5.7 Mercuiser - Voltage Drop During Acceleration

[mceagle]

Greetings! Need help with my 1986 5.7 Mercruiser / Sea Ray 22? Monaco. At idle, with no accessories on, blower, etc, the voltage at the battery is 12.8 volts. Applying power, (accelerating), the voltage at the battery and panel gage, drops and stays at 11.8 volts. I had the (Mando AR-150) alternator tested - results OK. The company who performed the test, let me borrow a re-built alternator which was tested before I left the shop. I installed the loaner and had the same results. Had the battery tested by two shops and was OK. Ideas would certainly be appreciated!

 

[Scott Danforth]

Re: 5.7 Mercuiser - Voltage Drop During Acceleration

are you using the gauge on the dash to read this, or a separate quality multi-meter?

 

[mceagle]

Re: 5.7 Mercuiser - Voltage Drop During Acceleration

are you using the gauge on the dash to read this, or a separate quality multi-meter?

I am using a digital VOM connected to the battery and observing the dash mounted gauge. The dash gauge replcates the actual VOM reading.

 

[Scott Danforth]

Re: 5.7 Mercuiser - Voltage Drop During Acceleration

I would suspect you have a loose connection or corroded connection, a bad wire / cable, or the plates in the battery are shifting.

 

[mceagle]

Re: 5.7 Mercuiser - Voltage Drop During Acceleration

Thanks for the insight and suggestion. I'll check connections etc this weekend.

I would suspect you have a loose connection or corroded connection, a bad wire / cable, or the plates in the battery are shifting.

 

[mceagle]

Re: 5.7 Mercuiser - Voltage Drop During Acceleration

I would suspect you have a loose connection or corroded connection, a bad wire / cable, or the plates in the battery are shifting.

After a bit of though, some ideas on the subject for comment….

The cable connections to the battery are clean - I'll have to look closer to determine the overall condition of the cable and the connection at the starter but if that was an issue, wouldn’t the performance of the starter would be effected as well?

If the B+ output connection from the alternator to the starter was compromised, then the voltage measured at the alternator would, theoretically, not drop during acceleration? Of course if there were bad plates in the battery, the effect would be the same regardless of where the voltage was measured…?? Thoughts?

 

[Bt Doctur]

Re: 5.7 Mercuiser - Voltage Drop During Acceleration

most shops spin then at 1725 or 3450 rpm`s. what ever your rpm`s are when the voltage drops make a note of.
crank is 6 inch, alt is 2 inch 1000 crank rpm= 3000 alt rpm. if your losing voltage at 3000rpm you would need to spin the alt at 9000 rpm to see the voltage drop from bad internal windings in the alt rotor windings.

 

[mceagle]

Re: 5.7 Mercuiser - Voltage Drop During Acceleration

most shops spin then at 1725 or 3450 rpm`s. what ever your rpm`s are when the voltage drops make a note of.
crank is 6 inch, alt is 2 inch 1000 crank rpm= 3000 alt rpm. if your losing voltage at 3000rpm you would need to spin the alt at 9000 rpm to see the voltage drop from bad internal windings in the alt rotor windings.

Thanks for your post! Very interesting information. My tach is not operational, however, the voltage drop occurs, my estimate below 2,000rpm - the boat doesn't reach plane before the drop.