Weird hot spot on Alternator

[TomG]

Yesterday, I was doing some work on the mighty A-4 in my "new to me" old Tartan 30. I had not run the engine, but I had the battery switch on to run the blower. Because there wasn't a load on the battery after turning off the blower, I left the battery switch on. The battery was on for probably 20 minutes or so when by chance I put my hand on top of the alternator. I was surprised to find it very warm to the touch at one spot right here:



Any ideas why a cold engine would have a hot-spot on this part of the alternator? Nothing else was even warm. The ignition was off and the key was even hanging on the hook.

Tom

[hanleyclifford]

The fingers are on the output terminal. If you had been running the engine I would recommend that you renew the terminal since it looks questionable and may have resistance. But since you did not run the engine but had the heat buildup nonetheless, it would seem that current is flowing back to the alternator. Maybe the regulator is energizing the field without the engine running - not good. Disconnect that terminal and test to see if current is flowing without the engine running.

[TomG]

Hanley,

Thanks for the tips. I was doing some work on the boat yesterday, and followed your suggestions. First thing I did was check the hot spot with the battery off... it was cold. Then I turned the battery switch on with the ignition key off... it started getting hot. I then disconnected the output lead and checked the voltage. No volts were at the output terminal on the alternator but terminal on the orange output wire read 12.7V. It seems to me that the output wire should only be hot if the ignition switch is "on." Is this correct? Is this a symptom of a bad ignition/mis-wired ignition switch or is this the way it is supposed to be?

It just seems strange to have the alternator hot with the ignition switch off.

[hanleyclifford]

Tom - Your problem is in the alternator. With battery switch on you should have a live wire running to the alternator, but the wire should not be feeding current into the alternator. Take the alternator to a shop for testing of diodes unless you can do it yourself. Hanley

[roadnsky]

... I then disconnected the output lead and checked the voltage. No volts were at the output terminal on the alternator but terminal on the orange output wire read 12.7V...

Tom-
Where is the OTHER end of the Orange wire connected?

[TomG]

Hanley,

Thanks for the diagnosis. I'll get the alternator to the shop in the next few weeks.

Just so I understand where you're coming from, you say it's normal to have battery voltage to the output terminal with the battery switch on and the ignition switch off. I was operating under the assumption (I know, I know) that the output lead from the alternator is routed through the starter switch. I was thinking that with the start switch off, the alternator output lead should/would be dead. It would be connected to the battery with the start switch "on" to provide power to energize the alternator's field and then remain connected with the start switch "on" to charge the battery, once the engine was running. Otherwise, it seems that you are always exciting the field when operating on battery power with the engine not running. That's what I thought was going on here. Thanks for your help.

Jerry,

Tom-
Where is the OTHER end of the Orange wire connected?

Good question. I think it goes to the starter switch, but haven't had the time to trace it out yet.

[ndutton]

Tom,

Here's an Atomic 4 wiring drawing I saved some time ago. You'll see that the orange wire is energized from the battery switch through the ammeter (if you have one).

[hanleyclifford]

Neil - Thank you for that excellent illustration. Tom - Neil's diagram appears to show an externally regulated alternator. This is not the case for your set up. Your regulator is internal and the alternator shop will find out how current is getting thru that post when it should not. It is not certain that the field is being energized - but something is.

[TomG]

Thank you all for the great advice. I really appreciate the time and wisdom. I also really like the diagram... a picture is worth a thousand words!

[rigspelt]

Here's an Atomic 4 wiring drawing I saved some time ago.

That is one sweet image; it's all there (A4 small sailboat engine/charging wiring). Our boats all have variations, but it sure is easy to explain a large number of issues using that diagram. Should be posted in a sticky or something.

[JOHN COOKSON]

Tom

Try this:
Leave the key out of the ignition. While someone else flips the main battery switch from off to on you watch the ammeter and see if there is a small needle deflection. Any needle deflection would indicate current is flowing down the orange wire to a short or something haywire in the alternator.

With the battery switch on the red wire to the ammeter is energized. Me thinks current is flowing across the ammeter down to a short in the alternator. (same as Neil)

It is time for bench testing\repair of the alternator.

TRUE GRIT

[roadnsky]

Boyz...

Note that Neil's drawing represents the stock 35A Motorola Alternator.
At the bottom of the drawing it states:
"Alternator has a solid state regulator attached to it, with field wire and ground connected under it"
In Tom's pic it certainly appears that he has this alternator.
If you look closely, you can see the regulator screwed on the back.
Both it and the gasket are painted, so it's a little hard to spot, but it's there.

The alt should be checked out as everyone suggests.
I just wanted to point out that Tom has a standard set up and it's just possible that somewhere there is a short or a mis-wire. Worth looking for...

[TomG]

UPDATE:

Just got off the phone with Mammock's Electric here in Annapolis. They bench tested the alternator and found a blown diode. They also found the alternator regulator was under-charging by one whole volt. Looks like Hanley nailed it.

Thanks to all.

Now the question is, do I keep this 37amp alternator as a spare and upgrade to a 55 amp model, or just leave well enough alone? I know what I should do and I know what I probably will do...

[hanleyclifford]

Tom - Whether you step up to bigger alternator depends on how you use the boat. If you are a long range cruiser you should have a spare on board. It is possible to get really elaborate in this area (as I have) but don't waste money if you don't need to. Hanley

[Administrator]

Tom:

That "one whole volt" was probably costing you half the capacity of your battery(ies). If you were managing the way things were, I doubt you'd see much of a difference in upgrading to a larger alternator.

Bill