New Ignition module

Craig Donaldson

New Member
Jun 30, 2022
8
Boat Info
1988 SeaRay 250cc 7.4l 454
Engines
Mercruiser 454 7.4L Alpha 1 Drive
Hi Guys, quick question, I replaced my ignition module on my 1988 searay 250cc with a mercruiser 7.4l 454. The boat has a wht/red, wht/grn, and a black wire. The new module had a blk, wht, and red wire. I hooked the wht/red to the red wire, the wht/grn wire to the wht wire, and the black wire to the ground wire on the module. The boat starts but shifting is hard and sticks in gear. Should I have hooked the black wire from the boat to the wht/grn wire, (which is where I think it might have been originally) and then grounded the black wire off the new module?
 
Sound more like the shift interrupt switch isn't doing its thing
Ive considered that too. The black wire ohms straight to ground. Is it possible that the black wire controls the shift interrupt switch? I didn't have any resources with me to figure out what the wires go to and didn't want to hook a ground to something that wasn't supposed to be grounded.

Another thing, I can be in forward, shift into neutral and still be in gear. When I shut the boat off and restart it, ill start in neutral. Same with reverse.
 
Any chance anyone knows were I can get a service manual for the boat, or schematics of the ignition circuit and shift interrupt circuit?
 
Found what I needed to know at 1:39. I have to hook the black wire, which comes from the shift interrupt switch, to the white/green wire. Just info incase this helps anyone else some time.

 
Found what I needed to know at 1:39. I have to hook the black wire, which comes from the shift interrupt switch, to the white/green wire. Just info incase this helps anyone else some time.

Sorry I couldn't help. Thanks for getting back and posting what you found. Let us know how it goes.
 
So hooking up the black wire from the boat to the white/green wire didnt work. The boat just cranks with the wire hooked up. If I disconnect the wire, the boat starts. Then if I connect the wires back while the boat is running, it dies. Maybe test my shift interrupt switch? Im making an assumption that this black wire from the boat has to do with that switch. I would swear that it came off the wht/grn side of the old coil.

Also a separate issue, the boat runs hot if I run it above 3500 rpms and seems to be working extra hard to get up to speed (25/30 mph).
 
Do you have the Thunderbolt 4 ignition (module mounted on the dist) or the Thunderbolt 5 (module mounted on a bracket) ? Should be the TB5.

I have the Mercruiser service manual #15 for 88-92 V8's if you need it. It is in PDF and too big to post here. PM me with your e-mail and I will send it over to you.

I also have a manual (with ignition curves) for the Thunderbolt 4 and 5 ignitions
 
Last edited:
Do you have the Thunderbolt 4 ignition (module mounted on the dist) or the Thunderbolt 5 (module mounted on a bracket) ? Should be the TB5.

I have the Mercruiser service manual #15 for 88-92 V8's if you need it. It is in PDF and too big to post here. PM me with your e-mail and I will send it over to you.

I also have a manual (with ignition curves) for the Thunderbolt 4 and 5 ignitions
I believe it will be the tb5. Its mounted to the right of the distributor,down low.
 
Update on the problem. The black wire apparently is an extra wire in the wire harness for a ground. I worked on the motor from the other side and found a white/green wire that came from the shift interrupt switch. I got the two wires mixed up when I reinstalled the ignition module. Originally I believe what was happening is the black wire was loose enough and long enough that it would wiggle when Id hit a wave and touch one side of the ignition module, therefore grounding out the system and shutting off the boat.
 

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