3.0 Mercruiser Tach Question

nrf414 · Mar 7, 2010

Hey Everyone!

Today was a beautiful day in the Northeast with temperatures nearing 60 degrees. So, I decided to take the boat out of the heated garage and see if I could get it to start. It ran great last year and I have had the battery on a trickle charger for the last 2 weeks. So I hook up the water, hook up the battery and check my gauges. My Tach gauge went crazy when I first turned the key to the on position. It went backwards to -4500 RPM and the volt meter moved a hair but not showing any voltage. The battery showed 14V when measured. What would cause a tachometer to spike in reverse to the level it did? Is this the signs of a shorted tach or a bad ground along the way?

All accessories had barely enough power to operate: i.e. bilge blower ran at less than 1/2 the speed it normally does, lights were dim, radio worked, horn would not sound.

I spent some time taking voltage readings and going over the diagram in the manual and everything looked ok.

Engine is 1994 3.0 Mercruiser AlphaI GenII
Boat is 1994 Bayliner Capri 1952CN

Jeepster04 · Mar 7, 2010

Re: 3.0 Mercruiser Tach Question

Did you check the Voltage at the blower to see what volts it was actually getting while running?

Got any corrosion on the battery terminals?

Blujay96 · Mar 7, 2010

Re: 3.0 Mercruiser Tach Question

Sounds slot like you have a bad conection somewhere

glennwolf · Mar 7, 2010

Re: 3.0 Mercruiser Tach Question

I would start by removing the battery and cleaning the connections. It is not good to leave batteries not in use in boats durring long periods of non-use.

nrf414 · Mar 9, 2010

Re: 3.0 Mercruiser Tach Question

No major corrosion on battery terminals or cables, the battery has been out of the boat since the fall. I cleaned the cables with a steel brush, but I think I'm going to try some Coca-Cola on a toothbrush just to ensure they are clean.

I did not test the voltage of the blower when running. I realize there's a short somewhere and I need to go back over the Mercruiser Service manual and wiring diagrams after the re-cleaning of the terminals.

It just blew my mind to see a tach go in reverse. EMI was my other suspect.

Bondo · Mar 9, 2010

Re: 3.0 Mercruiser Tach Question

I cleaned the cables with a steel brush, but I think I'm going to try some Coca-Cola on a toothbrush just to ensure they are clean.

Ayuh,.... That covers the battery ends,... What about the Other ends,..??

Windykid · Mar 9, 2010

Re: 3.0 Mercruiser Tach Question

Does sound like a main ground connection problem.

nrf414 · Mar 10, 2010

Re: 3.0 Mercruiser Tach Question

Yeah, I'll have to check both ends and ensure they are clean. It's supposed to rain for the next 4-5 days so I probably wont get to work on it til next week. I'll keep you guys posted for the solution when I find it.

rodbolt · Mar 10, 2010

Re: 3.0 Mercruiser Tach Question

trust me, you do not have a short.
shorts are simple to find,just watch where the smoke is coming from.
what you have is a loose or dirty connection or possibly a battery with an internal open circuit.
the voltmeter is telling you it has low voltage, its also what makes the tach crazy.

havasuboatman · Mar 11, 2010

Re: 3.0 Mercruiser Tach Question

The first thing you need to do is load test that battery. Just because a battery has 12 volts, doesn't mean it's good. Especially right off the charger.
But you can't do any accurate circuit testing with a faulty battery.

I/O WALDO · Mar 11, 2010

Re: 3.0 Mercruiser Tach Question

Possible problem at the main engine harness connector,un-plug it and look. Gotta connection issue somewhere!

nrf414 · Mar 20, 2010

Re: 3.0 Mercruiser Tach Question

alright guys I took a wire wheel brush and thoroughly cleaned the terminals at the battery end and checked them where they meet the engine. All looks good. So I tested the voltage when the key was off and found 13.5 Volts at the starter and solenoid. As soon as I turned the key to the on position it dropped to 3.05 volts. So I'm going to have the battery load tested at the local auto zone.

So right now I'm going to rule out:

Battery Load tested
Test Ignition Switch Terminals
Check Main Harness Plug

Any other places to look?

Fishermark · Mar 20, 2010

Re: 3.0 Mercruiser Tach Question

May as well wait till you find out if the battery is bad or not... I suspect that will be the real problem. If the battery turns out to be okay, (doubtful) - then you start checking the various connections.

nrf414 · Mar 20, 2010

Re: 3.0 Mercruiser Tach Question

Got the battery load tested and it had 0 CCA and low voltage....

70 bucks and 1 Deka Deep Cycle battery, a few revolutions of the engine and

Bada bing bada boom. She's running again!

So in retrospect, I have no idea what caused the death of the other battery.

It was 3 years old and used regularly last summer. Either way this is a cheap fix considering what it could have been.

Strangely enough my trucks battery died this year too. I must be a battery killer.

I'm off to continue prepping for its spring voyage. After working a ton of hours this week, this is a great reward!

Good luck to all that haven't made it this far, see ya out there.

Fishermark · Mar 20, 2010

Re: 3.0 Mercruiser Tach Question

I see this all the time with batteries. It seems like it used to be they would start showing signs of death long before actually dying. Now it seems they work well up to the point where they don't do anything. I don't know if that is a good thing or bad!

But glad you found the problem.

3.0 Mercruiser Tach Question

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