So the season comes to an end and so does one of my engins as far as spark goes... Went to drop it in for a last run and no start ! Gas is there but no spark from the coil ! Wire at coil reads about 10 volt but battery shows 12.4 ??? Replaced the sender in the distributor and the coil itself and zero spark still... But again does show 10 volt with ignition on ... Does the coil need to read 12 or above too to Mean the thunderbolt to get going or ??? Wonder where to go from here - do I try and figure why the wire only gets 10 volt or just forget that and move on from that ? Would it run with only 10 volt I wonder ? If so I will move on to the nxt possibilities ?
If it was the kill switch , would the engin still turn over or just a dead key like it was in gear kinda deal ? I'm at a standstill - no idea where to go from here

try to switch from one engine to another

I have never been happier then the day I dumped that thunderbolt stuff.

It needs over 10v while cranking, my guess is it has about 8 while your cranking and 10 with the key on. Most kill switches are not wired to the starter just ignition. May have a loose or corroded wire. Check the cannon plug for loose pins.

mercury has a troubleshooting flowsheet,it makes it easy to find the problem.

I have 525 sc with thunderbolt 4 with rev limiters. I had a similar problem with low voltage due to a bad limiter. The merc troubleshooting chart doesn't include the rev limiter that's external on tb4 ignitions.

I have the hp books, hotrod ding your marine 350 book and it states the IV's did not have a limiter but the v's do..... Which is correct? Cause my new motor won't spin past 4700, which is where the stock 260 merc stayed..... And the new motor has way more potential.

I had something similar as well. Rev Limiter went bad and would cause a no spark condition.

Sometimes the TBIV's had rev limiters, sometimes they didn't. I worked on a boat where one motor did, one didn't.

Great. Thanks. Guess I have to put in an aftermarket ignition

post your research and let us know the outcome