Alternator output QSB 5.9

[john maybeck]

I have repowered with a factory recon QSB 5.9. The Alternator doesn’t not excite until I am underway and am up to @ 2300 rpm. Then I get 13.5 V + (-) at the helm which remains relatively constant at any rpm. Belts are tight and battery isolator is operational. If I jump the isolator Alt post to a 12 volt terminal it wakes up the alternator and I get the correct output at low rpm. Does a single lead self exciting alternator need to reach high rpm before it produces proper voltage?

Topic

[Tony Athens]

There is NO SUCH THING as a “QSB alternator”…..Your same alternator, regardless of what is is, could come on many different engines……………Not less than 5 possible factory QSB alternator options & none of them say “QSB alternator”.. .. Let’s ID the EXACT alternator.. That requires very good pictures and maybe an engine serial number too if what you have came from the factory on the original build..

Tony

[john maybeck]

1 Wire Alternator

Thanks Corey,
I understand the voltage requirements needed to excite the alternator but I don’t understand why the alternator will wake up once the RPM reaches higher levels which is fine as it then starts to charge the batteries. There is a bayonet connector on the isolator marked ” IG ” which would suggest that it be connected to the ignition circuit and once closed would provide 12 v to the alternator to wake it up.
The peculiar thing is that the original engine , which failed at 675 hours did excite the alt at low rpm on initial start. All wiring stayed the same.

[Corey Schmidt]

Sensing Issues with 1-wire Alternators

From our diode isolator article:

Some 1-wire “self exciting” alternators rely on a direct connection to the battery to sense and provide charge as they do not have a separate lead that can be connected to sense battery voltage. In this case a standard diode isolator will not work as the diode itself will block access to sense voltage from the batteries. In situations like this, it will be necessary to obtain a battery isolator that has a voltage output the alternator can sense… this is sometimes termed as a “Alternator energize input”. Diodes with an energize input feature a special current limited energize input that will power the B+ when the engine run/stop switch is closed.

How to Install a Diode Isolator with an Alternator

[Author]

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