hot alternator

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Veteran Member
Feb 19, 2016
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CHB 34'
We have a 34 CHB with a single 120 Lehman. On my last visit to the engine room, (with engine cold/switches off...ran a week previously) I felt the alternator was hot. Is it connected up wrong, or what? We have used it like this for a while...since I replaced due to an old low output on the original unit about 50 hrs ago.


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Just a wild guess, but it is possible the field is being energized when you think the field is turned off. This could create an active field current that creates heat. I can't tell if it has a separate field energize connection so it could be internal meaning a failure in the field circuit. Again, this is just a wild guess. If it does have a separate field wire, disconnect it and see if heat remains.
The back of that alternator looks a lot like my Leece Neville 90 amp in which case it's likely a 1 wire (self exciting) version. No seperate energize circut just a + from battery and a - to ground. In some models the ground is to case and some to engine (check continuity between the case and neg terminal to see which you should use). Mine is never hot when not running so I'd suspect an internal fault.
Maybe there is an ignition wire that turns on the field current and somehow the ignition wire stays energized even though the engine is off.

I suspected it might be a "one wire" alternator. Then a short or bad diode could leave the field energized even when shut down. You should be able to take the alternator in to a car parts store and have them run a test on it. If it is an internal short then you will need to replace or rebuild it.
I had the same thing on a new alternator. Mine would be very hot days after I ran the boat. I disconnected the red wire and all is well, runs fine, charges batts, etc.
A diode is like a check valve for electricity. It only allows electricity to flow in one direction only. A diode bank converts the alternator AC current to DC. One wire alternators and many others leave the alternator connected and rely on the diodes to stop the current flow when the engine is not running.
You can remove the diode pack and test them one by one to see if current flows both ways. Replacement diodes can be found on the internet. IMO alternators are not hard to disassemble and fix.
Probably Youtube videos on alternator repair.

Some part sources:
The diodes are bad. This is usually caused by switching your main battery switch while running (if you don't have a field disconnect-type switch). If your battery switch is original, I doubt if it has the field disconnect function. I had the same problem on my 50' Trader. Ben
It may be a more complicated problem, but more likely than not the regulator is on all the time driving the field and generating heat. And draining your battery too. You will have to do some sleuthing on the particular device, but there usually is a wire that is intended to be connected to the engine's ignition that turns on the regulator. That wire is probably connected directly (directly or indirectly) to the positive battery terminal on the alternator thereby leaving the regulator on all the time.
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