95 TransAm Won't Start, mechanic at a loss

[PurplePig]

I have a 95 Transam that won't start. My son replaced the radiator and water pump then three days later it just wouldn't start. It tries to crank but doesn't. Our mechanic replaced the distributor, cap, and rotor. He installed a new brain box. He took it to GM and they scoped it and said the security module needed to be replaced. He replaced that and nothing. Then he replaced the VATS. He showed me that there is intermittent fuel injection pulse and he doesn't know why and it is generating code 18. I did some research and found that code 18 says:

• crank signal circuit

ECM fault injector circuit (vin P-5.7L)

He's checked wires and grounding. The strange thing is that if he unhooks the new distributor and rotates it by hand it starts. Any suggestions?? The PurplePig is sucking the money out of us at a rapid rate!