This is not a computer issue. Since you removed the starter relay and it still keeps cranking, the starter solenoid is stuck. You can verify that by measuring the voltage on the smaller wire on the solenoid. When you release the ignition switch from the "crank" position, if there's still 12 volts on the solenoid wire, we have to figure out where it's coming from. You should find 0 volts there. Then, the only way the starter can keep cranking is if the switching contacts in the solenoid are stuck on. There's two ways that can happen. One is those contacts are partially burned away and the contact disc on the plunger is getting stuck in between them.
The more common causes are the shaft the drive gear rides on is seriously rusted, although that usually causes a failure to engage at all, or a warped solenoid. GM has had some trouble with the solenoid warping causing the movable core to get stuck. You may be able to identify that by banging on it when it doesn't disengage. Be careful though when you do that. GM redesigned their starters, beginning with 1987 models, to no longer use field windings bolted to the case. They use permanent magnets that are glued to the case. Hitting the case can break the bond. At that point the starter won't work and it won't be suitable to be rebuilt.
Monday, March 2nd, 2015 AT 3:23 PM