Shift interupt switch?? - Mercruiser 140

[boozer1966]

1984 Mercruiser 140, serial no. 6873947, Alpha MR

I think the shift interrupt switch may be failing and cause an interruption in ignition when the boat warms up. Is there a good way to test the switch? The boat runs well on the water for about an hour an then develops a miss that I cannot for the life of me source. I switched to a Pertronix ignition system thinking that would cure it, including 12v coil. It did not. Plugs and wires are new. I cleaned the switch this weekend as best as I could, and it performed ok for about two hours before developing the same problem. I know, progress, not perfection. I can't replicate the same behavior on the muffs in the driveway.

Can the switch interrupt switch be cleaned/tested?

Can I purchase the switch by itself? (which I can't seem to find online)

Or do I have to buy the entire plate assembly? (which is pretty expensive)

Many thanks in advance for your responses.

 

[JustJason]

Re: Shift interupt switch?? - Mercruiser 140

The switch is an all or nothing deal. It either shorts the ignition coil out, or it doesn't. If your having just a "miss", it would not be the shift switch.

 

[boozer1966]

Re: Shift interupt switch?? - Mercruiser 140

I guess I should have been more descriptive. It is a miss that ultimately renders the boat inoperable. As in the engine does not run. Once it cools off, it will start back up.

 

[Bt Doctur]

Re: Shift interupt switch?? - Mercruiser 140

a coil going bad will do that.like JJ said, the switch is all or nothing. You could remove a wire for testing but it will make it difficult to get out of gear. Have you checked for spark when this happens? Clogged fuel vent will cause fuel starvation too.

 

[boozer1966]

Re: Shift interupt switch?? - Mercruiser 140

a coil going bad will do that.like JJ said, the switch is all or nothing. You could remove a wire for testing but it will make it difficult to get out of gear. Have you checked for spark when this happens? Clogged fuel vent will cause fuel starvation too.

I replaced the coil. But it behaved exactly the same with the old coil. Have checked the fuel vent. No problem there. I understand the switch being all or nothing, but could it not be all or nothing in some intermittent fashion. The reason I ask is that the boat ran much better and for longer after I cleaned the switch.

 

[JustJason]

Re: Shift interupt switch?? - Mercruiser 140

I don't see what you cleaned as the contacts are internal to the switch, and the housing of the 2 halves of the switch are riveted. But if you want to test your theory just disconnect the switch. And while your out and about boating, if you need to shift out of gear and back into neutral just shut the engine off first.

 

[thumpar]

Re: Shift interupt switch?? - Mercruiser 140

Check the coil voltage when you are having the issue. If you have voltage pull the wire that goes to the distributor and test for spark to the block from it (don't get in the path). If you have spark there and still no start test at the plug. If that all checks out I would suspect fuel issues.

 

[boozer1966]

Re: Shift interupt switch?? - Mercruiser 140

Check the coil voltage when you are having the issue. If you have voltage pull the wire that goes to the distributor and test for spark to the block from it (don't get in the path). If you have spark there and still no start test at the plug. If that all checks out I would suspect fuel issues.

I have 12+volts at the coil (Pertronix), and spark. I have tested the fuel system, and I don't think the problem is there. For example, when it first starts acting up, I can turn on an electrical device like the blower or activate the trim, and it goes haywire and gets worse.

 

[thumpar]

Re: Shift interupt switch?? - Mercruiser 140

Maybe a corroded connection somewhere. Check all your grounds.

 

[boozer1966]

Re: Shift interupt switch?? - Mercruiser 140

Maybe a corroded connection somewhere. Check all your grounds.

I have checked and re-checked my grounds. I even took apart the wiring harness and have checked each wire individually for any obvious problems.

I disconnected the tach lead from the coil last time to see if the tach was the problem. Nothing.
Any suggestions on what to check next time the problem comes up. Power at the coil was fine.

 

[bigdirty]

Re: Shift interupt switch?? - Mercruiser 140

just gona throw this out there... maybe a carb/fuel problem? Is there an electric choke? I had a similar issue once, which i thought was electrical, but in fact it was a carb issue (sticking metering rod) and it had similar symptoms.. My other thought is maybe the alternator is failing? Low voltage will cause all kinds of crazy issues with the whole electrical system..

 

[Bondo]

Re: Shift interupt switch?? - Mercruiser 140

1984 Mercruiser 140, serial no. 6873947, Alpha MR

I think the shift interrupt switch may be failing and cause an interruption in ignition when the boat warms up. Is there a good way to test the switch? The boat runs well on the water for about an hour an then develops a miss that I cannot for the life of me source. I switched to a Pertronix ignition system thinking that would cure it, including 12v coil. It did not. Plugs and wires are new. I cleaned the switch this weekend as best as I could, and it performed ok for about two hours before developing the same problem. I know, progress, not perfection. I can't replicate the same behavior on the muffs in the driveway.

Can the switch interrupt switch be cleaned/tested?

Can I purchase the switch by itself? (which I can't seem to find online)

Or do I have to buy the entire plate assembly? (which is pretty expensive)

Many thanks in advance for your responses.

Ayuh,.... Where'd ya pull the power from for the ignition swap,..?? Did it replace a Points fired ignition,..??

What'd ya do with any unused wires,..??

 

[boozer1966]

Re: Shift interupt switch?? - Mercruiser 140

I pulled it from the wire for the choke. It was doing the same thing before the conversion too. I have searched for loose wires and can't find anything obvious.