98 expedition will not start doesnot seem to be battery it is alrm system on truck

Does pin 16 at datalink connector have voltage? Should be hot all the time, fuse protected. You may be way ahead of me on testing this?

Maybe one way to tell if computer is awake, turn on key, check for ref voltage at any sensor that uses a ref voltage. If no ref voltage, check computer voltage and ground circuits.

You could pick one system that has a problem, then check voltage--ground and control where applicable. It may all lead back to computer for some reason, but, I can't tell at the moment?

I would look at the main power fuse in the main fuse block. Could possibly have a fried relay.

Is power getting to the starter by way of the neutral safety switch ?

First thing I would do is have someone qualified disconnect the alarm and stereo and start over.
Chances are you have circuits cross-connected and something is draining the battery.

Sounds like there is air in the brake system and or the brakees need replacing check your brake pads if ok have the brake system bled.

1. An electrical leak; something that draws current even when the ignition is off.
2. Poor electrical connections. Check the cables and terminals for cleanliness and tightness.
3. Some other problem with the charging system. I suggest that you have the charging system tested.
There is also the possibility that the new battery or alternator are defective.

hook up a fuel pressure guage and check it at start up, key on pressure should build and hold , if not check it could be regulator of leaking injector

Yes. You have to take the car to a ford dealer and have the new key programmed. This should solve your problem. Good luck.

its a dead battery

There are ride height sensors in the rear that send a signal to the ride height/load leveling control module, one of these ,may be defective. Have Ford dealer run an active test of the system, they will find out what the part is that has failed, otherwise you are guessing, that equals wasted $$$