Question on GM alternator wiring on older Cummins 3.9M

Hi Folks,

I have had my home-built boat in the water for 3 summers of limited use/ testing, but due to other priorities, never bothered to see why the alternator was not charging - it has a 1991 Cummins 4bt - 3.9marine engine and it doesn't need alot of electricity - boat is bare bones right now so i just put a battery charger on every couple of weeks so it would start ok.
Engine was ordered new originally (have serial number/ original order codes) with a Bosch K1 alternator but i assume it was changed in a previous life/ boat as the one i have resembles an older/ common GM alternator, with the terminal 1 and 2 spade tip connectors.
As it was not charging I originally thought it was a self-exiting 1 wire alternator *and as tyhe en gine sat for a couple of years without starting - based on youtube videos, i thought if i jumped terminal 1 to the charging/battery connector it would then exite it and work. It did not.
So I bought a new GM alternator and same thing.
Next, after seeing pics on the net of GM alternators with a short (red) lead from terminal 2 to the charging/battery connector, I tried this... Much better - about 13V at the panel voltmeter, and 12.75 at the large 8D battery. But, I could not shut the engine off (it has been converted at panel from key and pushbutton to a car style key previously). When i turn off the key, the engine still ran -until I pulled the short lead off of terminal 2...
So, I thought as it seemed to like seeing 12V at terminal 2 I should be able to run a wire from the key switch (switched side) to terminal 2 - thinking that now it would charge properly and when i turn the key off terminal 2 would not see the 12V (like when i pulled it off to shut down the engine) and therefore let the engine shut down...Wrong. It seemed to charge with 14V at the panel voltmeter, but much less or none at the 8D battery - and it would not shut engine off either... i had to pull lead off terminal 2 again..to shut off the engine...
Help please!!!
Note - wiring harness was butchered ab it before me but i only see one wire- red near the alternator - which i have connected to the charging/batt terminal....

 

You need to draw out a wiring diagram and ID the alternator. Many people change alternators from stock. I use GM CS-144s on my mains. Some pics of common GM alternators. Also SI wiring diagram. Once ID'ed, lots of info on GM alternators on the web. Most can be converted to single wire. SI type need the resistance of the charge light or some other resistance. SI are the most common I find in boats.

 

Thanks very much for the detailed response!
I determined that the new (and likely old) alternator is a 10SI -
I have the engine and panel wiring diagrams from Seaboard marine's website, but I am not finding wire which should go to the engine charging light from the alternator terminal #1. I am not great at reading wiring diagrams so i cam confused to if it looks like it should be a brown or red wire??

Also, i traced out the wire from the charging/ batt terminal of the alternator (which was yellow) but in the end was spliced to the purple wire in the harness.
So I am thinking this is wrong and I should run a new (red) wire from the charging/batt terminal to the red wire on the starter, correct?

As mentioned above, I did have the terminal 2 wired to the charging/battery connection and it did charge, so by this account and the diagrams on the SI series, I believe i need this connection - however, the big downside was the engine would not shut off - so I am hoping if I find the correct wire (red or brown) which should go to the charging lamp, that this will be controller by the ignition and thus allow the engine to shut off??

Thanks in advance for any further help
Note just looked again and took attached pic. I only see a tan wire in harness (for temp sensor) and not a brown wire... and the wiring diagram / plug / harness does not show a brown wire - it is only pictured/ listed in the sensor/ alternator connection diagram.... where is the brown wire found, if it is NOT found in the engine harness?

 

UPDATE - I ran a new red wire from the alt charging/batt terminal to the hot starter lead (from battery) and ran a new red wire from term 2 to the alt charging/batt terminal. It now charges at about 14V and the engine successfully shuts off with the key.

My only concern (unless someone raises other concerns..) is that the charging/ batt terminal and terminal 2 are now hot all the time, even with the key off (as they are basically connected to the positive battery post). I do not yet have a main battery on/off switch installed and was just leaving the battery connected.
So, is this wired correctly and if so will this drain the battery?

Also, should i have fuses in-line anywhere here (i.e. between the starter connection and the alternator charge/ batt terminal and if so, what size?

If it is wired correctly now, I can add a main battery switch without much trouble - just haven't gotten to it as my cabin / console design will likely change this winter if I have time. I have a used main switch and small panel and had planned on running temporary lights to get through the summer anyway, so i could do this now...

 

I changed it up a bit and ran the alternator charge wire directly to my large 8D battery, installed a battery switch (wired to the 8D as battery 1 and a smaller, also flooded battery as battery 2). I can start with either battery and can charge batt 1 by itself (default) or can choose both and charge the smaller battery when desired without worrying about blowing the alternator if I switch to off by mistake.
I know there are newer, other nice ways to wire batteries and charge them with battery combiners, etc - but i think this will work ok for me, using what equipment I already have and not spending too much more on a partially finished boat...
I am open to other advice and input though!!

 

The + terminal on the alternator is normally hot. I turn my alternators on with an oil pressure switch. When the engines start, as soon as the pressure hits about 5 psi, the light circuit is energized. In the diagram I posted, the pressure switch goes where the ignition switch is shown. It's a good idea to have switches isolating your batteries so if at anchor, you can't run down the starting battery. You can charge multiple batteries that are switched off by using a diode connected across the terminals. A diode only allows current to travel in one direction. There should be instructions on the web or Youtube. There are many electronic things you can build or make that are useful. A 50 amp diode is less than $10 on ebay.
Everything should be fused. Otherwise in a dead short, the wire may overheat and catch fire, and catch other things on fire. Fire on a boat is worse than on land. If the fire gets out of control, where do you go? The fuse should be rated 30-50% more than the load. As you get your boat finished, think about an emergence battery near your wheel. So in a 12v system failure you can still use your radio. Mine has a SPDT switch. Normally radios, lights and bilge pumps run off the 12v house battery system in the engineroom, but by throwing the switch I still can power them. I use a diode across the terminals so when normal charging is happening the emergency battery is being charged.

 

Thank you for the additional advice and pointing out specific concerns. I also have a 40 year old sailboat, which I did alot of work on - between reading Don Casey's book on sailboat / systems maintenance, and working on the sailboat over the years, I am aware that (at least on older, original boats) there was not alot of fusing of main cables, etc. I have added some over the years as I replaced batteries, added an electric windlass, etc.
Although I am a jack of all trades, and master of none.... i feel that doing the work oneself makes it easier to fix or rig up something in an emergency, as you tend to know your boat systems quite well.
This winter I have to weld in floors (currently juts have cheap OSB/ply floor panels) and may move the helm/Console as a small cabin roof and head works through it's design... once these things are more permanent I will do a better job of laying out cables, switches, panels, etc.
Thanks again for taking the time to answer my questions!