Merc 75 HP Mystery

"Hello all , I am new here and

"Hello all , I am new here and could use some help on figuring out a problem with my merc.Last season it ran fine but the voltage gauge on the dash intermittently went haywire.It would climb quickly to 18 volts{ pegged } and then drop back to normal at around 12 to 13.Did this just about every trip.Also , when the needle jumped up it would trigger my overheat alarm,but I don't believe it ever really overheated.Anyway , I recently got a nice vector charger and tested the battery.I got a fault code "F01" which means there is an internally shorted cell.The manual says if you get that code,throw the battery out.My question is ,if my battery really has a shorted cell,could this have been the cause of my voltage needle going nuts , OR....maybe it was the other way around - I have a bad stator ,and it CAUSED the battery to go bad? At any rate , with all this,the battery worked and cranked just fine last season.Now I'm at a point where I don't want to spend the money on a new stator if it is just the battery that is shorted out."

"I just LOVE a good mystery!

"I just LOVE a good mystery!

Which came first, the chicken (dead cell) or the egg (bad alternator circuit)? Hard to say, but you'll soon find out after the battery is replaced, right?

Good luck, and Happy New Year!

Jeff"

"not sure because I did not te

"not sure because I did not test the battery untill two nights ago.It's a catch 22 - I could get a new battery and possibly damage it again if it is my stator or some other issue with the motor.Or I could replace the stator or rectifier and not really need to.Guess I should go for both fixes.Also , if the battery has a dead cell , why would it crank over for me just fine?"

The stator and rectifier can b

The stator and rectifier can be tested! Good trouble shooting goes before replacing parts by guessing.

"I doubt that you will ruin a

"I doubt that you will ruin a new battery if you run it together with a bad charging for a short while. Just make sure it doesn't run dry. On the other hand, I think you also have some problem in your charging or circuit system.
First, check all the connections between motor and battery. A bad connection can act as if the battery was disconnected, and shoot the rectifier.

A shorted cell is mostly due to age and conductive lead deposits in the bottom. It acts just as if you had a jumper cable on that cell and you will lose 2 volts. The charge regulator will read 10 volts and tries to boost the charging to 13,7 volts. It increases the current to max charge current, and might get overheated after a while.
If you still have the problem after replacing battery and cleaning the connections, continue with the charging system.
Robert"

"Well I wasn't convinced t

"Well I wasn't convinced the battery was bad so I put it on my vector for a second time.This time the fault code read "F03" which is an "open cell" OR a sulfated battery.I proceeded on recondition,or "desulphate mode" for 24 hrs and then put it on charge.It charged on about 2.8 for approx 8 hrs and then read "full" and the charger shut off. Maybe I don't have to throw it away after all but I don't know why the charger first read F01{shorted} then the next day F03.So are you saying the battery itself could be the cause of my voltage needle going nuts or does it sound more like an electrical circuit problem? Bare with me I am not too good with this kind of stuff."

"Test the cells with an acid t

"Test the cells with an acid tester (you know a glass tube with a float). You can easily see if the cell "takes charge". there will be a significant difference between good and bad cells.
With a short in one cell, that particular cell will not take charge. If you have an open cell there will be no charge current in any cell (cells are connected in series). Do you get 18 volts if you measure the voltage directly on the battery poles? If so, I would suspect a temporary discontinuity inside the battery (the battery would "swallow" a high voltage with an increased current).
Robert"