New alternator difficulty

[footrope]

I just bought a new alternator to replace the stock(?) 55 amp. The old one was wired for an Alpha external regulator. The new Leece-Neville 51 amp alternator is nearly identical - although it has an on-purpose tap for the tachometer. I wired it for the Alpha regulator and when I turn on the key I get no green or red light on the regulator - and the battery voltage drops pretty badly.

This doesn't seem right - but the wiring checks out. I haven't tried to start it yet - just doing smoke tests. Any ideas? Thanks...

 

[ref_123]

Two suggestions...

Having no clue to that particular regulator but being a somewhat electronics person, I'd check two things. The key in my assumptions is that drop in voltage:

1) Polarity - your diods somewhere are getting a hit;
2) Short circuit somewhere, which may amount to a stuck rotor or something.

First is easy to find, and pray no diodes are dead so far;
Second - not so easy to find... But probably did not cause any drastic trouble yet... If wires are not fried, at least.

And of course, I may be wrong.

Good luck,
Stanly

 

[rssailor]

Wiring ideas

Looks like you need to check a few things. First I am assuming that the leece neville unit is a "P" type alternator (controlled by an external regulator). Ok if this is the case, then check to see if you are getting power to the regulator on the ingnition wire. Check the sense wire as well for power. Next check to make sure your DC negative wire has a good conection. After that lets move to the alternator. Check the output wire with Ancor's wire table to make sure it is the correct size. Look at a west marine cataloge by the wire and consult the tables for the wire length that you have. Make sure this is from the alt. to wherever the output on the alt. is to where the output wire ends. Next make sure to add the length of wire from the alt. to the DC negative block or battery, not to the engine block. Alternator's externally regulated need their own ground wire.
What kind of voltage drop are you getting and where are you measuring and comparing to figure out that you have a voltage drop. Ryan Moonglow Ericson 25+ owner Ryan's Marine

 

[footrope]

More information

Thanks for the ideas. I am not sure how to tell if the alternator is "P" type, but the folks that sold it to me showed me how to disconnect the integral regulator and wire it for the external one. The old alternator had a disconnected integral regulator, too. The new one is an 8MR2049KA. I went to the Prestolite website and they don't mention anything about P-type or how to attach an external regulator.

I took notes on the wiring when I removed the old alt. so I think the connections are right. In fact, the charging system worked fine. But I just had my house bank alternator fail while on vacation and I wanted to avoid the failure mode there, which was caused by a shorted rotor which then zapped the Alpha regulator used on that bank. So I was trying to get the old alt. rebuilt - but it was cheaper to buy new.

I went back and tried turning on the key with the field disconnected. The voltage drop did not occur and the regulator lights indicated all was well. I am suspicious of the low resistance across the field of the new alt. ~2 ohms; nearly a dead short. I measured the old alternator field resistance - it was about 60-200 ohms. Dirty old brushes and slip rings I guess. So possibly there's a clue there. Any thoughts on that?

I'm not sure what to do next. I may call the Prestolite technical folks, and I have a line up to the Xantrex folks in Arlington, WA who now support and repair the old Cruising Supply "Alpha" regulators. (They repaired both of mine - I managed to blow the second one while troubleshooting the other alternator.)

I'll post a resolution - and thanks again.

 

[footrope]

Happy ending

Well it was a tiny bit embarassing, but I found out yesterday that the new Prestolite alternator is fine. The problem was apparently my eyesight and my troubleshooting technique.

I had installed the gray brush/slipring cover they gave me in place of the integral regulator. In the process of tightening the terminals on it, I changed the orientation of the connectors underneath so that they shorted. Even though I was aware of the possibility and checked visually several times and "seeing" they were not, they were indeed shorted. I didn't bother to remove it completely and use the ohmmeter on it.

I hope I didn't hurt the regulator. Again. Possible sad story there.
*** Update - Regulator is not damaged, even after shorting across the field wire twice.

No charge for that and a quick output check from the folks at Romaine Electric in Seattle (on Sixth Ave S near the ballparks). They have a big starter/alternator rebuild operation. I recommend them as capable and kind to DIY folks like us. When I brought my ancient 55 amp to them for preventative maintenance, they recommended a new one since it would be cheaper. It was a really good deal because they credited me for the core exchange (~75 bucks) and then handed me back the old alternator. It needs some cleaning up - and I bet I could get new brushes and bearings from them and rebuild it myself if I wanted to. If the bearings are press-in I bet they'd do that for me. Now I have a spare alternator to carry around.

Years ago, when I had more time than money, I did that with a starter motor that had worn-out bearings. I did the disassembly/reassembly and a rebuild shop kindly sold me bearings and put them in the frame for me for nothing.

I'll remember the wiring checks recommended by Ryan, if I need them.