Alternator Issue ? - Help Please

[DAKA]

The mechanic who has looked after our boat for the last 6 years or so is temporary out of service so I need to DIY.

36ft Flybridge 2 x 300HP diesel (Cummins-Mercruiser D300 ( 4.2L)



Max RPM 3 800
We usually Cruise RPM 3 100 (22-23 knots)

Boat happily cruising in calm waters and the RPM on Starboard suddenly dropped to 2 600 - no black smoke , we lost speed.
Increased RPM on Port to compensate and kept on the plane while I checked downstairs instruments ( Flybrdige doesn't have volt meter other than voltage shown on GPS Navigator which was static at 12.7v but I dont know which engine powers this ).

Lower helm has manual gauges so not 100% accurate
Port - 13-14 v ish stable
Starboard - 6-12 v ish bouncing
Oil/temp all good

After 1 NM all picked up and back to normal

When we arrived at the Hamble I changed all filters (engine , pre and air filter)

Ran the starboard engine in neutral on the visitor berth and RPM stable at 3 100 but Voltage still bouncing about between 6-12 volts

Removed Alternator and thick wires and cleaned every connector except two thin wires which I don't know what they do and they looked a bit insignificant and I didn't touch the suppressor
Alternator was rotating freely / bearings smooth.
Fan belt tension was good before and after

On return journey smooth seas all ran perfectly smoothly , voltage stable at 12 v which is a bit low.
On return to own berth I ran it neutral 3 100 rpm and the voltage again bouncing 6-12 v

Do I need a new alternator ?

Can a very low alternator Voltage drop so low that the engine loose RPM ?

Both engines run off the same battery starter bank and the batteries appear good, both engines start

 

[PaulRainbow]

Take it off and get your local automotive electrical people to check it.

 

[kashurst]

A low alternator voltage can make an electronic engine do lots of strange things. Not recommended.
As per PaulRainbow take the alternator to be tested. They can sometimes be repaired but it is not really DIY because of the parts etc.
Fingers crossed it's just the brush assembly playing up.

 

[markc]

I had a dodgy alternator a couple of years back and I spoke to these guys. They suggested it was a failed regulator, sent it to me and it took about 10 mins to swap out. Cheap as chips and no need to remove alternator - might be worth doing this first. The Starter Motor & Alternator Company Ltd

 

[DAKA]

I had a dodgy alternator a couple of years back and I spoke to these guys. They suggested it was a failed regulator, sent it to me and it took about 10 mins to swap out. Cheap as chips and no need to remove alternator - might be worth doing this first. The Starter Motor & Alternator Company Ltd

Thanks Kashurst, Paul and Mark.

The alternator is tucked behind a bracket ( I can’t just replace any bits in situ) so I will have to take it off and take it in for testing/repair.
You have given me the confidence that it is the likely culprit.

 

[DAKA]

Thanks Kashurst, Paul and Mark.

The alternator is tucked behind a bracket ( I can’t just replace any bits in situ) so I will have to take it off and take it in for testing/repair.
You have given me the confidence that it is the likely culprit.

All sorted thanks for all your help .
Just as an update in case anyone else has a similar problem

I spoke to The Starter Motor & Alternator Company Ltd as Mark suggested and they were very helpful.

As the alternator was 2009, I decided to replace it with one from The Starter Motor & Alternator Company Ltd

The guys there were very helpful , they sent a brand new replacement that looked identical to the old one.
They gave me a number to ring if I needed help while fitting it (fantastic service ) , as it happened I didn't need help as it was a straight swap

Both engines and volt meters now running smoothly.

Conclusion

A faulty alternator can affect engine revs / performance .

 

[PaulRainbow]

All sorted thanks for all your help .
Just as an update in case anyone else has a similar problem

I spoke to The Starter Motor & Alternator Company Ltd as Mark suggested and they were very helpful.

As the alternator was 2009, I decided to replace it with one from The Starter Motor & Alternator Company Ltd

The guys there were very helpful , they sent a brand new replacement that looked identical to the old one.
They gave me a number to ring if I needed help while fitting it (fantastic service ) , as it happened I didn't need help as it was a straight swap

Both engines and volt meters now running smoothly.

Conclusion

A faulty alternator can affect engine revs / performance .

Good to here , thanks for the update.