Reconditioned alternator not charging

I have a second hand Bukh 36 bought from the dealership, and when i fired it up the alternator was only charging at 12.25 volts. I took the alternator off and sent it back to Bukh. Apparently there had been a reversal of polarity or something similar which had fried the regulator. It came back to me reconditioned, tested, and with a heavy duty regulator.

I put it back on, wires as they were when i bought it, and the multi-meter was reading less than a volt!

The rest of the wiring is pretty simple. Any ideas on where to look next before i pay for an auto/marine electrician to come and have a look at it would be greatly appreciated.

 

So you bought a used Bukh with a wiring problem, fried the alternator, had it repaired and tried to fry it again!

The basic alternator has one large terminal and a small one. The large one is ALWAYS connected to a point where there is 12 volts from the battery to be charged. The most logical point is the (also large) terminal on the starter relay.

The small terminal is less simple. Many marine alternators should have this terminal connected to 12 v when the key switch is turned to ON. That is the way the alternators on a Mercruiser or Volvo Penta work.
But the others want a small pilot light connected in series to the switched 12 volts, like it is done in all passenger cars. Sometimes it is not just a lamp, but also a ballast resistor across it.
The lamp is lit as soon as the switch is on, feeding a little bit of current to the alternator to energize the rotor windings and build up a magnetic field. As soon as the engine starts the light goes out because the alternator generates it's own power.

If you connect such an alternator that requires a lamp to +12 volts directly, the regulator dies.....

 

Is that one of the alternators that need to be polarized when you install it?

 

Alternators produce 3-phase AC that is rectified by 6 diodes into approx. 14 VDC. The only requirement is that the rotor has a magnetic field; which side is north is irrelevant.

Several decades ago there were still some DC generators used; they could be wrongly polarized to produce a negative output voltage. But they are extinct, except a few museum pieces.

 

Thanks for the input. i got someone to come and have a look at it, and it was nothing more than a bad earth connection from the alternator. Its charging very nicely at 14 volts now.

 

An aside on Gonzo's post about polarizing.
When we put a Generator or a Regulator on some of the older cars with Generators, before Alternators, we had to hook everything up and short across the Regulator with a pair of Pliers.
I believe we shorted the "B" terminal to the "F" field terminal.
It worked, and if you didn't do it, Sometimes it worked, and if it didn't you just shorted the two terminals out and it did work.

Not knowing anything about Smoke n' Mirrors in those days, I was really confused when the Alternators came out. If you shorted those two terminals the Alternator suddenly wasn't any good?
When the Regulators dissapeared I was even more confused.

 

Thudplucker ..get unconfused ..you can have a dynamo neg earth or post earth( or have one of each and get 12 and 24v) but an alternator is neg earth only because of the electronics ..you usually need a warning light to energise it and 14 v is not enough you need 14.4 as a minimum or your battery life is much reduced ..at 13.6v you will only get 6 months

 

I think I know what your saying. Our lingo is a bit different.

As time went by and with some education, I found out about it all.
For instance the old Chrysler products had a Positive ground, everybody else had a Negative ground. I was sure glad when that difference finally went away.

On that subject, I recall an instance in a Tech school. We were told that all our schematics would be Negative ground, and all demonstrations would be the Conventional Current flow theory. Pos. to Neg.

But....One of the instructors was an old WWII, Tube type guy. His training was to start at the ground and move toward the Power supply.
We got into a circuit, tapping a voltage off one end of a Zener, and using it to trip an SCR (Crowbar circuit) where his old theory's failed him.

He was ankle deep in chalk dust from starting over repeatedly. He'd used up all his erasers and was badly in need of a cup of coffee, before he finally gave up and a student got up and finished the demonstration.

I realized he was right, but it was difficult to demonstrate it his way. Just one other form of "Language" barrier.