Alternator Problems

[valand]

Hello, I have a problem with my alternator and as was installed by the previous owner, I have no documentaation about it, neither model or manufacturer. Yesterday I run the engine and I smelled burnt platic. Stopped the engine and checked around and this is the situation...
My alternator is a 100 a/h with an external charge controller, there are 3 studs used of the alternator, one little one and the bigger POS and NEG.
The thing I do not understand is that what burnt is a wire connected between POS and NEG with a fuse holder. The fuse inside is a bit different than normal ones, it has a coil that ends on a little metallic bar. What I found was the wire on the negative side completely melted so I reconnect it, put back the "fuse" and it blew immediately. I wonder if there should be a diode or something else because a fuse between POS and NEG is surely going to blow, unless this one is a kind of diode or some sort of regulator...
Anyone can give me some clue ??

[donradcliffe]

Do a google search on zap-stop. If you don't understand it, don't run your engine again until you do. Always be sure that your battery selector switch is set to a real battery bank while the engine is running.

[djmarchand]

The wire between the pos and neg is most likely a Zap-Stop. This is basically a diode that keeps from frying your alternator diodes if you accidently turn your battery switch to off while the engine is running.

The alternator will work fine without it. Remove it until you can replace it with a new one.

David

[valand]

Thanks, I found what it is and understand what it is supposed to do, but is the alternator supposed to work if this is blown ?
I didn't run the alternator with batteries disconnected, and now is not charging at all, so I guess I have a problem...

[donradcliffe]

Simple alternator test for externally regulated alternators--monitor the alternator output voltage while taking a wire from the positive terminal of the battery to the field connection on the alternator while the engine is running. This essentially bypasses the regulator, and calls for full output from the alternator. If the output voltage doesn't jump up at least half a volt, the alternator is dead, and in your case I would suspect the diodes are fried.

The zap-stop is supposed to limit the alternator voltage in the event that it is run with the batteries disconnected. However, it really only works with internally regulated alternators, which sense the alternator output voltage (making the voltage spike only a short-term transient).

With some externally regulated alternators, the regulator voltage sensing wire is connected to the battery, not the alternator. If SOMEONE turns the battery selector switch to off (it only takes half a second), the external regulator does not notice that the alternator voltage is spiking, and continues to excite the alternator to full output. First, this will melt the the zap-stop, then all that energy the alternator is putting out with nowhere to go will raise the output voltage until the diodes are fried.