A127 alternator - intermittent fault

[WotEver]

6 hours ago, Lee1831 said:

Hi did you get this sorted in the end. I have the same problem. If the battery is fully charged the. There's no problem. As soon as I start the engine with a low battery the lucas A127 starts to get hot and belt starts smoking. The rev counter stops working. 

Sounds like a loose belt. Have you adjusted it? 

[Tony Brooks]

6 hours ago, Lee1831 said:

Hi did you get this sorted in the end. I have the same problem. If the battery is fully charged the. There's no problem. As soon as I start the engine with a low battery the lucas A127 starts to get hot and belt starts smoking. The rev counter stops working. 

That is to be expected if the belt is worn, slack or not matched to all the pulley grooves. Alternators do get hot at maximum  output - uncomfortable to keep your hand on - and also need a lot f power from the engine to spin them. The smoking tells us that the belt is slipping. The revcounter is fed pulses from one of the phases so if the belt is slipping the revcounter will give odd readings.

 

Step one - Check the belt is not bottoming out onto the base of the crankshaft or alternator pulley grooves. If it is sort the pulleys out, if not get a new belt and tension it correctly about 10mm deflection under moderate finger pressure in the centre of the longest belt run.

 

Step two - check for faulty battery cells - individual cells being dryer then the rest, individual cells gassing a lot more than the rest, much hotter local areas on a battery, or a horrid rotten eggs smell. All when on charge and checked every half hour or so.

 

Step three - get a multimeter set to 20 volts DC (200 V DC for a 24 volt boat) and measure the voltage an hour after shutting down, just before you go to bed (in the dark) and when you get up but its still dark. Post them here. If you find it is less than about 12.2 volts at any time then you may still have shorting battery cells or over discharged batteries so that means looking at your electrcity use/ bank size/charging regime.

[ditchcrawler]

On 23/08/2013 at 13:58, Mike the Boilerman said:

 

 

P.S. I'll be taking the old battery boiler out anyway soon. I find it hard understand what it is for and I don't like things I don't understand! In additon, there is no manual for it and google searches for it reveal nothing on the net.

A kestrel by any chance

[rogerc]

I have a similar problem with my 127A which I think is an internal alternator fault. The charge light comes on intermittently and the voltage goes up to 15-16v when the light comes on the exciter terminal is at 10.5 v and the output 14.7v hence light on light 💡. I have had the alternator fully checked no issues reported. It was a pretty sophisticated tester JBT3 I think with a diagnostic printout. Said in good good regulator and diode bridge. condition I fitted a new regulator just in case but it made no difference. 
I think there are some diodes in the exciter circuit. Can I test them? Can I get replacements. I am concerned because diodes rarely have intermittent faults. 
The alternator is charging well. Maybe overcharging. To stop the buzzer I have fitted a switch to short the bulb it doesn’t seem to affect the alternator but gives me peace☺️
many suggestions welcome

[Tracy D'arth]

11 minutes ago, rogerc said:

I think there are some diodes in the exciter circuit. Can I test them? Can I get replacements. I am concerned because diodes rarely have intermittent faults. 
 

I would ask why. What are they connected to? One I could understand for the warning buzzer using diode logic.

 

You can test them with a multimeter  set on ohms, there should be a low resistance one way and high if you reverse the probes.

 

The acid test is to disconnect the D+ at the alternator, connect a 5w bulb from D+ to B+ on the back of the alternator. If the bulb comes on with ignition, start the engine. It should go out and the batteries charge. If this is all working as it should, you must have a wiring fault or those mystery diodes.

[rogerc]

The diodes are inside the alternator 

[rogerc]

Sorry I should have made that clear. 
thanks for your input 

[MtB]

On 11/08/2020 at 00:47, Lee1831 said:

Hi did you get this sorted in the end. I have the same problem. If the battery is fully charged the. There's no problem. As soon as I start the engine with a low battery the lucas A127 starts to get hot and belt starts smoking. The rev counter stops working. 

 

In the intervening eight years I did get it sorted, thank you!

 

What happened was the engine completely failed, and during removal a scab fell off the baseplate and the boat began to sink. Barry Tuckey was summoned with his crane and stuck it on the bank. Simon and David took the interior out, chopped the rotten base off and constructed a new baseplate, footings and put the interior back in. Meanwhile Richard rebuilt the engine and Sarah put it back in. 

 

And amazingly after all that, the problem was gone! 

 

 

[Tony Brooks]

2 hours ago, Tracy D'arth said:

I would ask why. What are they connected to? One I could understand for the warning buzzer using diode logic.

 

You can test them with a multimeter  set on ohms, there should be a low resistance one way and high if you reverse the probes.

 

The acid test is to disconnect the D+ at the alternator, connect a 5w bulb from D+ to B+ on the back of the alternator. If the bulb comes on with ignition, start the engine. It should go out and the batteries charge. If this is all working as it should, you must have a wiring fault or those mystery diodes.

 

If he does the red bit the light will probably come on as soon as he has made the connection, no need for the ignition to be on because the bulb will be fed form B+ and earth through the rotor and regulator. Otherwise, a good test.

 

 

[Tony Brooks]

1 hour ago, rogerc said:

The diodes are inside the alternator 

 

Yes, although there seems to have been a number of designs for the diode pack the field/auxiliary diodes used to be, and may still be, three little black "electronic style" diodes. The six main diodes are set in the two heat sinks with a little wire stuck up and don't look much like any other diode. A failed field diode can cause high charging voltages because most alternators sense the charging voltage from the output of all three.

 

I suspect a dry solder joint where the field diodes are soldered to the diode pack. Hence, it conducts when cold and without much vibration, but once things have warmed up, expanded, and been shaken a bit the connection goes bad.

 

The diodes are NOT in the initial exciter circuit, but do provide the current for the rotor/regulator once the alternator has been exited/started charging.

 

You can buy new diode packs and they are not too difficult to fit PROVIDING you are good at soldering and have something more than a silly little "electronics" iron.

 

Edited August 25, 2021 by Tony Brooks

[rogerc]

Thanks I’ll have a check for the dry joint and have a proper look at the diodes