Programming Spare Keys

I have a 2001 Lincoln Town Car I bought used. It came with one coded key. I went to a locksmith who made up two additional coded keys. All 3 coded keys work after the programmed them with a computer tool.
Later, I made up a fourth coded key and tried to program it myself according to the instructions in the owners manual. I put the first key in, turned to ON for one second, took it out, put the second key in to ON for one second, and put the new key in to ON for one second, but the new key didn't start the car, and afterwards, none of the keys would start the car for several seconds.
I have a few questions.
1) What did I do wrong? Did I not leave the keys to ON for long enough? 2) Why did no keys start the car after the programming failed? Is this normal? Is it just the time is takes for the PATS to reset itself? After maybe 10 seconds, all three keys that I had before worked. 3) Do I run any risk of deleting the keys from memory if I try this process and it fails? In other words, can I be left with NO keys that will start the car?
Thanks!
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Try it again and leave each key in for a few more seconds. I don't think you're going to screw anything up. Here are instructions (see Programming Method "C"). http://www.fordf150.net/howto/patskey.php
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Turn the keys on long enough to allow the theft light to prove out.... Doing things too fast can send the module into 'anti-scan' mode and it will take a few moments to re-establish equalibrium

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Thanks! That worked.
The owners manual just says to turn the keys alreast one second. One second doesn't seem to be long enough.
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