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TOPIC: Alternator charge wire to mbss?

Alternator charge wire to mbss? 03 Jul 2017 15:07 #1

  • sketch96
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I know I saw another thread on here before about this but I can't seem to find it now. So here goes again. I took a break from fibreglass to have a look at the wiring harness on my motor. Getting ready to pull the intake and heads. The charge wire from the alternator has been cut at the back of the motor and a wire spliced on that heads up to the common terminal of the mbss. I know it is supposed to go to the starter motor. But thinking about it would this really be a problem? All I can see that it would do is shorten the run that the charge would have to take to get to the common terminal of the mbss to be able to charge whichever battery is selected. The only down side I can see is that there would now be an extra wire running up and connecting to the common terminal of the mbss.

What are the thoughts on this? I am going to buy some heavier gauge wire to replace the spliced wire that the P.O. put in. I will route it back to the starter as original if need be. The original wire looks like an 8 gauge. I'm going to put in a 6 gauge. May upgrade to a larger alternator later. (I have a 65amp now.....up from the stock 55amp)

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Doug
1995 7.4l 2859 Bravo ll
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Prince George BC

Alternator charge wire to mbss? 03 Jul 2017 15:41 #2

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sketch96 wrote: .................. The charge wire from the alternator has been cut at the back of the Engine and a wire spliced on that heads up to the common terminal of the mbss.
I know it is supposed to go to the starter motor.
Yes.....I agree..... it should connect directly to the Common cable post at the starter motor. Schematically, this is identical!
This keeps the charge lead short and with less resistance!



But thinking about it would this really be a problem?
No..... not if the charge lead size has been increased, and if the circuit is protected against damage, etc!

All I can see that it would do is shorten the run that the charge would have to take to get to the common terminal of the mbss to be able to charge whichever battery is selected.
Ah.... but consider the size of the common cable!

The only down side I can see is that there would now be an extra wire running up and connecting to the common terminal of the mbss.
BINGO!
I would put it back to the OEM configuration!

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Rick E. Portland, Oregon
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Alternator charge wire to mbss? 11 Sep 2017 12:51 #3

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Well i finally realized why the alt charge wire had been changed from the oem configuration. Reassembling the boat i took the mbss off to add another circuit and the charge wire from the alt bypasses the mbss and runs to an isolator. I didnt know that it was an isolator......assumed it was something to do with the onboard battery charger!!! All this now after i ran a piece of #6 back across to the starter to put it back to factory!!!! Now in the process of changing it all back again!lol!

The isolator looks kinda cheap to me. Its a guest model. Anyone have any experience with these? I have a motomaster isolator kit i bought to install on my last boat but never installed it. It is larger and has fins for cooling. Looks better than the guest model thats in the boat now.

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Doug
1995 7.4l 2859 Bravo ll
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Alternator charge wire to mbss? 11 Sep 2017 13:56 #4

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Isolators are of Old Technology. They also offer a voltage drop of approx .7 volts.
You will have much better results by using an ACR or VSR.

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Rick E. Portland, Oregon
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Alternator charge wire to mbss? 11 Sep 2017 15:18 #5

  • Centerline2
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I agree that for the sake of your batteries you need to get rid of the old style isolator and get an acr/vsr (same things, different terminology) in its place... as Bounty said, the old isolators cause nearly a fill volt of drop across the terminals at the Isolator. (thats before you take into account any voltage drop due to the length/size of the wire)

this means you are starting off with a discharged battery, using it in a discharged state, and never bringing it up to full potential... which is unhealthy and not conducive to a long lived battery....

after switching to an ACR you will immediately notice that your motor cranks faster, and you may also notice that all your electronics remain on and working thru the moment while the engine is cranking...

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1989 Bayliner 2556, 5.7 OMC Cobra
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Alternator charge wire to mbss? 11 Sep 2017 15:31 #6

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Dad had an isolator on his big boat, freaking thing slowly sucked the life from the battery it was supposed to isolate :pinch:
Go back to original or go ACR as an upgrade. If your like me and your boat sits on a trailer plugged into shore power (onboard smart charger doing its thing) you probably don't even need an ACR. Been very happy with my "keep it simple" wiring with MBSS and no ACR yet.

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Alternator charge wire to mbss? 11 Sep 2017 15:40 #7

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+ 1 on the ACR. Keep the new wiring and abandon the old. Isolators are wired differently than a ACR. ACR is wired into the stock system. With the ACR primary run to start secondary to house.

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Alternator charge wire to mbss? 11 Sep 2017 21:58 #8

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Good to know! I had already wired in a new #6 chrge wire to the starter and nicely taped it all into the harness. Wasnt really wanting to remove it all to put it back to the isolator. I will remove the isolator altogether and put in an ACR in the near future. Will just be diligent with the mbss for now. Thanks for the replies!!!

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Doug
1995 7.4l 2859 Bravo ll
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Alternator charge wire to mbss? 11 Sep 2017 22:26 #9

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yachtman wrote: + 1 on the ACR. Keep the new wiring and abandon the old. Isolators are wired differently than a ACR. ACR is wired into the stock system. With the ACR primary run to start secondary to house.


with most common brands, the ACR will use the same wires as the old isolator...(heavier gauge wires may be a good upgrade at this time) but with an addition of a small ground wire from one of the batteries to the ACR.

the main difference in the connection being that the charge sources that used to run to the center post of the isolator will now connect directly to one of the batteries, (usually the house battery is the best choice for all charge sources to be connected to)... the automatic charge relay does the rest.

there are some diehard/oldschool people who like to use switches for the task of putting the charging source where they want it, and with an ACR, switches can still be added for combining the batteries in whatever combination one chooses.
personally, I like to know my batteries are always being tended to by whatever charge source is active, no matter what im doing or how sidetracked or forgetful that i might become.... and if one has solar on their boat, the acr becomes almost necessary to keep all the batts charged properly....
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