Some tb4 have a rev limiter called overspeed module under the amplifier module. The limiter is wired between the amp and coil or pickup and amp, can't remember. If you find a old TB4 cable without the limiter, install it to bypass the limiter for testing.

I had all 3 go bad this year. 2 went within a week and the third a month later. Symptoms were low coil voltage/ dead ignition and the third had a off idle miss.

if you suspect low voltage to coil as athe problem run you a quick temporary jumper lead from battery positive to coil positive,just for the test

Blue Merc racing engines have a rev limiter. Black Merc engines do not have a rev limiter. This is for TB IV ignitions.
If you were hitting the rev limiter, you would definately know it. Its feels like engine has misfire.

Well - after bypassing the kill switches, swapping the thunderbolt units ... Still zero spark coming out of the coil ! Turns out all it was - bad ground on the thunderbolt !! Bypassed the ground that attached to the bolt on the exhaust - put a solid ground wire to the engin block and now getting enough spark to light up the moon :). Thanks for your help guys - much appreciated !!

glad you got it figured out,as i stated before,the mercury flowsheet makes it easy to diagnose,the dist ground is one of the first things they say to check.