** Resolved ** 1990 Mercruiser 4.3 Quits after running for a while - no power at coil

[ndtish]

I have..
1989 or 1990 (the boat is a '90 but the engine might be an '89)
Mercruiser 4.3L V6
Engine Serial Number D396424
Thunderbolt IV system

Problem:
The boat will run for a while, in the shop running at idle it will run for quite a while maybe 30 minutes before the problem occurs, last year was the last time I had it on the water and on plane it would only go a few minutes before the problem. What happens is the engine just quits. If you wait a few minutes it will start again and run for another few minutes under power or longer on the muffs.

What I have figured out so far:
I wired a test light from one side of the coil, I don't know if it is the plus or minus for sure, I think it is the plus side, to the engine block. With the key on the light lights, while the engine is running the light stays lit. When it quits the light goes off (no power to the coil). I have turned the key to off and on serveral times after this happened once and the power would not return to the coil. Then I tried to start the engine again and almost as soon as the engine started turning power came back to the coil and it started again. I think the timing of the power returnind to the coil might have been a coincidence because I have tried to start it before when this has happened, before I had the test light attached, and I would have to try it serveral times before it would start again.
At first I thought it might be fuel related but there always seems to be gas in the carb after it quits, and after further testing, the fact that there is no power to the coil, I am pretty sure it's an electrical problem.

I have a Clymer shop manual with some wiring diagrams but I can't really tell how everything works together. From what I can tell there doesn't seem to be much between the battery and power to the coil besides the ignition switch. I am thinking maybe I need to bypass or dissasemble and clean the ignition switch. Does someone know this system well, and can tell me if there should always be power to the coil if the key is on? Is there something else that would cause this problem?

The problem has been challenging to diagnose because it is intermitent and will run for a long time in the shop without quitting.

Thank you for any direction you can provide.

 

[ndtish]

Re: 1990 Mercruiser 4.3 Quits after running for a while - no power at coil

Re: 1990 Mercruiser 4.3 Quits after running for a while - no power at coil

Lots of views but no advice yet. I will be trying to recreate the problem tomorrow by messing with the tether and ignition switch as well as wiring under the dash. I will post again with the results.

 

[stonyloam]

Re: 1990 Mercruiser 4.3 Quits after running for a while - no power at coil

Re: 1990 Mercruiser 4.3 Quits after running for a while - no power at coil

Lots of views but no advice yet. I will be trying to recreate the problem tomorrow by messing with the tether and ignition switch as well as wiring under the dash. I will post again with the results.

I'll give it a try, Don't know much about t-bolt, but look at the purple wire. That is the one that provides voltage to the coil in a Merc. Next time it stops, see if you have voltage on the p wire. The p wire also powers your instruments, so if they are working your ignition switch is probably OK. Take a close look at your stop switch, (if you have one) in the control. I think the purple is the key to your problem, so check all the connections, including the pins in any connector.

 

[ndtish]

Re: ** Resolved ** 1990 Mercruiser 4.3 Quits after running for a while - no power at

Problem Resolved - It was infact the kill switch, tether, what ever you want to call it. With a test light attached to the coil I was able to move the wires around on the back side of the tether and get the light to go out. At first I thought is was just the connections but I cleaned and sanded them all up and it still would loose power when messed with. Took an ohm meter to it on the bench and by moving the leads coming out of the tether switch just right could get it to go to open. For now I wired the leads together for a test and it seems to have resolved the problem. Took it out for a couple hours today with the tether bypassed and it never missed a beat. Now I suppose I will get a new tether switch to fill the hole in the side panel and all should be good.