Thunderbolt IV ignition problem

[pstwrt1989]

I have a Thunderbolt IV system I’ve been going back and forth with since last season. Have replaced cap and rotor, ICM w/ a used one and the original ignition sensor in the distributor with the updated sealed three wire design as opposed to the two terminals and now have an MSD Blaster 2 coil on it and it still won’t start. Have put a spark checker on the coil wire and it seems to have good spark there. Granted I was by my self so I couldn’t see it very well since I was turning the switch But I could hear it popping over the engine turning over. Put the spark checker on one of the plug wires and had a very very weak spark barely bridging the gap and it was very orange.

my question is that I know the shift interrupter will ground the output pulses momentarily but has anyone seen one go bad and cause a no spark or weak spark situation similar to mine? As I said I am getting at least some spark out of the coil can’t be sure how good it is since I couldn’t see it very well from where I was.

 

[alldodge]

With a TB4 just disconnect the shift interrupter and that takes it out of the mix

What motor serial number?

 

[pstwrt1989]

Don’t have engine SN. It’s at my shop and I won’t be back there until tomorrow. It’s a ‘94 GM 5.7L. I checked to make sure the detent on the interrupter wasn’t sticking doesn’t seem to be but idk if there’s something in the electrical portion that’s keeping grounded or at least partially grounded at all times

 

[alldodge]

For now year is good enough.
Use this to troubleshoot. and this way you can be back at the motor when testing. Can make it spark by rapidly touching Wht/Grn wire to ground. The TBV is same as TB4 with troubleshooting

 

[Fun Times]

Also using the video below as a general guide, try bypassing the ignition key while cranking/starting the engine so you're closer to the engine and to help check if maybe the starting problem is up near the helm area.

Yes a bad/incorrectly adjusted, corroded wires/connections, etc. to the Mercruiser shift interrupter switch system (and 12volt power and ground system too) could affect the spark system form having no to weak spark situations.

*A weak battery and/or cables too.
* Weak grounding to any of the electrical system sides.
*Aging Spark plug wires and rubber boot connectors (Leaking plug wires/boots at plugs or coil/cap)..., And/or spark plugs / bad plug gapping.
*Common issue of this one seems to had been an incorrectly installed distributor cap...Sitting sideways/crooked eyed.
*Rotor not all the way down and/or the rotor wheel hitting the sensor.
*Are you getting around 12.6ish volts while cranking the engine over to start?
*Pushed out/bad connection wire pins to the TB4 ignition module and other components of the ignition system.
* Bad boat tachometer...Internally / externally grounded.
* Incorrect coil being used (as of now too??)
* Weak engine compression test numbers could keep it from starting and may show other starting symptoms too.