Alarm not recognizing my key

Every time I lock my car and try to unlock it again to get in my alarm goes off. Any idea why it does this? Its the stock anti theft system

im guessing you have the keyless entry with the keypad on the door, that its not a remote keyless entry system, and that the car you are talking about is the 89 in your sig. they key doesnt have some kind of chip in it that the car can recognize. you have to manually input the code to unlock the doors and disarm the alarm.

To turn off the alarm, you need to use the keypad or FULLY rotate the key in the lock. Assuming your key is good, I would suggest checking the lock cylinder and the cylinder switch.

You are not the only one.

I have the same problem with my 92 SC. Dealer can't help. So took it to custom stereo shop, they 'say' the problem is the key cylinder switch. This part is not available through any Ford dealer, so I am told. I am looking for the part but many suggest that I put a remote keyless entry on. Until I find the right part, I will continue to enter through the passenger door, or use the keyless entry. Try entering through the passenger door and this will narrow your search down. Alarm or key switch?

Well, if you want the easy fix, ground the dk.green/purple wire in the door harness/kick panel, and that should disable the alarm permanently. The only other fix would be to set up a relay that would pulse that wire upon unlocking the car from your keyless remote.