90 hp Ignition Question: Thunderbolt VS Prestolite

[10.90 Hot Rod]

Hey there, Just seeking any opinions please. I have a 1990 90 hp that has Prestolite ignition that drops off a cylinder between 3000 and 4000 rpm and then comes back and runs great. I've gone through all of my resistance checks and have found nothing suspect. I'm thinking of throwing the kitchen sink at it by changing the entire ignition system over to a complete thunderbolt system I have from a 91 donor motor I bought. Any thoughts ? Is the thunderbolt a better or more reliable system? I think the Thunderbolt it is a Merc system. It may be more economical and less of a hassle than trying to fool around with the prestolite, and I know the thunderbolt is good.
Thanks

 

[Scott Danforth]

Thunderbolt was used on Merc Motors. Prestolite on Chrystler/force motors

there are no provisions to put a merc ignition on a non-merc motor.

 

[roscoe]

Welcome to iboats.

These motors were made by Chrysler until 1984, then US Marine until Merc bought them in 89/90/91.
Well, your motor was made by Mercury during the transition year(s).
It's parts are almost all from the old Us Marine Force parts bin.

I'm not sure if an ignition swap is possible or not, will it fit ?

I suspect you would need to swap out the trigger, stator, flywheel, and the rest of the ignition parts.
Will they fit ?
There were several combinations of parts used during those years.

Sorry I can't be more specific, but its been a long time since I've worked on those, and my shop manuals went with the last motors I sold.

What is wrong with the donor motor?

Perhaps better to fix that one ??

 

[jerryjerry05]

The donor 1991.
It does use the Mercury ignition.
It's possible it can work?

​You'd need to swap everything.

​First thing I'd check is the flywheel.
If that fits?? then the rest probably will too.

 

[10.90 Hot Rod]

Thanks for all your input. According to the manual and the two engines that I have, this engine could and did come with 3 different ignition systems through it's production years.. The earliest is the chrysler prestolite, next was the merc thunderbolt (when they bought chrylsler), and finally the CDM system which I can only assume is Merc. The engines themselves ( all castings and details) are identical and one year apart. Yes the flywheel/trigger/stator/CDI box and coils will all have to go together. My only question was as to the reliability or quality of one over the other

The donor motor has a piston where the ring has caught a window and is toast.
Thanks for replying.

 

[jerryjerry05]

Reliability???
All 3 units are good.
​Prestolite made the older products and they were good.
Then Mercury took over and used the rest of the older Prestolite stuff.
​Then they started using their products.

​The only thing I don't like about the Mercury is the connectors.
​They can heat up and burn under the plastic cover.

 

[pnwboat]

I converted my 1988 125 HP Prestolite ignition system to the 1995 and later Mercury Thunder Bolt CDM ignition system and haven't looked back. Had to change the flywheel, stator, ignition trigger mount and ignition trigger itself. Also had to get the CDM coil wiring harness, and CDM mouting plate. Been working like a champ for the past 8 or so years. The coils (CDM) are available used anywhere from $10 - $50 each. Much easier to trouble-shoot too. Jerry mentioned that the Barrel connectors are an issue. The only connection that I had an issue with was for the ones on the Regulator/Rectifier. I cut the barrel connectors off and replaced them with solder/crimp eyelets and a terminal strip. Been working fine.

Original Prestolite Ignition system.


Later Mercury ignition system.

 

[JonKeilman]

I converted my 1988 125 HP Prestolite ignition system to the 1995 and later Mercury Thunder Bolt CDM ignition system and haven't looked back. Had to change the flywheel, stator, ignition trigger mount and ignition trigger itself. Also had to get the CDM coil wiring harness, and CDM mouting plate. Been working like a champ for the past 8 or so years. The coils (CDM) are available used anywhere from $10 - $50 each. Much easier to trouble-shoot too. Jerry mentioned that the Barrel connectors are an issue. The only connection that I had an issue with was for the ones on the Regulator/Rectifier. I cut the barrel connectors off and replaced them with solder/crimp eyelets and a terminal strip. Been working fine.

Original Prestolite Ignition system. View attachment 267903


Later Mercury ignition system.
View attachment 267905

Hey sir, is there anyway you can send me the part numbers for that exact conversion. I have the 1988 125hp force engine as well and would much rather swap from prestolite to the cdm. I know this post was 5 years ago but I'm hoping you're still on here.

 

[kbh121956]

Hey sir, is there anyway you can send me the part numbers for that exact conversion. I have the 1988 125hp force engine as well and would much rather swap from prestolite to the cdm. I know this post was 5 years ago but I'm hoping you're still on here.

This is an old thread from 2017! I would start a new one.

 

[Nordin]

I think pnwboat still on here, but start your own new thread.
You will get more help from the forum readers.

 

[jerryjerry05]

I can see wanting to do the change but the cost would stop me as soon as I tried to get a new flywheel.$650 +
I had the Prestolite system on 5 different forces and the only problems was a sheared flywheel key and a trigger wire that broke under the shrink wrap.