neck while someone else turns the key. If you cant hear it run then you are most likely looking at a bad pump, pump relay or fuse.
If you can hear it run then you need to check for fuel going into the motor. If you have or can get a fuel pressure guage hook it up and verify there is fuel pressure at the fuel rail when the engine is cranking. If not then you probably have a clogged fuel filter. If you cannot get a fuel pressure guage then you can take out a spark plug and see if it is wet after cranking the engine. If its totally dry then you arent getting fuel into the engine and we need to look at the fuel system and some sensors.
If you have good pressure or wet plugs it could still be a sensor but we need to check spark first. Remove a spark plug wire and put something metal in the end of it so that it contacts the metal inside the connector but also sticks out the end of the wire. Then lay the wire on something metal under the hood where it wont get damaged and the metal sticking out is about a 1/2 inch away from the metal its laying on. Try to start the van and look to see if there are sparks arcing from the end of the wire. If there is no spark then we need to troubleshoot the ignition system. If there is spark and you already verified that there is fuel, we need to look at the spark plugs and sensors.
Let me know what you find and if you need any additional help or have any additional questions.