My buddy has 1987 454/370HP mills with the old merc ignitions, with the big ignition amplifier. Early T-bolt setup.

He was cruising along last week, and engine just died. Turns out he isnt getting spark. Swapped coil from good motor, nothing. He bought a new ign sensor in distributor, still no spark. Getting 12v at coil, actually at both the hot and ground side of it. Other engine is same way, and that one runs. Swapped amplifier boxes, no luck there.

One odd thing we noticed, is with dist cap off, key on, when you pull rotor off, coil sparks. Also, he is getting a 5 volt reading from one of the distributor sensor wires. The good motor does not get this reading.

This one has me stumped. I am guessing something must be shorting out somewhere. Removed tach wire, thinking that could be it, but nope. Even ran a jumper from a separate 12v source to coil, and still nothing. I dont get it.

I chased a similar problem this spring.

Check your grounds. I had a voltage drop when I turned the engines over and spark was weak. Do you have 12volts when you turn it over?

If you don't already have this, the link will give you the TBIV troubleshooting chart. I've had two of those older style large ign. modules give up on me. One went intermitently and the other just died.

http://www.marinemechanic.com/site/page251.html