Alternator / Rev Counter Problem

[Baddox]

The battery warning light has remained lit after starting my boat’s engine recently. The voltage measured at the battery remains at 12.6v or less and the rev counter bounces around at 2~400RPM despite then engine sounding to be at the usual speed.
Increasing the revs for a few seconds gets the voltage up above 13V and the rev counter then works. After that, the problem goes away for the day, even after restarts.

Today, the voltage was low again at start but for the next hour the tacho was maxed out at above 4000RPM, even though the engine was running no where near that fast. The engine hour LED meter was dim and flickering too.

I’m going to start checking for bad connections from the alternator, probably by voltage drop. Are there any other pointers of what to look for and where to look?

Thanks for reading.

 

[RichardS]

The battery warning light has remained lit after starting my boat’s engine recently. The voltage measured at the battery remains at 12.6v or less and the rev counter bounces around at 2~400RPM despite then engine sounding to be at the usual speed.
Increasing the revs for a few seconds gets the voltage up above 13V and the rev counter then works. After that, the problem goes away for the day, even after restarts.

Today, the voltage was low again at start but for the next hour the tacho was maxed out at above 4000RPM, even though the engine was running no where near that fast. The engine hour LED meter was dim and flickering too.

I’m going to start checking for bad connections from the alternator, probably by voltage drop. Are there any other pointers of what to look for and where to look?

Thanks for reading.

The tacho is usually connected to a small wire coming out of the back or even the side of the alternator. There will probably be a bad connection probably a spade or bullet connector somewhere hear the alternator.

Richard

 

[VicS]

and check the circuit from" ignition switch" through the warning light to the alternator. There may be a resistor in a parallel circuit as well esp if the" bulb" is an LED.

Also check the alternator brushes.


Check everything basically !

 

[William_H]

The current through the warning light is a starter (initial excitation) current for the alternator. When it starts to generate power another set of diodes takes over from the current from the light so light goes out. Your symptoms suggest that the problem is with the alternator itself. Not the tacho which just uses an AC output generated by the alternator. The bright warning light would normally indicate a good circuit through the regulator brushes and field coils. The brushes are a common point of intermittent failure. Light on would indicate a failuire of alternator drive (belt) or a major failure of the alternator but I feel sure you would know if this were the case.
So logic fails me here. As said check all wiring connections. A short to negative ground of the excitation wire from panel to alternator might give these symptoms.
If you want to check volt drop then use a multimeter on low volts scale (2v) when the laternator is apparently running OK to check volt drop between the big +v output terminal on alternator and the battery +ve terminal. Assuming it is charging some amps then this voltage drop should be less than about .2 volt. that should check switches and wiring. You perhaps also should check in a similar way the volt drop between the alternator body and battery negative.
Of course if you have complicated smart charger control then it could be anything. I would always advocate fitting an amp meter to an alternator battery system to see exactly what the alternator is doing although volt meter can also indicate. good luck olewill

 

[Billjratt]

It's not unusual to have to give the alternator a burst of revs to get it started. Are you starting the engine with the throttle closed and are the idling revs lower somehow (thick oil/cold sump? This would explain subsequent successful warm starting later in the day.
Just give it a blip and don't worry if it actually does charge properly once started. - do you have an ammeter fitted to verify that it is actually delivering full current once the light goes out?
There's a possibility of a bad connection on the rectifiers in the alternator, some had a problem with dry solder joints as a result of overheating. Do you think it takes longer to reach 14.4volts (or whatever is written on the regulator/brush assembly) - It's a 3 - phase machine so you lose 33% for each dud diode.

 

[wotayottie]

The battery warning light has remained lit after starting my boat’s engine recently. The voltage measured at the battery remains at 12.6v or less and the rev counter bounces around at 2~400RPM despite then engine sounding to be at the usual speed.
Increasing the revs for a few seconds gets the voltage up above 13V and the rev counter then works. After that, the problem goes away for the day, even after restarts.

Today, the voltage was low again at start but for the next hour the tacho was maxed out at above 4000RPM, even though the engine was running no where near that fast. The engine hour LED meter was dim and flickering too.

I’m going to start checking for bad connections from the alternator, probably by voltage drop. Are there any other pointers of what to look for and where to look?

Thanks for reading.

The rev counter picks up an unrectified signal voltage from one of the stator coils. I cannot see any way in which that signal could multiply in frequency when generated so my guess would be a fault in the tacho or in the wiring from tacho to alternator. That said, the alternator isnt starting to charge as it should which could be a fault anywhere along the circuit from battery through the on switch and the warning light to the field coils or in the stator coils. In short almost anywhere!

In your position my first action would be to wire direct from the battery to the D terminal of the alternator and then start the engine. That way you bypass the instrument panel on/off switch wiring and if everything then works OK you know the problem is in that wiring. If the problem stays then you know its in the alternator . Take it off and send it for testing.

Mind you, what do I know. It took me almost a year to chase down a similar issue which turned out to be the diodes breaking down under excess temperature but working when cooler.