Alternators need engine revs to work

[Cliffhanger]

I have a catamaran with two Volvo diesel engines. When I start the engines the alternators don't start charging until I rev the engines a bit in neutral. If I don't do this the charging alarm goes off. Once started it will idle without a problem, though I haven't checked I suspect the output is not what it should be.

I have to admit I only have a sketchy idea of how alternators work.

I am guessing the field coil is not getting enough voltage and therefore does not create enough field to get the stator charging properly. Or could it be the regulator not working properly?

So, does anyone have an idea of what is going on here?

[MikeHoncho]

Start the engine with a 12volt load already on to see if it still does it. Check the actual volts it making when it does do it compared to what is showing on the gauge.

[Wotname]

Originally Posted by Cliffhanger

I have a catamaran with two Volvo diesel engines. When I start the engines the alternators don't start charging until I rev the engines a bit in neutral. If I don't do this the charging alarm goes off. Once started it will idle without a problem, though I haven't checked I suspect the output is not what it should be.

I have to admit I only have a sketchy idea of how alternators work.

I am guessing the field coil is not getting enough voltage and therefore does not create enough field to get the stator charging properly. Or could it be the regulator not working properly?

So, does anyone have an idea of what is going on here?

The alternator has a minimum "spin up' speed and this is usually below the engine idle rpm but is dependant on pulley ratios. Below this speed, the alternator is not self exciting and relies on a small rotor current provided by the vessel's battery via the charge lamp. Once the spin up speed is reach, the alternator becomes self exciting and the field current is provided by the alternator stator coils (via the 'field' diodes). It remains self exciting should the alternator drop below the 'spin up' speed.

At a guess, your alternator is not reaching it's 'spin up' speed at your engine idle speed or the initial field excitation current via the charge lamp is slightly too low.

[more]

Originally Posted by Cliffhanger

I have a catamaran with two Volvo diesel engines. When I start the engines the alternators don't start charging until I rev the engines a bit in neutral. If I don't do this the charging alarm goes off. Once started it will idle without a problem, though I haven't checked I suspect the output is not what it should be.

I have to admit I only have a sketchy idea of how alternators work.

I am guessing the field coil is not getting enough voltage and therefore does not create enough field to get the stator charging properly. Or could it be the regulator not working properly?

So, does anyone have an idea of what is going on here?

Alternators need service. cleaning inside cleaning wire connector. also on control table of VP. for first time try with WD-40.i hate when this happen because lot work for lose connection. ussualy bad connection is on side of volvo penta control table

[wiekeith]

Are the drive belts correctly tensioned?

[Gary Matthesen]

I learned about the spin up the hard way years ago. I was on a long trip, checked battery condition once anchored. Saw the Tach not showing RPMs, panicked I had no output. Ordered a new alternator at the local Brewers, had it delivered 2 days later. Thanked him for saving my vacation. Installed, and still no tach reading. OMG! Just dropped a grand for nothing. Reved it up a bit, and all is well, need to excite the thing.
So now I have a back-up paper weight, and learned a bit more about my boat.

[Stu Jackson]

Originally Posted by Cliffhanger

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I have to admit I only have a sketchy idea of how alternators work.

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There is a way to deal with this. Google 12V Alternator Handbook. I found a pdf file to download which will teach you things you can learn.


If you can't find it, pm me with your email address and I'll send it to you.


Good luck, good answers here.

[donradcliffe]

What you are seeing is typical for a one wire alternator. It just needs a bit of revs to self excite.

[Disailor]

Alternator PDF is here: http://www.autoshop101.com/forms/alt_bwoh.pdf

[Boatyarddog]

Originally Posted by Cliffhanger

I have a catamaran with two Volvo diesel engines. When I start the engines the alternators don't start charging until I rev the engines a bit in neutral. If I don't do this the charging alarm goes off. Once started it will idle without a problem, though I haven't checked I suspect the output is not what it should be.

I have to admit I only have a sketchy idea of how alternators work.

I am guessing the field coil is not getting enough voltage and therefore does not create enough field to get the stator charging properly. Or could it be the regulator not working properly?

So, does anyone have an idea of what is going on here?

Pretty normal, field excitation issue.
Install a slightly smaller pulley on the alternator, your belt size will change to so watch that.
Boatyarddog

[Cadence]

I don't think you have a problem. Goose the engine briefly to get the alternator excited as you have been doing.

[svfinlandia]

Originally Posted by Cliffhanger

I have a catamaran with two Volvo diesel engines. When I start the engines the alternators don't start charging until I rev the engines a bit in neutral. If I don't do this the charging alarm goes off. Once started it will idle without a problem, though I haven't checked I suspect the output is not what it should be.

I have to admit I only have a sketchy idea of how alternators work.

I am guessing the field coil is not getting enough voltage and therefore does not create enough field to get the stator charging properly. Or could it be the regulator not working properly?

So, does anyone have an idea of what is going on here?

What you’re seeing has been standard operating procedure on my boat ever since I have owned her. Same with the old alternator that died a couple of years ago and the new one that I replaced it with. Both Standard 100 watt automobile type alternators.
I usually give the diesel A little throttle when I start it anyway, so it’s not a problem for me.

Al, S/V Finlandia

[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.

[colinalleck]

I had same issue

Suggest you check that all your engine warning lights are illuminating correctly when you go to start engine

This dummy learnt that these lights created “dummy load” used to excite the alternator

Good luck

Col

[seabreez]

Originally Posted by Cliffhanger

I have a catamaran with two Volvo diesel engines. When I start the engines the alternators don't start charging until I rev the engines a bit in neutral. If I don't do this the charging alarm goes off. Once started it will idle without a problem.So, does anyone have an idea of what is going on here?

First thing to do is with the engine off... try and turn the alternator pulley by hand. If you can turn the pulley by hand... the belt needs to be adjusted. Next take a voltage reading. Engine off battery voltage should be 12.89 volts. Then start the engine... battery voltage should be in the range of 13.2 volts to 14.7 volts. The alternator starts charging almost immediately after start up. The older ones used to start charging after the revolutions were higher then would charge at lower rpms. By your description, it seems your belts need adjustment. good luck... Happy New Year !!