Volt drop

[Bman03878]

I have a 07 5.7 mpi towsport. When running lately i noticed the volts were running low. Normal is between 13 and 14. Lately it’s 12. If boat sat for a week battery was dead. I disconnected radio, bilge, lights, blower, accessory, and horn. Anything that uses the battery. Nothing changed the draw. I did put a new battery in it and gave it new cap and rotor. So one day while it was running I disconnected the battery to see if alternator was bad. Boat didn’t stall so it’s good. I noticed though that the volts went up to where they are supposed to be. When reconnected battery they went back down. Even rpm dropped. Thought I had a bad battery. Bought another one and all was fine volts returned to 13-14 again. A week later went to start and battery was weak again. Now volts are running down again. Any thoughts on what this might be. Thanks for any help

 

[Bt Doctur]

shutting the battery switch to off when the motor is running will cause a spike in the voltage maybe destroying anything that is on not to mention blowing out the alt.
Remove the alt and have it tested or replaced or go buy a voltmeter and do the testing correctly . ECM`s are quite expensive

 

[Scott Danforth]

So one day while it was running I disconnected the battery to see if alternator was bad. Boat didn’t stall so it’s good. I noticed though that the volts went up to where they are supposed to be.

you probably fried the voltage regulator in the alternator by doing this.

never disconnect an alternator when running. normally they keep trying to up the output (because they cant sense the voltage) until they glow blue and then go up in flames.

 

[Bt Doctur]

And it`s such a wonderful blue too

 

[achris]

Once you have replaced your alternator, and anything else that got fried when you disconnect the battery on a running engine :facepalm:, get a multimeter set to the amps range and put it in series with the battery postive. This will tell you how much current is being drawn with nothing on. Then chase down the problem.

Chris....

 

[Bman03878]

so with all the comments of frying everything electrical, everything electrical still works as it should and the alternator is still producing the same amount of current to charge the battery. any other suggestions

 

[H20Rat]

You got really, really lucky you didn't fry anything. That normally doesn't happen, voltage will shoot up past 20v and take out every piece of electronics connected.

Anyway, re-read achris's post. Get a multimeter and hook it up inline. Do NOT touch the key. Just see what the draw is, and then start unplugging/pulling fuses until the draw drops to nearly zero.

 

[achris]

The fact the nothing got destroyed may very well indicate a bad alternator... ???

Chris....

 

[H20Rat]

Not impossible, and your alternator itself could be responsible for the parasitic draw if there is some type of internal short in it. But... as you saw, you still had power out of the alternator to power the ignition when the battery was disconnected.

Figure out the parasitic draw problem, it might lead you to whatever else is wrong.