Thunderbolt IV ignition problem

[sgm26]

My mercruiser quit abruptly when I tried to get up on plane.

When I got home I found I was getting no spark to the plugs or out of the coil. I have a manual so I started troubleshooting.

I was getting 12 volts at the coil. so I went to the distributor.
The book said to check the red.white post on the distributor and if no power go to the bullet connectors. However there was no external connector so I went straight to the bullet connector, and at first got no reading from either side.
At that point it appeared to be the module. When I learned the price of the module and that they seldom fail, I replaced the coil. it would still not start.
I went back and checked the R/W bullet connector and got 11 -12 volts when I used the battery as a ground.

I am now thinking that it is the ignition sensor.

Any ideas or recommendations

 

[incrediblened]

Re: Thunderbolt IV ignition problem

Hey. I just have a suggestion before you drop the big bucks to switch the ignition sensor. I just had problems getting my Thunderbolt IV system started (4.3 Mercruiser). My shift interrupt switch was grounded out. Disconnect the switch. See if it will start.

Are you sure that it's spark?

Good luck. Ryan

 

[Bt Doctur]

Re: Thunderbolt IV ignition problem

Did you remove the tach lead ,grey wire on the neg side of the coil. Did you "scratch" the green/white lead to ground with the key on?

 

[sgm26]

Re: Thunderbolt IV ignition problem

Thanks

I will check the grounding on the shift switch.

I did not disconnect the tach wire because that was listed under the Thunderbolt V in the guide. My engine does not have a r/w post on the distributor as the book said a IV does but the carb cover says thunderbolt IV, and the ignition module looks live the IV. I will also try disconnecting the tach.

Are you talking about grounding the w/g wire while looking for a spark from the coil? I did that and got nothing.

Also, my new coil says use with an external resistor. It's a Sierra coil, and when I look it up on the site with the number on the box, it does not use an external resistor. I'm thinking that the wrong coil was in the box.

Thanks for the help and please get back to me

 

[sgm26]

Re: Thunderbolt IV ignition problem

Ryan

Thanks It was the shift interrupt switch. There was a green/white wire going into it and, I disconnected. Started right up.

I had seen mention of that switch in a forum but it was not in the trouble shooting chart in the manual.

I'm still not sure of about the external resistor for the coil but it starts so if it runs Ok, it's not an issue.

Again great Forum and thanks Ryan You saved me big bucks.

Going forward, this is worth passing on to lots of people because if I had known this I could have fixed the problem in a few seconds and avoided being towed.