New alt and batts. Voltage drops with higher rpm

[ov10mech]

Hello,

I have been battling a frustrating 12v problem on my boat for some time. 1995 Sea Ray Sundancer 33

I just had the port side alternator completely rebuilt and they fully tested it. It’s a Mando 55A alternator.

I also have 3 DELCO Voyager Deep Cycle Batteries brand new (95 AH Each) .Two are wired in parallel on the port side where all the accessories draw off and for starting port side. The third battery serves the starboard side engine only. I have taken off all the batt connections , cleaned and re tightened everything down. I put a 135A load tester on each battery to make sure they were ok. Everything tested ok under 10 sec load.

Everything charges fine at the dock with on board charger. I always check the voltage as soon as I unplug the shore power . Port and starboard side show a little over 13v (engines not running and nothing drawing)

Now, if I start the port engine and monitor voltage it’s steadily a bit over 14 which I believe should be telling me the charging on that side is ok.. at idle anyway.

Here is where it gets interesting. When I start increasing RPMs my voltage actually drops on the port dash gauge. I recorded a short video of this here:

http://www.youtube.com/watch?v=dzaqDyjZ3q4


To rule out the gauge being bad I also put a multi meter on the back of the gauge and it is indeed dropping in the 12s when RPMs go up. BUT, If I put a meter on one of the port batteries while revving it does not fluctuate at all.


So my symptoms are usually dead batteries on the port side after parking for dinner for 2-3 hours . The only thing running is the fridge. I would think on 2 fully charged batteries this should be no problem. When I go to start the boat I sometimes have to jump the port side using the emergency switch that takes juice from the starboard battery. My theory is the more I use the boat, the more the port side drains since I effectively don't charge above idle and leaves me little capacity when docked away from my slip.


So at this point I don't know if its possible I may have hooked something up wrong when i replaced my rebuilt alternator or if I should be looking elsewhere?

Sorry for the long post.. Just wanted to include as much info as possible. If you have any other tests I can run I am all ears.

 

[njmello]

Sounds like a bad diode in the port alt. I'd take it back to the rebuilder and have it bench checked.

 

[mistercomputerman]

Also, swap the alternators between engines and see if the problem follows the alternator or stays with the engine/battery bank.

 

[Bt Doctur]

Mando`s like to go open in the windings at high rpm.most shops test at 3600 + or - .Have the shop test it at a much higher rpm to see if the voltage drops off.I`d replace it with a GM 1 wire.

 

[ov10mech]

Thanks for replies. I have heard a lot about swapping for the GM one. I did find at WOT 4000 RPMish that the voltage does increase to 14ish but I am rarely in that range. On that port side I need to be able to run a couple of fridges and a stereo so I need an alternator that is going to push some power at lower RPM and charge the 2 batteries up for me. Can the GM one do that better than the Mando? Is there any bracket changes, and does anyone have a link to the GM one that replaces the Mando 55? Thanks for all of your help. This has been frustrating and costly. I have another post going in the 330 forum and took some gauge pictures today while I was out cruising today.

http://clubsearay.com/showthread.php/12648-Official-330-Sundancer-Thread?p=615688#post615688

 

[ov10mech]

Sounds like a bad diode in the port alt. I'd take it back to the rebuilder and have it bench checked.

Had marina install another one from shop and its a tad better, but not much. Trying to figure out what is normal for these Mandos..

 

[2000SEARAY280SS]

have you checked the voltage at the batteries? i have had to replace two of those teleflex voltage gauges.