Electrical deficit???

I need some input on this! I have an 87 SN (351pcm) with the standard alternator (30 amp???) and internal voltage regulator. I'm running a marine battery (die-hard) as well. My boat seems to slowly discharge as I run it. Over about a 2 month period of running on the weekends the battery voltage slowly works its way down to about 10 volts, the boat horn barely blow, the stereo gets a little weak, and the running lights are dim. For accessories I have digital depth, Pro-pass, and FM stereo (they are powered up after start up and run constantly). I ski a lot so there are a lot of engine starts as well. Even tho my voltage gets down to 10 volts the boat still starts although it doesn't crank as fast.   I've checked my voltage at the battery (in the past) and it was 13.5 volts. The battery is properly serviced, and the belt tension is up on my alternator. There is no belt slippage, although the alternator is making a squeaking sound that you can hear at idle (always has). Is it possible to have a charging alternator and still have an electrical deficit??? I never see 13.5 volts at my volt meter (when running on the water), however I do see 12.5 volts after my battery maintainer has been plugged in over night in the garage. Or,is it possible to have an alternator thats putting out 13.5 volts but very few amps? (Correctcrafts really need an amp meter not a volt meter). I'm considering upgrading to the 50 amp alternator from Discount Inboard Marine that is suppose to be a bolt on replacement. Has anyone done this mod? Any idea's about what is going on????

Hi skyhawkflyer

An idea for you.

Why don't you put in a Delco single wire alternator.

It's a simple hookup and the mounting pattern is the same as your original Prestolite.
You just have to drill out the bottom hole on the alt slightly so you can use the original mounting bolt.

You can find them from reputable rebuilders on EBAY for about $65.00 or so plus shipping for a rebuilt marine single wire alternator

I've had one in my boat for quite a while now, works great and if you ever need parts they are easy to get unlike your Prestolite.

The Delco has an internal self exciting voltage regulator so you can ditch the external regulator that goes with the Prestolite.

Somebody is bound to say to get the 3 wire Delco, it's a little harder to hook up the wiring and for a boat without much for electrical loads, the single wire works just fine.

Now you have one idea, maybe somebody will have some comments after reading this but that was my solution after my alternator died and it's worked great for years now.

Keno

Sounds like as mentioned, you have a leaking diode in the alternator. Remember 13.6v is the mininum voltage running. A delco 3 wire does not contain the internal flame arresting as a marine unit does. The single wire should be no problem. The 10ga output wire you now have on the engine is fine.

Hi skyhawkflyer

Well,at least I generated some discussion for ya'.

I'll bet that like Jeff and Nate said you have a bad diode so it won't put out full capacity.

Also as far as the 3 wire, they make a marine version of that too. I have one sitting in the garage that is definitely "marine". About the same price as the single wire for a rebuilt.

keno

Thanks guy's. I can do all of that, I'm putting it in the water this weekend. I do have a very nice VTOM so I can tap in and read the amps. Something in the system isn't up to par.... but I think we're honing in. I'll get back to you sometime over the weekend with results, Thanks TJ
(Vince as SkiDim said I should have the original 45amp alternator so it should be capable of handling all accessories)

Measuring the voltage at the alternator stud is the way to go. When it didnt recover on the gauge, was it still 14v at the stud? Your alternator is fine if you turned all accessories and it remained at 14v at idle. If the alt were bad, the voltage would drop to 12.5 or less at the stud. Sounds like we have a voltage drop issue. If you take a voltmeter and measure between the - of the battery and the engine chassis, then between the + of the battery and the + accessory bussbar feed under the dash, and between the - batt post and the - bussbar under the dash. These readings will tell you the voltage drop between those items. You cant go by the voltmeter on the dash for accurate readings due to voltage drop in wiring and meter inaccuracy.

14 volts is fine. Everyone may be a little different depending on the regulator and sense wire. The MAIN goal is that the battery better be above 13.5v in order to work properly. You may get 14.5v or higher at the alternator stud if you are drawing a big load due to the voltage drop in the output wire. Anything above 14v or so will do nothing but boil your battery. Also, you will find the voltage higher when the alternator is cold and then will reduce some when it heats up. This is normal. Mine runs at 13.7v at the battery after warm up. Dont always go by the voltage reading at the stud, as it may be extremely high if the batt is low and taking a charge. May get 15v at the stud and 14v at the batt. Good luck on the trip!!

Thanks Jeff, We've been flying pretty heavy in the Mississippi area. I'm on my way to Stennis in a few minutes. Disaster relief!