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  #21   IP: 174.192.26.225
Old 05-17-2019, 09:20 PM
tac tac is offline
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I should mention this afore forgetting it.

Your isolator and alternator failure may have been caused by opening (disconnecting) the alternator output while the engine was running. This causes a momentary (microseconds long) spike which results from the conversion of energy stored in the stator windings to voltage (Faraday’s Law). You will notice on the front of your disconnect/selector switch it says, “BEFORE SWITCHING TO OFF POSITION STOP ENGINE”. Well, that’s why.

But wait! There’s more!

Many “ordinary” rotary switches, and some old battery selector switches, have what are called “Break-Before-Make” contacts. That is, if the switch is in position 1 and its contacts are closed, when you turn it to position 2 the contacts first open (Break) before they reach the 2 position, where they then close (Make). As you can see, if you are in position 1 on battery #1, and the engine is running, when you switch to position 2 on battery #2, no matter how blinding-fast you are, the contacts will open, the alternator output is opened, and POP! goes another diode (or was that a rectifier?).

So most of these battery selector switches today are “Make-Before-Break”. That is, on switching from position 1 to 2, the contacts remain closed to position 1 until the switch is in position 2. Then, with the switch contacts closed in position 2, the contacts in position 1 can now open. Voila! The alternator output is never opened, and All's Well.

Well, almost all. You can still kill a running alternator by switching to the OFF position. Stop the engine first. Then switch to OFF.

Last edited by tac; 05-17-2019 at 09:25 PM.
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