OMC 4.3l conversion to Thunderbolt 4?

[dyoung00]

I have an 1990 boat with the OMC Cobra 4.3l with points ignition. I have come into possession of a complete Thunderbolt 4 distributor, coil and harness from a slightly newer pre-Vortec motor. I thought I would be brilliant and swapped out my old points ignition for the electronic Thunderbolt 4. I have the engine running but cant seem to get it to run well. It misfires horribly. I cant get it to idle speed to set base timing. Does the OMC shift interrupt circuit need to be modified or what? I am a little stuck so any one who has tried this I could use any help. Thanx. Darren

 

[Don S]

Re: OMC 4.3l conversion to Thunderbolt 4?

Your OMC has an ESA (Electronic Shift Assist) unit. How did you go about incorporating that? Something the TB4 system never had to deal with for sure.

 

[dyoung00]

Re: OMC 4.3l conversion to Thunderbolt 4?

I used the original OMC wiring without modifying anything. The positive and negative to the old coil are connected to the new coil along with the positive and negative from the tb4 module. I did not take the esa out of the loop. I thought that was only engaged during shifting so I thought it should not affect the motor while I was just running to set up the tb4.

 

[dubs283]

Re: OMC 4.3l conversion to Thunderbolt 4?

my best guess would be to refer to the appropriate mercruiser manual that the t-bolt 4 ignition system you have pertains to

check out the wiring diagram for that and set it up that way

it may be the ESA from the OMC set-up is causing your troubles and you could have to modify your shift bracket to incorporate the mercruiser shift interrupt switch instead of using the OMC ESA

 

[Don S]

Re: OMC 4.3l conversion to Thunderbolt 4?

I used the original OMC wiring without modifying anything.

There is your problem, the ESA module from the OMC system will not work with the TB4 system.
The ESA has to go, and you have to bypass the shift interrupt switches from the ESA, to work like the Mercruiser system to make it work.

 

[dyoung00]

Re: OMC 4.3l conversion to Thunderbolt 4?

Thank you Don. I kind of knew that was the issue but was reluctant to start cutting wires. Just needed a second opinion. I have the complete Merc harness so I will get this thing wired right and let you know how it goes. I know you can buy electronic kits that are probably more simple but, this was free to me so what the hell.