Thunderbolt IV. 1986 454 inboard

[alldml2001]

what are the symptoms of a bad control box?
Do they act up shutting engine down intermittently or do they just die ?

 

[dubs283]

Control box as in ignition module/control or remote control box?

If your saying ignition there's a couple good write ups on the thunderbolt ignition including troubleshooting in the stickies, top of forum page

 

[Scott06]

what are the symptoms of a bad control box?
Do they act up shutting engine down intermittently or do they just die ?

Most of the time they just die but have seen folks with intermittent miss due to ignition control module.
here is the trouble shooting guide would verify the pickup sensor in distributor is not the issue first

How Mercruiser Thunderbolt ignition systems work.

Mercruiser Thunderbolt ignition systems. Mercruiser introduced the Thunderbolt IV onto their engines in 1982, alongside the introduction of the 1-R drives. The TB-IV system was used until the introduction of TB-V mid-way through 1996. Thunderbolt V was an improvement on an already exceptionally...

forums.iboats.com

what symptoms do u have