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Alternator problem part 2

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Hi mate, yes I remember this from the other site.....

A wiring diagram would come in real handy around now.....

Can you get a voltage reading from the wire that connects to the battery terminal on the alternator and a voltage reading across the battery?

Hi, we were getting the same reading of 15v on both, or do you mean just testing the alt cable when not connected to battery
These pics are one of the new ones and the original
The alternator feed to the batteries went through a heat sink first, then on to the split charge unit
It made no difference when the heat sink and charge unit were bypassed, nor when a direct earth was run from the block to the battery

Sent from my SM-G981B using Tapatalk



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Have had the 3rd new alt tested and it's good

When power is removed from the red wire terminal (under test) it starts upping the voltage to 15.4, fine when connected

So sounds like you were on the button with your diagnosis suzook

That red wire going to the reg/rec, does it com from the back of the ignition switch ?

All the alternators came with 2 flying leads, red and purple, which connect to the wiring harness via bullet connectors

The fault/break will be between the connector and the ignition switch?

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Hi mate, effectively it can come from the switch, although could be relay fed etc, that's where the wiring diagram comes in. What would also be an easy fix would be to replace the flying leads, much chinese stuff uses very poor wire, so making a couple of leads out of proper automotive grade cable wouldn't go amiss.

To find the other end without a schematic, you will need to be belling out (long length of test lead, disconnect and connect to test lead and "bell" for continuity with multimeter). Once found, disconnect and test with a "megger", this will give you an idea of how good your wire is... Alternatively, run a replacement wire and see if it cures it....

Hope that makes sense?

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Also, if it is fed from the switch, see what voltage you are getting from the switch, a worn contact inside the switch that drops the voltage output is quite common. You could either replace the switch should this be the case, or install a relay to feed direct from the battery and use the now redundant wire to activate the relay

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Getting confused now

Traced the red flying led from rear of alt (it connects to a green harness wire via a bullet con) this goes into a wiring centre and is made off with heat shrink


Hasn't been connected to anything since I have owned the boat

Run a temp cable from battery  to red flying lead on alt and voltage drops from 15.4 to 14.5

The other flying lead on alt gets battery voltage via a relay once engine is started


Edited by jonnyswamp
incorrect info
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I would connect the red to back of the switch then, I have a feeling the relay is the charge light relay, so with engine not running it supplies pos feed to the alt, soon as alt is running polarity reverses putting light out.....

Not sure why someone would disconnect the red/green wire, it's obviously supposed to be connected as bourne out by your testing


What engine/ alt is is? Will see if I can find a wiring diagram

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That purple wire was giving the alternator battery voltage via a relay when engine was started 

The red wire on the right is a temp one from the battery

When wired like this the battery voltage was 14.5v

I removed the temp red cable, put the purple one in its place and the voltage is still 14.5

Is this OK

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The only info I have was a sicker on the original alternator  (mando)which said Replacing LPA01926 This was cross matched to the one that's on there now

The engine is a 230hp mercruiser (black engine) on a 2004 boat

Do both of those flying leads need to connect to something, obviously the right hand one (red/green) hasn't been for some time 

Thanks for your patience zook

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exc will be exciter. this is the one that should be connected to your charge light, it goes pos frrd to alt light to alt, with the alt providing the neg, soon as engine is fired, then the alt outputs pos so the light is effectively getting =12V from either side and goes out....

The S terminal is your battery voltage sensing wire....

So what has basically happened is your alternator has been running flat out and died. Luckily for you 15V seems to be its cutoff overall or you'd have a lot of dead electronics....

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Something not adding up here then, nothing is connected to the exc terminal (battery light still goes out when started)

The cable that was connected, I put on the S terminal and all seems fine with an output of 14.5 v when running

I appreciate your time on this, but it isn't making sense 

I'm away tomorrow, but will go down Sunday morning and take a pic of the back of it, with terminal letters visible and maybe you can tell me what should go where and how/why it appears to be working now


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Common practice is to link the B terminal to the S terminal, which would explain why the red/green wire was not connected. The downside of this is it reads the voltage at the wrong end.....

The exc ia def connected to switchable power source, alternator lights can be a bit less simple these days

So if you link out B to S your dilemma will be solved, If you want a more accurate charge voltage, connect to the battery isolator switch, boat side.


I guess the two terminals are connected to some extent internall and probably provides enough to power the exciter coil

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13 minutes ago, Andy135 said:

Well done Swampy and well done Zook 👍

Is this our first fix of the new forum site then?

No, I diagnosed Fistys turbo issue for him. 
But we’ll done zook for sussing this one out. 

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