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View Full Version : Low Output on new alternator - HELP


prostar205
05-13-2006, 10:10 AM
I recently upgraded the stereo in my '02 X30. I added two Blue Top Optima batteries just to run the stereo. I also upgraded the alternator from its stock 51A output to 101A. Everything is working great except the alternator's output at idle, which is only 12.2V on each battery bank. At 1,500 rpm (engine), the alternator charges both battery banks at 13.8V and at 2,000 rpm, I am getting 14.2V at the batteries. I tried putting a smaller pulley on the alternator to get it to shin faster at idle but that did not improve the situation. The shop that did the work said I could put a variable voltage regualtor on the alternator but I have no room behind the alternator for that device.

Anyone have any other ideas other than getting a custom alternator built?

Workin' 4 Toys
05-13-2006, 09:55 PM
I come to a conclusion by your signature note : the possesed boat, However. If you "upgraded" the alternator to 100 AMPS did you increase the wire gauge?
Next, are you reading all your voltages with a tester?
Last, was the pulley a different size on the old one?

prostar205
05-14-2006, 11:50 AM
W4Ts -

Yes, all the wiring has been upgraded to 2 AWG wire - everywhere that comes in contact the alternator.

Yes, all of my voltage readings have been done at the battery with a Fluke Digital Volt Meter.

Yes, the new pulley is smaller than the original. The new pulley has a diameter of 2". I think the old one was 2.25".

I took the boat out yesterday for 4 hours. Under load, when I get the RPM above 1,000, the dash volt meter reads slightly over 13 V.

JimN
05-14-2006, 11:59 AM
I guess the next thing to do is disconnect the additional batteries and see if the voltage comes up. A regular alternator won't give you 14.4V at idle, so I wouldn't worry too much. Under load, you would get even less with either alternator.

Did you upgrade the engine ground cable, too?

prostar205
05-15-2006, 02:02 PM
Yes, both the engine ground and the positive lead have been upgraded to 2 AWG wire. The positive lead does directly to battery 1 (main starting battery).

At this point, I am going to leave it the way it is. If the batteries go dead over a period of time, I will look into having a custom alternator built with a variable voltage regulator.

Thanks for everyone's advice.

zberger
05-15-2006, 02:57 PM
Under a load with 3 batteries you should be getting around what you are.. the alternator is having to work 3 times as hard.

Answer the following questions for me..

How long is the wire from the alt, to the ground, also from the alt to your battery bank.

Do you have any sort of isolator or anything, because that will suck some voltage up.

I would be willing to bet if you take off the other 2 batts.. it starts reading around 13 at idle...

If you read off of each battery what reading are you getting.. when the motor is off, you should probably get around 13.4 or so.. if i remember correctly.

prostar205
05-15-2006, 03:18 PM
The 2 AWG ground wire is 4 feet long.

The 2 AWG poistive lead is slightly over 3 feet long.

There is no isolator. However, I do have a Perko Battery Switch wired between each battery bank. My understanding it that the battery switch allows me to choose which battery gets charged (1, Both, 2, off).

I have not put my multimeter on each battery bank to see what the battery voltage is when the engine is off. I would guess it would be around 12.2V.

zberger
05-15-2006, 05:55 PM
The 2 AWG ground wire is 4 feet long.

The 2 AWG poistive lead is slightly over 3 feet long.

There is no isolator. However, I do have a Perko Battery Switch wired between each battery bank. My understanding it that the battery switch allows me to choose which battery gets charged (1, Both, 2, off).

I have not put my multimeter on each battery bank to see what the battery voltage is when the engine is off. I would guess it would be around 12.2V.

ah, I am not a huge fan of the ol perko switch, but mr.diesel is.

In the installation I did, we used a napa isolator. it was cheap and worked for what we were going to be doing, and most importantly requires no effort on my part to remember to turn it on or off.

those grounds are definetely short enough, probably as short as they can be..

101 amp, still might not be enough to charge 3 batteries. I would prefer an alt with an automatic regulator.. less things to break imo.

how often are you gonna have the voltage turned up? probably all the time.

JimN
05-15-2006, 06:12 PM
One issue with isolators is that there is a voltage drop going through them, usually about .7Vdc. Significant at idle when the alternator isn't providing full output.

prostar205
05-15-2006, 06:14 PM
JimN -

I completely agree. That is why I chose not to use my isolator. I have one but will not put it in.

Regarding the Perko Switch, I plan on changing that out for the Borg Warner relay that Mr. Diesel used in his installation. I don't know if that will fix my problem.

prostar205
05-15-2006, 06:16 PM
zberger -

If you are referring to how often I will have the volume of the stereo up, probably 25% of the time which is the amount of time I ski and wakebaord behind the boat. The other time spent on the boat is with the family (6 yr old and 4 yr old) and the wife.