Warm alternator

I have been chasing a parasitic amperage draw. I have noticed my alternator is warm when on the shore power charger and either battery bank selected. I have measured 3.1 A in wires from the alternator. It is on the trailer, engine is not running.

I am thinking’s least a/some alternator diodes are shot. Am I missing something?

K

Is your voltage regulator energized?. Other than just the diodes, check whether the field wire is constantly energizing the alternator field winding.

Your field current wire should go through the ignition so that it gets switched off when engines are not in use. I have seen so many alterations to this over the years that i’m Never surprised to find one wired around the ignition switch.

Some alternators are turned on by a pressure switch instead of the key. When the engine is started, oil pressure causes the pressure switch to close the contacts. As the pressure switch gets old it can leak current.

Some auto style alts have an internal V regulator , which is supposed to energize the field when the alt is spun. AKA "single wire".

Perhaps it failed ?

Something definitely not right if drawing 3A sitting. Either bad diode inside, or as others posted the exciter circuit is left energized. Will take a little sleuthing to figure out which.

7.65 volts at the field terminal with key off, battery switch to start battery. 4.3 v on the regulator ‘S’ terminal.

Something is not wired right. What alternator? What regulator?

Good question. I cannot find any identifying information on either the alternator nor the regulator - except the ‘Motorola’ logo on some of the alternator components. The engine is a Perkins 4.236 circa 1984.

K

"Good question. I cannot find any identifying information on either the alternator nor the regulator"

Sounds like most of the life has been used.

Might be time to head out to a OTR truck supply or the local Detroit dealer and pick up a brand new 135A, $135 or so large frame alt.

The pulley will be an extra , and you will need to select a fan that matches the alt rotation direction in your boat.

A 3 or 4 stage V regulator, would set you up for the next 40 years .

To have an "ultimate" (idiot proof) system a Cole Hersey ignition switch with an ACC terminal wired to $18.00 RV solenoid to charge the house bank, and you are done.

It looks like a Delco 10DN. I had one until recently on a 1971 Buick. Swapped it out for a 94 amp 12si internal regulator alternator for about $50. The 12Si has significantly higher output at idle. The swap requires a minor rewiring to replace the alternator plug and eliminate the regulator.

Looks like a Ford car regulator. Probably not original to the engine. Power supply to the reg should be switched and go through a resistor (usually a light bulb) or switched with an oil pressure switch. Trace the wires out and see how it is fed.

Also try unplugging the reg and leaving the B+ term on the alt and see if the drain persists. If no drain with reg unplugged, the big rectifier diodes in the alt are probably fine.

Well, Ski, you were correct. I traced wires, ascertained the regulator was tango-uniform and found one locally at Napa - 1979 Ford F-150. Haven’t fired it up yet but there is voltage where there should be, and not where it ought not be....

Except!

There is a mysterious purple wire - part of the original factory wiring harness - that is hot with 12.6 volts and was connected to the downstream side of the ignition switch. It comes on with the battery switch. Everything works right when it’s not connected. It goes down into the engine room and disappears... or so it seems. I have not had a chance to spend the hours it will take to chase it down. Ah boating fun.

Thank you all.