2QM20 alternator wiring question - SailNet Community

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Old 08-19-2014
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2QM20 alternator wiring question

Wasn't sure whether to put this in the engine, diesel or electrical sub-forum, so it goes in here.

Now, I am trying to run all charging sources directly to my house bank, as I have an echo charger.

I've run into a bit of a situation here though - according to this diagram 4A-5 on page three of this PDF http://homepage.ntlworld.com/topcats...Alternator.PDF the charge lead goes to the battery via the starter, meaning that its only the starter battery that would get charged.

Now, I am wondering if I could replace that lead with another cable that runs right to the house battery, but I'm worried that the original run to the starter seems to go via the switch for some reason (maybe to make sure the charge lamp lights up)

Can I just disconnect the B (alt) to B (starter) cable and run it from B (alt) straight to the house bank? Or should I run that cable from the starter switch to the house bank?

I just don't want to blow any diodes.
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Re: 2QM20 alternator wiring question

That's what a battery switch is for...right? You have a 'battery lead' running from the engine to the starter battery. Disconnect it, and insert a battery switch at that point wired to each bank.
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Re: 2QM20 alternator wiring question

I'd rather not add a third switch in. I'm trying to set it up so it starts using the starter battery, but then charges the house bank (I have an echo charger) without need for manual intervention.

I'm wired up as this diagram (courtesy of Maine Sail)



so I already have a 1/2/OFF switch (the two position only being used in an emergency) and an ON/OFF switch for the starter
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Re: 2QM20 alternator wiring question

The B terminal on the starter and the B terminal on the alternator do not need to run to the same battery.
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Re: 2QM20 alternator wiring question

Quote:
Originally Posted by Alex W View Post
The B terminal on the starter and the B terminal on the alternator do not need to run to the same battery.
Perfect!
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Re: 2QM20 alternator wiring question

You shouldn't need to add the second switch, since there should (!) be a starter switch already, usually a key switch that powers the starter solenoid or starter relay and the starter itself at the same time.

In production boats, you can save a yard of heavy wire on every boat, by running the alternator output to the starter and then daisy-chaining it to the positive bus, instead of running to the bus and then back again to the starter. It really is that simple: this is done to save a yard of wire and a couple of extra minutes of assembly time.

You should be able to just pull the wires off the starter + terminal, and run a new one from that to the new starter battery terminal. Leaving the key switch on the old house bank, or moving that circuit as well. (If the main battery switch is OFF, the key won't start the engine unless you've also moved it to the starter battery.)

In any case, make sure to FUSE the new battery-to-starter wiring, usually just by putting a heavy primary fuse on that starter battery. The old standard was to use "fusible link" wiring, but that's considered a fire hazard now, even when it is run in insulated covers.
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Re: 2QM20 alternator wiring question

I think the only thing I need to figure out now is what wire the alternator as the voltage regulator 'battery sense' wire, as I'm guessing that if that was not on the house bank as well as the actual charge wire, Bad Things would happen.

Thanks for help all
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