The truck died on the Highway, Replaced the solonoid & alt. Tried the test when you start the truck then pull off the power lead to the battery. The truck still died. What can it be? It is an 89 F250 4X4.

 

Get a voltmeter and check the battery voltage with the truck off, and then with the truck running. If the voltage is higher with the truck running, then it is charging. The "pull the cable off test" must have come from the old days with points style ignition. That is a very risky test on a vehicle with electronic ignition and also dangerous to the EFI computer.