Prestolite Alternator Permanant Discharge

[SeamanStaines]

Hi,

I have just replace an alternator on one of my Yanmar 3JH3E engine. It was not the original hitachi unit but was a prestolite so I ordered and identical unit. When I first fitted it I had no output which I traced to a fuse blown on the 'sense' (marked exc) line which I replaced. Bingo, worked fine and I had the expected output.

But..... A few days later I was trying to work out why my wind generator did not appear to be charging. After much investigation I worked out I had a permanent 3.4A discharge from the main battery banks. After much fiddling around I eventually worked out it was through the 'sense' wire on the newly fitted Alternator.

The alternator has three connections, the main positive output which goes to the input of a splitting diode, the 'sense' connector which is connected to the positive side of the house batteries and a further connector to the ignition system (which I is marked volt sense).

I cant see why I would have a permanent drain through the alternator when the engine is off? The other engine, with an identical alternator does not exhibit this problem.

 

[pvb]

I had a similar problem a few years ago with my 90A Prestolite alternator. Turned out to be a faulty regulator - these are easy to replace.

 

[SeamanStaines]

Thanks, is there any way to test the regulator to see if that is the culprit?

 

[pvb]

I don't think so.

 

[pampas]

Assuming all connections are correct. Does the alternator charge and if so is the voltage at the battery within the limits of your charging system? If the voltage rises when given more revs the I would expect a main diode to be faulty.
Suggest removal and a test at your local auto electrical repair shop. testing should not be a lot of Dosh and you would have your answer in minutes.

 

[SeamanStaines]

Well, all works as I would expect when the engine is running, charging at about 55A (fairly low house batteries). This does drop at tick over bit goes to full charge at about 1200rpm and stays at that rate irrespective of engine revs.

Guess I will have to take it out :-(

 

[pampas]

Usually with one main diode faulty a decent charge is shown, the critical being the battery voltage which in my past life, rises to very high values and boils the battery,

 

[SeamanStaines]

Yeah,

Thats what happened to the alternator this one is a replacement for, fortunatly without boiling the batteries

 

[Bassplayer]

Problem is.... It's a Prestolite. QED. Terrible things in my experience. Always going wrong and cooking the batteries etc. Best used as a door stop/anchor after you fit a decent unit!!

 

[pvb]

Problem is.... It's a Prestolite. QED. Terrible things in my experience. Always going wrong and cooking the batteries etc. Best used as a door stop/anchor after you fit a decent unit!!

Your experience is unusual. Prestolite alternators have an excellent reputation.