1985 gmc vandura 2500 will not fire when trying

no
but the back fire could be caused by a failed timing belt or chain
a back fire is just exploding fuel in the exhaust or inlet manifold caused when a valve has not properly closed and fuel fire is allowed to escape

If you vehicle is parked on a slant and the wheels are slightly turned, the pressure from the weight of the car may be forcing you steering wheel to bind. Try tweaking the steering wheel in the same direction that the wheels are turned using your left hand and at the same time try turning the key with your right hand. Tweaking the steering wheel takes the pressure off of the ignition switch. If that doesn't work, try a different key if you have one.

Check battery connections and battery fuse. Battery may not be large enough for the size of your van

change fan clutch

There is most likely a cable that has come off or broken behind the heater controls

has no relay switch....starter has solenoid.....

I would bet first to check and change would be cap and rotor and coil. Lack of proper spark at proper time is the main issue, that or the timing chain jump a tooth due to age and wear.

My sincere apologies for the delayed response.
I am personally going back to answer your unanswered questions.

I do not know if you still need this information, but I am going ahead and answering it anyway. You may want to Print the Diagrams for future use.

CLICK on the following LINK. It has the Belt Diagrams you will need.
If you would: Before Clicking: Let me know if this helped you, or if you have any additional information or questions. Feel Free to contact me at FixYa.com!
Again my apologies...
http://www.2carpros.com/car_repair_information/year/1990/make/gmc/model/vandura/1990_gmc_vandura_drive_belt_routing_diagram.htm

there is a special tool to remove these you can get at Napa, heres the link:
http://www.napaonline.com/NOLPPSE/(S(hiremb55jk30vvegzxg1xzux))/Detail.aspx?R=SER3906_0055041161

If it's anything like my 1994 Vandura, you have a small panel of rocker switchers above the rearview mirror for turning your interior lights and other gadgets(tv, vcr, etc) off and on.

The leftmost switch on mine moves the electric bench seat up and down, *and* forward and back. The trick is that you have to hold it down to make it move.

With mine, even though there are four actions:
- slide seat forward
- slide seat backward
- fold seat down
- fold seat up
there is only one rocker switch.

So what you do is hold "up" on the switch and see what it does. If it slides the seat in either direction, up for your van is for both sliding forward *and* back. To do the opposite direction, just wait a second then press the same button the same way a second time.

If up makes it fold, it's the same principle. Hold the rocker switch depressed in the up position to make it fold up. Wait a moment and repeat to make it go down.

It wasn't intuitive for me either but once you know and play with it a bit it makes sense. Probably better than having four switches up there anyway :)

Good luck!