Van starts & shuts down after 3 seconds

The battery died from repeated start & quick shutdown. A few days later after reading posts, I installed a new dealer ignition switch with the wire harness. I did not change the ignition cylinder. I installed a new battery and the van started fine. I started & shut it down several times to check if it was operating correctly. Alternator appeared to be charging properly. Vehicle was performing normally. After 1/2 hour I shut it down to button up the covers and when I finished the job, it started doing the same thing over again. The only adjustment I made was to snap on the plastic ignition cylinder butterfly over the key insert. Does the ignition cylinder have some electrical component that controls spark & fuel? It seems some theft deterent system malfunction is at work? Can I bypass this some how?
Any ideas?

What year and engine and what van?
Ant security lights on key on and then go off or stay on?
After market anti theft or remote start stuff?

I'm Having the same issue on a 97 express. 5.7 votec. Someone tried stealing it once and messed up the ignition. It was hard to work the ignition but after a year I finally replaced it. The security light comes on with the key just like it always did. After the motor starts the light comes back on and the engine shuts down. There is no chip on the key and the cylinder has no wires. The little button with the two wires on top broke so I tried twisting them together. It didn't work. Is there something missing for the three wires on the side? I mean I don't see anything in there but that seems the likely place for a problem.

You both have the Passlock2 issues. You can fix it by using the resistor trick. You can look it up on here under PK2 or Passlock issues.

How does the passlock work?

This is what I know, The ignition was pulled once and re-keyed. I dont have the ignition but I have the key. There is no chip. And it's a replacement key from a hardware store. It always worked till last month when ignition would no longer roll over and if it did it wouldn't crank. It's just a simple cylinder. I did notice a silver spring laying on the floor after they pulled the original cylinder out. don't know if thats related or not. It throws no codes at all. When you start it it wont start at all you touch the gas pedal. The fuel pressure stays up around 62 all the time. It doesn't die down so the pump isnt being shut down. The security light comes on and goes off like normal after about 2 seconds. it comes back on as the engine dies out then goes back off again. it does start right back up without turning the key off first. I know it's the passlock doing it but how do I get around it. I've searched the forums here and only came up people having the same problem and cures posted at all.

Try searching some of the Cadillac forums on the net.
Its common for older cadillac owners to want to use a regular ignition key blank, instead of going for the expensive route of getting the correct chip.

There is a procedure where one can locate the orange PASS wire under the dash beside the steering column. One cuts the wire then installs a resistor in that wire that duplicates the resistance of the original key chip. Of course the trick is to find the correct resistance, or the problem will remain. The on-line instructions told you how to do this, but I cannot find the link to these instructions right now.
I read about it about a year ago, but this is all I can remember. Good luck.

Well I am not sure if it is the PK2 system or not. Usually when that system is tripped the security light just keeps flashing or stays on and doesn't shut off when you have the key on. You have stated that this light goes off but it comes on when it stalls? If that part is right then it COULD be the pk2. To bypass it you need to get to the bottom of the ignition switch. You'll see the main harness on one end but a small, 3 wire plug up near the key area. There should be a black or orange/black wire (ground reference), a yellow wire (resistance), and a white wire (circuit check). The yellow and black are the ones you'll use.

Now, since it won't start you can skip the part of reading the resitance to get the correct resistance. Get a resistor say something like 1500 ohms or so. Connect one end of the resistor to the black or orange black wire. Cut the yellow wire and put the other end of the resistor to the cut yellow wire going to the BCM (not ignition side). Then you'll have to put a charger on the battery. Turn the ignition on and try to start it. It'll stall out and that light should flash. Let it sit for 10 minutes with the ignition on then try to start it again. If it starts fine but if it doesn't repeat with the ignition on for 10 minutes again.

Normally this will take 20-30 minutes but I have heard some take up to an hour to reprogram. What you are doing is the same thing if you had taken it to a GM shop to fix it. This is the resistance relearn sequence.