Thunderbolt IV Sensor Wiring
 

I changed the sensors in my TB 4 ignition and in addition to the ground wire attached to the distributor bottom, there was another ground wire running from the white/green side of the sensor to the fuel fill neck. There is one on each engine and I can't find this on any wiring diagrams so my question is should it be there and what does it do? Engines are 87 350/Mags.

Some boats with the old red button kill switches used to ground the white/green lead on dist. Seems like the wire you have traced is a mistake. Basic ground from tanks,fills,ground plates and transom assy. are green.

The newer style Thunderbolt ignition sensors have a ground wire that the original sensor did not. Is that what your talking about?

Dave N - no it's not the large black ground wire that comes off of the sensor, this is a small black ground.
3M - I'll check it out further tonight. Is there any harm in leaving it in place? I don't want to start motors and fry anything.
Thanks,
Phil

There is no reason to ground fuel fill to sensor lead. The fuel fill should be grounded to tank. You are messing with fuel and electric.
Not a good mix. Best of luck.
Jim

Just replaced my tb IV sensor last night with new one. Are you supposed to leave the black ground wire that goes from underneath the distributor to the mounting bracket on ignition module??? The new sensor had a black wire on it so I grounded it where the other was right on the bracket...hmmm..??? Do I still need that other ground???

Deleted...started new thread.