Parts replacement

Here's the story for Caravans:

If you DO have the anti-theft system, you can use any Engine or Body Computer from the salvage yard, but from then on, that computer will only work on another van that also has the anti-theft system.

Both computers have anti-theft programming that can not be undone. If your van does NOT have the anti-theft system, you must find a used computer from a donor van that also did not have anti-theft, and that can be impossible to find out from the salvage yard since there's no easy way to tell unless the car was running when it was brought in. Even then, they might not have bothered to find out.

When you install the Body Computer with anti-theft programming into your van without anti-theft, it will teach it to the Engine Computer. At that point the van will not start because both computers are waiting for the disarm signal that's never coming. Both computers will have to be replaced at the same time. If you just the Engine Computer, it will immediately learn the anti-theft programming from the Body Computer as soon as the ignition switch is turned on.

In the cars this self-programming takes place either way from and to either computer. Things are a little easier with the minivans. If you buy a remanufactured Body Computer from the dealer, it will come without the anti-theft programming. It will work in any van as soon as it is installed but it will not learn the anti-theft function from the Engine Computer like it does in the cars. To initialize the anti-theft function, use the key to unlock the lift gate. That will send a disarm signal to the new Body Computer. That's what tells the new computer to self-program itself for anti-theft, but you don't have that switch in the lift gate if you don't have the anti-theft system.

Once you initialize the system, only the lights will flash when the alarm is triggered. The horn will sound too after you start the engine 20 times.