Tachometer / Ignition Problem

[300sflyer]

The tachometer on my starboard engine is suddenly acting up. All of a sudden it will not go above 3200 RPM, and yet below that RPM it functions as it should. It?s like there is some kind of mechanical problem with it preventing the needle from moving past 3200. I know for fact the engine is revving higher than that, as you can hear it in sync with the other engine. While in sync, the port tach will show 4000 RPM, and this one is still showing a max of 3200 RPM.

Anyone have any suggestions as to why?

I should also add that while at cruising speed, this same starboard engine will suddenly lose power like the ignition was turned off, but then suddenly run just fine again with just a split second delay. It may only do this once or twice within an hour, or sometimes not at all. Not sure if this is a related problem of not. I have checked the connections at the main wiring harness, and at the ignition coil, and all they are in good shape.

The engines are a pair of 2009 Merc Bravo 357's, with Thunderbolt V ignitions.

Thanks,

 

[Don S]

Re: Tachometer / Ignition Problem

Probably a bad tach. Being a 2009, take it in for warranty work. Or if it's new engines in an old boat, replace the tach.

Since you have twins, just swap tachs to test.

 

[300sflyer]

Re: Tachometer / Ignition Problem

Thanks Don. I was thinking of swapping the tachs as well. I will give that a shot and see what happens.

I am also going to install a new pair of ignition switches. I believe the ones in it now are original. This might cure the intermittent ignition problem.

 

[300sflyer]

Re: Tachometer / Ignition Problem

Bit of an update:

I swapped the tachs, and the problem followed the tach. Now the port side tach will not go above 3200, so I need to replace it. Easy enough to do.

I also installed two new ignition switches, and I still have an intermittent problem with the starboard ignition as described previously. I talked to the local mechanic at the merc dealer, and he said the problem could be many things, but getting it to do it while trying to find it, could be difficult and quite time consuming.

Any suggestions where to start looking? I'm thinking perhaps a bad ignition coil?

Thanks,

 

[Bondo]

Re: Tachometer / Ignition Problem

I talked to the local mechanic at the merc dealer, and he said the problem could be many things, but getting it to do it while trying to find it, could be difficult and quite time consuming.

Any suggestions where to start looking? I'm thinking perhaps a bad ignition coil?

Ayuh,.... More likely a Bad, or Corroded wiring connection...

Ya gotta do the shake, wiggle, pull test...
Shake, Wiggle,+ Pull at all the igntion wiring...

The Big cannon plug connecting the motor to the hull comes to mind....

 

[River's Revenge]

Re: Tachometer / Ignition Problem

Also look at cannon plugs or Any plug ins for that matter. Plugs can sometimes wiggle loose. ya look at em and they look fine only to be just seperated slightly, A zip tie keeps them in place if this is the culprit.

One more thought. and this might not be your application, But Some engines use a "Load sharing" electronic monitoring. If one engine is working harder than the other, it will drop off line. Generally they have a tolerance of anywhere from 5 to 10 rpms. (or close to it) If both engines don't play well with each other... well, you get the idea. One drops off line. and sometimes this drop (or hesitation) is just enough to match the other engine allowing it to "Load share" again, which allows that engine to jump back up and run just fine. These problems take awhile to find...uhg!

When you have two of the same, The swapping of parts are always a bonus, nice job with the tachs.

 

[300sflyer]

Re: Tachometer / Ignition Problem

Yup... I almost wish the engine would just die on me... Then it would be easier to find the problem.

 

[300sflyer]

Re: Tachometer / Ignition Problem

So I went to replace the tach, and sure enough VDO is out of business, and I ended up replacing all of the gauges with new Faria ones. So about $400 later, they now look nicer and are easier to read too.

I did find one loose connection on the engine starter, but the ignition is still dropping out for a brief second occasionally. There is no corrosion on any of the wiring, [not that I would expect any with new Merc engines] and everything else seems good and tight.

Aside from having the Merc dealer look at it, [minimum 2 week delay as they are very busy] I really don't know what else to try, aside from swapping ignition parts from one engine to the other.

Are any of the parts of a thunderbolt 5 ignition prone to failure?

Thanks,

 

[300sflyer]

Re: Tachometer / Ignition Problem

I finally got around to swapping the main wiring harness connectors, from the two engines. The leads were just long enough to reach to the opposite engines.

After doing so, the port tach now drops out intermittently, instead of the starboard tach. [Still the starboard engine]. One can feel the boat hesitate when the engine drops out for a half second or so.

If my assumptions are now correct, this would rule out any of the wiring on the boat itself, which only leaves the ignition components on the starboard engine itself.

Does this sound like a reasonable conclusion?

 

[6meter]

Re: Tachometer / Ignition Problem

Sounds reasonable. Swap the coils. Does it have kill teathers?

 

[300sflyer]

Re: Tachometer / Ignition Problem

Nope, no kill teathers.

 

[tigerfishes]

Re: Tachometer / Ignition Problem

not sure if it's even equipped,(this shows how little i know) but we had a similar issue with a church bus engine and it was the cam sensor... $15 part and about 10 minutes to replace but took us months to figure it out.
but symptoms sound the same... little more of a problem when it's your only engine though.
just my 2 cents... worth every penny... maybe.

 

[300sflyer]

Re: Tachometer / Ignition Problem

These are carbed engines rather than EFI, but thanks anyway.

http://www.mercurymarine.com/_media/pdfs/productbrochures/899883260_357MagBrvo4V_SS.pdf