Question 2 on the alternator.

[madmat99]

The alternator seems to drag the engine down at idle. The battery is fully charged. The belt is screaming like the battery is flat but is all new and tensioned. Voltage is 13.2 not running and 14.4 running so it’s working fine. if I flick the battery selector switch off and on it disappears straight away and all runs smoothly??
any ideas?
Yanmar 3gm30 all stock stuff.
Cheer

mat

 

[VicS]

If you switch the battery off while the engine is running you risk blowing the diodes in the alternator

 

[Moodysailor]

It's not just the volts that matter - it's what current (amps) the alternator is putting out. The alternator must be sensing a low voltage, and is therefore putting out the maximum possible current. Check all the connections, and look for the not so obvious things (such as loose terminals, bad crimps and corrosion).
And as mentioned above, it is not good practice to turn off the selector whilst the engine is running.

 

[William_H]

Check the voltage at the battery when this effect is obvious. Might find voltage quite high. Could be a bad regulator (or wiring) so alternator is convinced to run full power.
if not then no idea? ol'will

 

[LadyInBed]

If 'The belt is screaming ' that suggests that it is slipping. You say it's new and tensioned but I would check the tension again.

 

[Boater Sam]

Does the belt fit the pulley or is it bottoming?

 

[RichardS]

Does the belt fit the pulley or is it bottoming?

If the belt is new and is correctly tensioned and it is still screaming then it is, indeed, the wrong sized belt for the pulleys. Too thin a belt or too thin a profile are the most likely issues. The belt must be flush with the outside edge of the pulleys and have the correct profile.

Richard

 

[peteK]

Check the alternater Bearings are ok.

 

[[70521]]

It's not just the volts that matter

That might not be true depending on battery type.

AGMs and other technologies need charging at >14.2 volts.

 

[RichardS]

Check the alternater Bearings are ok.

I typed that but then deleted it as the noise vanishes when he isolates the battery.

Richard

 

[peteK]

If the bearings are slightly worn could be putting extra load on causing belt to slip but not enough to slip when no load on alternater.

 

[Moodysailor]

That might not be true depending on battery type.

AGMs and other technologies need charging at >14.2 volts.

I'm aware, but not sure what that has to do with my statement in reference to the OP's problem?? Maybe you can elaborate?

 

[Refueler]

Rarely read on forums now - but it was a common enough item in the past when people upgraded charging.

The standard alternator on an engine has a pulley ratio and power output set to be a good compromise of not loading up the engine but still providing reasonable charge rate. The belt being matched.
People then started upgrading alternators ... higher outputs. These then put strain on belts and pulleys. It was not uncommon for a high amp rate alternator to then 'screech' ..... the belt slipping as it couldn't handle the greater force needed. The remedy of course being swap out the pulleys to wider belt or add a second pulley to have a dual belt system.
Pal of mine 2 weeks ago swapped out his alternator on a 1GM Yanmar for a higher output alternator ... ignoring my comment that he would need to take care about pulley sizes and belt etc. He spent next two days trying to sort it out ... eventually having to source an unusual belt .... so far no 'screech' but I'm not so confident about it as he ! The pulley size being noticeably less diameter .. meaning the alternator spins faster and also puts more stress on the belt.

Just mentioning based on OP saying he has SCREAMING belt when charging..... and alternator appears to drag engine down at low revs. It just sounds so reminiscent of those old days .......

 

[madmat99]

Update.
took the alternator of and got it to a shop with a tester. When you spin it up all sorts of funny noises and voltage jumping around. Not bearings but something weird going on.
the are stripping it now and I’ll let you all know what the problem is in case it can help others!!

cheers all

mat

 

[Refueler]

Update.
took the alternator of and got it to a shop with a tester. When you spin it up all sorts of funny noises and voltage jumping around. Not bearings but something weird going on.
the are stripping it now and I’ll let you all know what the problem is in case it can help others!!

cheers all

mat

Sounds like you are the road to solution.

 

[RichardS]

If the bearings are slightly worn could be putting extra load on causing belt to slip but not enough to slip when no load on alternater.

Theoretically possible I guess, but not something that I have ever witnessed.

Richard