Chances are you're
not providing enough information. I'll bet that you have a newer starter on your car. You may or may not know the difference between the orig starter and the newer starters that have the relay on top of them. But, that's a good example of the problems you'll have with repairing your car and the help that you'll get in the forums if you don't provide
all of the relevant information.
Unless you say what is *different* on your car than stock, you'll often get wrong advice.
Check out your starter. Does it have place for JUST one wire? Or does, it have two wire lugs? One big (battery), and one smaller (start from the **KEY**).
If you have a newer starter and the smaller starter relay wire goes to the same terminal on your FENDER relay as the big starter wire, then that
WILL HAPPEN.
There's "little difference" between a starter and an alternator. The newer starters are more efficient. So, when the engine starts and before the stater disengages, the starter GENERATES VOLTAGE onto that Big wire. Now, if the smaller relay engage wire (on the starter) is connected to that Big wire, the voltage and current generated by the starter will be enough to keep the relay energized. So, the starter "hangs".
For the NEW type of starters, the smaller relay engage wire on the starter MUST go to KEY start wire. It can **NOT** be just connected to the big "starter" post on the FENDER relay.
If someone says "I did it and it works fine", tell them that they are WRONG!

A number of years ago, Ford started putting the same info on their Starter Upgrade *kits*. Before Ford did that, morons would argue to NO END that what I said was *not* true because they may not have HEARD the starter hang. - roll_eyes
BTW: It is a simple and
UNDERSTANDABLE mistake to make when doing a starter upgrade.