My Ford F-150 o/d off keep flashing and the car won't go fast it go slowly

Question edited for clarity, typos and words that are incorrect 'automator' to 'alternator'.
Question moved to model category.

Put a battery charger on it for at least 8 hours on 6 amp charge. Make sure the battery ground has a good contact using a digital multimeter. After 8 hours, start the car and run it.

Next day, try and start again. If the battery is flat, you have parasitic draw.

https://www.fixya.com/cars/r29903527-parasitic_draw_car_batteries

One further thought, does this old vehicle have a security system and engine immobilizer? Do you have a steady or flashing red light anywhere. You may be locked out of starting the engine because you disconnected the battery. In fact Goggle says it does have anti-theft.

1995 f150 theft light flashing, cannot start!

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It's easier than you might think. In most cases, the anti-theft system on a Ford F-150 can be reset by turning the vehicle to "accessory" mode and leaving the key in the ignition for several minutes until the anti-theft light shuts off.

Feb 10, 2021

Ford F-150: Resetting the Anti-Theft System - VehicleHistory

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Will not crank how ? What happens when you turn the key to the start position ? Nothing or the engine spins around ? Does the security light flash fast ? anti-theft system problem . Anti-theft system problem will keep the engine from cranking .

A few seconds, or like 2 seconds. Is the security like flashing? The light could be a red dot on the top of the dash. Have the codes read, takes a scanner. Check engine light on when it is running, Also check the codes, code reader will do this. Does the engine just shut off or does it bog down, like sputter till it dies?

was it running before you replaced the alternator and battery or did it not run,and you thought that the battery and alternator was the cause of it not running. if a motor cranks and not start the battery and alternator are good. you need to check to see if the fuel pump is working and if you have spark

It can be faulty fuel pump or clogged fuel filter or its issue with starter or battery or may be problem with sensors.There are few possibilities that can cause these problem,there are few help links provided related to these issue. Click the link below and refer the troubleshooting procedure to solve the problem:-----
Engine cranks but will not start? http://technoanswers.blogspot.in/2012/02/engine-cranks-but-will-not-start.html
-------- Landcruiser prado won't start http://technoanswers.blogspot.com/2012/10/landcruiser-prado-wont-start.html ------- 2007 ford expedition engine cranks but wont start http://technoanswers.blogspot.com/2012/10/2007-ford-expedition-engine-cranks-but.html

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: Engine Cranking Fast but not Firing up? http://whatisbyme.blogspot.in/2012/05/1999-lexus-gs300-engine-cranking-fast.html

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1997 Oldsmobile Cutlass Ciera: The Engine cranks but not start? http://schematicsdiagram.blogspot.in/2012/04/1997-oldsmobile-cutlass-ciera-engine.html

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Mazda MPV cranks but will not start? http://technoanswers.blogspot.com/2012/01/mazda-mpv-cranks-but-will-not-start.html
-------- MAZDA MPV WILL CRANK BUT NOT TURN OVER TO START? http://technoanswers.blogspot.com/2012/01/mazda-mpv-will-crank-but-not-turn-over.html
--------- Vehicle Cranks Fast but Won't Start http://whoisbyme.blogspot.in/2012/07/vehicle-cranks-fast-but-wont-start.html

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How to test Crankshaft Position Sensor? http://fordrepairhelp.blogspot.in/2012/03/how-to-test-ford-focus-crankshaft.html

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These details will help,keep updated for more help.

your starter is bad.

start engine then disconnect the neg terminal and if engine keeps running the alternator is working ,if engine stops then it isnt working .

does the engine turn over fast when you try to start it are dose it turn slow . if it is slow to turn over it could be the starter are the battery has a bad cell and also make sure the alt is charging .

Could be a problem with your ignition switch. This happend to me. Battery shown it was charged, then while driving, the gauge slowly would drop. There could be a wire that is shot inside the ignition switch.

It sounds like an automatic choke system. Let it warm up for 5 minutes and then give it gas up to 2000rpm and release.