We have a Universal M3-20 diesel engine
with the stock Prestolite internally regulated alternator
which seems to put out 14 to 14.2 volts constantly no matter what the batteries state of charge is. It never drops below 13.8 and usually is around 14. This alternator
was just totally rebuilt this spring, new everything except the case and windings. I want to protect the diodes from accidentally shutting off the batteries (this necessitated the spring rebuild) and also to be able to turn off the alternator so it doesn't fry our little house bank which currently is two group 24 dual purpose 12 volt but will shortly be two Group 27 Trojan 12 volt deep cycles. We have minimal electrical
requirements as we have no icebox
, use LED lights
, have a 4 stage charger
, etc., etc.
We have a separate starter battery
wired direct to the starter with its own disconnect switch. The alternator voltage sense wire is on the alternator output so it sees the house bank, not the starter battery
. The starter battery is charged by a Xantrex Echo charger
off the house bank. We really don't need the Perko switch, but it is central in the panel, would leave a big ugly hole if not there and it does give us the ability to run the house off one battery if we have a dead cell or something in the other one. We carry an AWG1 jumper cable if we ever need to boost the starter from the house.
I replaced our Perko battery switch (8501) with an identical Perko switch that has the field disconnect circuit in it (8503). The field disconnect circuit takes the feed from the purple wire feeding the EXC (Excite) terminal on the alternator to the F1 terminal and then from the F2 terminal returns to the EXC terminal on the alternator. Then I put a manual switch in the line from the Perko field disconnect F2 to the alternator EXC terminal.
I thought that by breaking the power on the EXC terminal with the manual switch I could turn off the alterator output when the batteries are known to be topped up and not needing charge thus avoiding boiling them to an early death. But when I installed all this stuff and tested it, nothing seems to happen, the alternator keeps on putting out 14 to 14.2 volts even after I turn off the EXC circuit with the manual switch I added.
So I am guessing that this alternator only needs voltage on the EXC terminal until it starts making power, then somehow it is getting voltage on the field internally and no longer needs anything from the EXC terminal.
This raises two questions:
1./This indicates that the diode protection on the Perko switch is useless since breaking the field feed from the purple wire seems to do nothing, is that correct?
2./How do I turn this alternator off?
I know the real solution is a P type alternator with an external regulator
, but we are 5 years and 10 feet of boat away from being able to justify this equipment
. I just don't want to be buying
new house batteries every year.
Thanks in advance for any assistance.