I turn the key and the security light flashes

OK. you had the battery disconnected. Here is what you do. Turn on the key. The security light will begin to flash. Wait (about 10 minutes) with the key in the ON position. The security light will stop flashing. TURN OFF the key. Then, turn on the key and start the car. Security system is supposed to prevent theft for a 10 minute delay. This usually discourages any thief. This will work, You just need to be patient with the 10 minutes. It seems like an hour when you wait ten minutes for that security light to stop flashing.

Check if security light is on, if so, must reset. To reset, put key in ignition and turn to on position. Security light will start flashing on and off. Leave key in on position until security light stops flashing and turns off completely. This will take approximately 10 minutes. Then turn key off and take out of ignition. Put key back in and start car. Should now start

Likely a bad fuel pump. However, if the security light is flashing, leave the key on until the security light stops flashing. Ten minutes with the key on. When the light stops flashing and stays on all the time, turn off the key. Now, start the car. You may be surpised. If your battery went dead and you charged it up, the security prevents you from stealing your own car! Have fun.

if that has a chipped key for the security system,try another key,that will let you know if its a bad key or the reader is screwing up.

turn the key on and wait till the security light stops flashing ( this could take as long as 30 minuets ) then turn the key off then back on but do not try to start the engine and wait till the security light stops flashing ( again this could take as long as 30 minuets ) then turn key off then turn key on and start engine.

You need to reprogram the security system. Hook up a battery charger. Turn the key on for 11 min. The security light should go off. Turn the key off for 5 sec. Turn the key on for 11 minutes again. repeat this three times. After the 3rd time the truck should start

try disconnecting the battery should reset the system

If you want to initiate a new master key.... 1. Insert new key into ignition cylinder and turn it to the ON (RUN) position. The security light on the dash will flash for fifteen minutes.
2. When the security light stops flashing, you have FIVE minutes to start the next step. Turn the ignition cylinder OFF and then back to the ON (RUN) position. The security light on the dash will flash for fifteen more minutes.
3. When the security light stops flashing, you have FIVE minutes to start the next step. Turn the ignition cylinder to OFF and then back ON (RUN) position. The dash will flash for fifteen more minutes.
4. After the security light stops flashing the third time, the new key has been programmed into the vehicles computer and will operate the car. It has replaced ALL the previous electronic key codes and is the ONLY key programmed into the vehicles computer.

The dealership should be able to do this for you without a current key, of course this means towing the vehicle to them. See if they will sell you a key and you program it yourself.

hope this helps. I found the above info at FordF150.net

your ignition switch is bad for now though turn your key on for at least ten minutes until the light stops flashing this will reset your security system after stops flashing turn key off and then start the truck

Take the Original Ignition key out to the car. Get in it and turn the key on. Every light should light up like your going to start it. The factory security light will flash. Leave the ignition on until it stops flashing. Should take about 10-15 min to do it. once it does, shut the key off and then turn it over to start the car.