Alternators need engine revs to work

[svfinlandia]

Originally Posted by cottonsail

There's a device called a "tiny Tach". The transducer clamps to an injector line and picks up the pulse. Add 12 volts and you have a tach.

Apparently my old Perkins 4.236 has the tach pick up somewhere other than the alternator, because when my old alternator seized and threw the belt the tach was still working.
I never have been sure where the tach gets its pulse.

Al, S/V Finlandia

[Bill Seal]

From a dead start, the "idiot light" used to be used to give a +12v kick to get it self exciting. LED lamps don't draw enough current to get things going. As was mentioned above, just goose the throttle upon starting.

[MartinR]

Completely normal, as others have written.

[GordMay]

"The 12 Volt Doctor’s Alternator Book" ~ by Edgar J. Beyn
Free ➥ http://www.nautickiforum.com/biblioteka/alternatori.pdf

Allso by Edgar J. Beyn:
The 12 Volt Doctor's Practical Handbook [For the Boat's Electric System]
➥ http://www.melody-in-blues.org/downl...lthandbook.pdf

The 12 Volt Doctor's Project Book ➥

[Scaramanga F25]

A marine mechanic told me that it is best to charge the batteries at 1,200 RPM and to blip the throttle occasionally.

[seabreez]

Originally Posted by Scaramanga F25

A marine mechanic told me that it is best to charge the batteries at 1,200 RPM and to blip the throttle occasionally.

That mechanic knows what he is talking about. At 1200 rpms is the optimum charging rate / speed / less wear and tear...

[team karst]

Originally Posted by MartinR

Completely normal, as others have written.

my twin VP's do same about half the starts... a quick throttle shot quiets the alarm and lamp. The tach works, so there is some amount of ac voltage in the stator, but not enough to silence the alarm.

[team karst]

Originally Posted by GordMay

"The 12 Volt Doctor’s Alternator Book" ~ by Edgar J. Beyn
Free ➥ http://www.nautickiforum.com/biblioteka/alternatori.pdf

Allso by Edgar J. Beyn:
The 12 Volt Doctor's Practical Handbook [For the Boat's Electric System]
➥ http://www.melody-in-blues.org/downl...lthandbook.pdf

The 12 Volt Doctor's Project Book ➥

Edgar did a nice job. I love the hand drawings! page 110 on the excite current story.

[Cadence]

Originally Posted by team karst

my twin VP's do same about half the starts... a quick throttle shot quiets the alarm and lamp. The tach works, so there is some amount of ac voltage in the stator, but not enough to silence the alarm.

If both are doing the same thing, you don't have a problem.

[more]

Originally Posted by svfinlandia

Apparently my old Perkins 4.236 has the tach pick up somewhere other than the alternator, because when my old alternator seized and threw the belt the tach was still working.
I never have been sure where the tach gets its pulse.

Al, S/V Finlandia

magnet rpm sensor on engine flywheel

[Nicholson58]

If your alternator has an external charge controller such as a Balmar it will ramp up output over time. It will not start immediately.

It’s also possible that the pulleys are wrong and the alternator turns too slowly.

[Paul Carey]

Is it possible that sometime in your boats life , that either inexperienced crew or charters have switched battery selector switches to off when engines running & therefore destroyed alternator regulators .....then subsequent repair/ replacement has been using very similar tractor , digger alt or just regulator/brush pack that give out approx 0.7 volt less than marine version which give higher o/p to overcome ZIG unit ..?

[Cynara]

That is what ours did. Been so long ago I can't remember what the reason was, but it waited for the oil pressure came up

[donradcliffe]

The boat I just delivered had a legacy Xantrex alternator controller. It waits for about 2 minutes after engine start to switch on enough field to get the tach to display, then cuts in the charge cycle about 2 minutes later. It was a bit disconcerting until I figured it out.

[Cheechako]

It's not uncommon. I've had a few that were like that. Never worried about it.