The current for the starter has nothing to do with the ECU. There's a difference between a "no-crank" and a "no-start". You could remove the ECU from the vehicle, and the starter would still crank.
Here's a brief rundown of what happens when you turn the key:
The ignition switch has a momentary switch in it that closes a circuit when the key is turned to START. This energizes the starter relay coil. The coil turns into an electromagnet, pulling the relay armature towards it and completing another circuit to the starter solenoid. The starter solenoid is responsible for engaging the starter motor drive gear with the flexplate or flywheel, as well as closing another set of contacts. This second set of contacts is what actually supplies the current to spin the starter motor.
Why such a Rube Goldberg means of turning the starter? It comes down to current, and the size of wires and switches needed to carry it. The starter motor requires a couple of hundred amps to spin the motor over. This is why the starter motor has such huge cables running to it. In order to have a low enough resistance to carry the current without welding themselves together, the solenoid's contacts are pretty big. However, the solenoid requires less current to operate, which is small enough that it can be switched by the starter relay. Again, the relay is a pretty beefy piece of equipment compared to the ignition switch, but, again, it takes less current to operate than it's capable of switching. This current is small enough to be carried safely through the small wires and contacts in the ignition switch.
I would get a good wiring diagram and probe the coil terminals of the starter relay socket. You should get about 12V there when the key is turned to START. If you get nothing, there's a problem with the switch or wiring going to it. If you do have 12V, check the starter solenoid contacts. You should have 12V there, too. If you have 12V all the way down, try hitting the starter with a broom handle, plastic mallet, hammer handle, etc. If this gets the car to start, the starter solenoid is probably the cause of the problem. No solenoid action, no starter current.
I would put the ECU way, way down on the list. I can't tell you how many people on here have replaced the "computer" because the car was acting strangely and they didn't bother checking grounds. ECUs themselves rarely fail for no apparent reason unless they've been burnt, overheated, submerged, etc. They're really not wear items. 9 times out of 10, the problem is in the wiring somewhere. Replacing the ECU usually only lightens your wallet. And no weight reduction anywhere helps a car that isn't moving...
Edited by B10alia, May 2, 2013 at 04:22 pm.