28V AC Output from my Alternator

[affinite]

I'm trying to track down the cause of an intermittent problem with the tachometer and coolant temperature gauges on my boat.
Periodically they both flutter for a minute or two.
My first thought was a slipping alternator belt (although I'm surprised that the temp gauge also flutters in that case) so I do intend to replace the belt as soon as I can but I also measured the AC voltage present on the alternator output.
I measured 28V AC on a cheap multi-meter.
28V AC sounds wrong to me - is that OK or should I consider changing the diode pack?
(BTW It is a 12v Alternator)
Thanks

 

[VicS]

What is the DC output volts from the alternator?

cross check with another meter

is the alternator battery sensed. If so check the connection

Disconnect any enhanced charging gizmo or booster

Suspect the regulator

Fluctuations may be due to bad connections or alternator brushes

 

[TernVI]

I'm trying to track down the cause of an intermittent problem with the tachometer and coolant temperature gauges on my boat.
Periodically they both flutter for a minute or two.
My first thought was a slipping alternator belt (although I'm surprised that the temp gauge also flutters in that case) so I do intend to replace the belt as soon as I can but I also measured the AC voltage present on the alternator output.
I measured 28V AC on a cheap multi-meter.
28V AC sounds wrong to me - is that OK or should I consider changing the diode pack?
(BTW It is a 12v Alternator)
Thanks

Personally I'd look at it with a scope, but it does sound roughly consistent with losing a phase, either a diode or a stator winding or a connection.
I would not read too much into the exact numbers from a cheap meter, they can read nonsense with anyhing that's not a 50Hz sine wave.

Instead of just changing the diode pack, why not test it, diode by diode?
The problem could also be the internal regulator damaged.
The periodic variation does not sound so much like a simple diode fault?

It may be easier to just change the alternator.

 

[affinite]

Thanks both
food for thought ... not a bad idea to keep a spare alternator on board but have you ever tried working at the front of the engine in a Moody 44 engine compartment

 

[Billjratt]

I've got a simlar problem with a VDO revcounter on a new Beta engine. The revs flutuate for the first part of a journey and then settle just as if they are being affected by the rate of charge. The current and voltage is stable and as expected throughout the process. I'll 'scope it to check the waveform sometime soon. Meanwhile I set revs to suit SOG!

 

[chriswesterman]

Had this on alternators before. Its down to the cheap multi meters which are no good for measuring AC ripple on top of a DC offset.

The cheap ones measure AC by simply rectifying the signal, so you get the Alternator DC O/P, dropped by a diode, then averaged. ( so lets say 12V). This is then used to calculate an effective AC voltage which would produce this. ( with no DC offset)


As @TernVI says the only way to test is via a scope.

 

[VicS]

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[William_H]

It is unlikely the AC voltage from the centre connection of the rectifier is too high. More likely it is OK as is. However the fluctuations could be caused by an intermittent charge from the alternator (which would also give intermittent AC output to tacho.) An amp meter measuring charge from alternator would show we would imagine the fluctuations. Most likely caused by bad (dirty) brushes and slip rings of alternator. Sorry usually accessed at the back (opposite end to the pulley) so no easy. ol'will

 

[affinite]

Thanks all
Comments re multimeter were expected so I decided to buy a replacement alternator
I want to carry a spare anyway.