Alternator warning light problem

[WindermereColvic]

Towards the end of the first engine run of the year, the alternator warning light on our Sole Mini 26 came on. I ought to point out that in order to get the engine started for the first sail of the year, i had to clean a corroded fuse holder in the supply to the keyswitch. Were going back to the boat on Wednesday, and thinking about it, i don’t really know where to start in diagnosing the alternator problem. What could suddenly go wrong that would put the warning light on? Any suggestions on how / what i should check?
Thanks, Matt

 

[steve28]

Sounds like the alternator has a fault now, this can be down to the feed to the instrument panel being bad/disrupted. its like turning the key off/on when the engine is running.



Steve

 

[earlybird]

Assuming that the drive belt hasn't become very slack, I'm inclined to the alternator fault suggestion, although switching the "ignition off/on shouldn't affect it, neither should the corroded fuse. If the light came on "just like that", then it might be worn brushes/ corroded holders in the alternator, esp if fairly old. Probably easy/cheap to repair. Otherwise, I've no specific suggestions.
Edit just thought, I'm not sure about brush suggestion, the light wouldn't come on;-- I think!!!

 

[WindermereColvic]

Thanks for the suggestions so far chaps. The alternator is fairly new in hours terms, 700 in 10 years, but has lived in a fairly damp environment. What do you think i could do to test for the things suggested? In terms of where to stick the multimeter probes?
Perhaps this should be a separate question, but there is something else that's puzzling me about my set up. The wire that remotely senses the battery voltage is connected to the Out terminal of the 1 2 B switch. The two batteries are charged through a split charge diode. So, which position should i put the 1 2 B switch when i want to charge both batteries? Ie, which one should i set it to sense the voltage from? Im hoping this will allow me to play round the with system more and understand it, so i have more chance of fixing it.
Matt

 

[VicS]

I can only find a fairly poor wiring diagram and so far studying that no flashes of inspiration.

Check all connections ... just possible that if you lost the power supply to the switch ... that fuse holder again ... that the warning light would come on. Check you have a power supply at the switch preferably using a bulb and a couple of leads rather than a digital multimeter.

Alternator brushes as already suggested.


Re your split diode system. It does not much matter which battery you sense. I take it that the alternator output goes direct to the diode splitter and that the outputs from the splitter go directly to the two batteries. I would say sensing directly from one of the batteries is preferable to sensing at the common terminal of the 1,2,off, both isolator. Tony Brooks in his tb training notes says, " There is much debate as to which bank it should sense off. As long as you ensure all master switches are turned on whilst the engine is running, it is probably marginally more advantageous to sense from the bank most likely to be discharged. If you do this, any voltdrop in the charging circuit carrying the highest current will be taken into account."

Separate isolator swtiches for each battery would be better that the 1,2, both, off thing especially when using a diode splitter.

Look at Tony Brooks' diagram HERE


There is also a basic section on alternator fault finding that might be useful
HERE

 

[Billjratt]

I assume the problem you have now is that the light doesn't go out when you spin up the alternator?

If the light is the same brightness as it always was,(with 'ign' on and engine stopped) it is unlikely to be a brush/field/regulator problem, more likely belt slip. (but could be diode failure in the field rectifier stack, or an open circuit stator coil.)

There is a chance that the original problem was an overvoltage in the alternator, where the voltage generated for the field circuit became much higher than the battery voltage - enough of a potential difference to glow the ign light (but usually dimmer.) That would indicate a regulator (or its connection) failure.

You asked about measuring...
preliminary - black lead to alternator body(or neg post if isolated), red to battery neg, = zero ohms
Black lead to alternator + , red lead to batt+ 0.6v (set to diode test)

Black lead common neg (probably engine) red lead Ign/61 or whatever (where the wire to the ign light connects to the alt - set to volts, will rise from zero slightly when ign is turned on, SHOULD rise to 13v-ish as engine is started

Tell us...

 

[WindermereColvic]

I can report back now.
Thanks to everyone for your help, very, er Helpful!
I took the alternator off to gain access to the panel supply fuse and, whilst it was off cleaned all the connections. I also cleaned the warning light connections behind the panel. Everything worked fine, the tacho worked properly and the batteries were showed 14+ volts. I did notice that there is a wire from the alternator ‘out’ terminal to the starter motor supply , via a terminal on the starter solenoid, the s/m supply goes back to the 1,2,B switch. This would indicate that whichever battery is selected gets the full alternator voltage (15+, i think as the regulator senses from the 'far' side of the split chare diodes) The other wire from the 'out' terminal, goes to the split charge diodes.To have two different voltages going to the same battery seems a bit odd, so no doubt I’ve missed something.
Ta, Matt

 

[VicS]

To have two different voltages going to the same battery seems a bit odd, so no doubt I’ve missed something

If the system is wired as you say and I have understood you correctly then it is a bit of a mess. I nearly said that the 1/2/both/off switch is bypassing the diode splitter but its not that simple.

You dont say which battery the alternator sensing is connected to but when your selector switch is set to that battery it will bypass the diode that might otherwise feed that one and the alternator out put will be controlled at the normal 14 + a bit volts The other battery will be recieving a charge via the other diode so it will be getting 0.7 volt less.

When the selector switch is set to the other battery ie the one without the sensing connection. The battery with the sensing connection will be charging via its diode and as a consequence the alternator will be controlled at an enhanced voltage ( 15 ish) but the selecor will be bypassing the other diode so the other battery will be getiing the full 15 volts applied. ie overcharging.

If you set the selector to both then both diodes will be bypassed and both batteries will be i in parallel will be charged at 14+ volts.

The wire from the alternator output to the terminal on the starter solenoid should have been disconnected when the diode splitter was fitted.
Then the batteries will be always be charged simultaneously via the diodes directly from the alternator which will be controlled at 15 volts in order to give 14+ at the batteries.

The engine electrics will be supplied via the selector switch from which ever battery you choose with it.

You do not say how your domestic circuits are supplied. (I'd like to know that before you actually go ahead and remove that connection between alternator and starter solenoid)