Alternator not producing power

[Punk Stig]

Morning all- alternator on vetus 4.65 stopped producing power last night- the alarm sounds on the instrument panel, the power warning light stopped coming on last week when turning ignition on but as no alarm was guessing a short on the pcb or bad led (which his built into the pcb) 
I removed the instrument panel and there was corrosion on the pcb so cleaned that all up, hoping they were related I put the instrument panel back together and tried starting again- engine starts and runs fine, but still have no power from alternator and the alarm going off (still no light on the panel though)- any advice on where to look?

Bottom wire has never been attached in the 4 years I've been on boat

[Punk Stig]

I think the belt is fine- if I really up the revs the alternator will kick in, but I've never had to do it so high before- does that suggest belt is slightly loose or to do with current temperature or simply that alternator is on its way out?

[nicknorman]

With many types of alternator, the initial excitation is provided by current through the warning light. If there is no current through the warning light then the alternator may still kick in eventually at high revs, due to residual magnetism.

So this fits your symptoms fairly well. You mention “LED” on the panel, are you sure it’s an led and not a small incandescent bulb? If the bulb had blown it would explain the symptoms. Or a disconnect between the bulb / panel and the D+ terminal. If it is an LED, there may be a big resistor somewhere on the panel that passes the current as a substitute for a bulb (LEDs pass too little current).

Check for a low voltage of a few volts between the D+ terminal and battery -ve with the ignition on but engine not running. If it is not there the problem lies with he panel or wiring. You could also try momentarily touching a short bit of wire between the D+ terminal and the B+ terminal (fat red wire) with the engine started and running slowly. This should cause the alternator to start charging, again showing the problem is with the panel or wiring, not the alternator itself.

[Tony Brooks]

The fact you can get it to energise with lots of revs and you say no warning light indicates a problem on the warning lap circuit OR worn brushes.

Take the yellow & green wire off. Turn the ignition on - engine off. Make a very temporary connection between that wire and battery negative or any bare, clean metal.  If the lamp comes on then the brushes have probably worn out. If the lamp stays off there is an open ciruit between the terminal you pulled off, the bulb and the ignition switch terminal feeding the bulb (assuming all the other lights come on).

Another test is to turn the ignition on and connect a voltmeter between D+ and battery negative/bare metal. It should read 12V ish with the ignition on and zero with the ignition off. 12V = brushes, zero = circuit fault. I refer the functional test above because jusr a dirty or loose connection may give some voltage.

If it is a circuits fault manipulate the large multi-plug in the main engine wiring harness, if the fault clears take the plug apart & clean/ tighten all the individual connectors.

 

I do not like the look of the bottom terminal, it seems to be a main negative connection but may well also secure the diode pack negative to the alternator body making it an earth return alternator. It looks as if it may be loose and is rattling about so rust particles  have blown around it. If it is loose then it would break both the charging and warning lamp current paths. I would take a very close look at it.

That brown could be caused by overheating burning the plating off the connection. If so it is probably loose in the diode plate or alternator body.

Note what Nick says about an LED requiring a parallel resistor.

Edited January 19, 2018 by Tony Brooks

[Punk Stig]

Cheers guys, I'll be doing those tests tomorrow- definitely led and not incandescent bulb- they're built into the pcb

The bottom wire has never been connected since my ownership and was taped up!

[RLWP]

My guess <WARNING> is the taped up wire is a disused positive output cable. It points in the right direction, it's about the right size and it's red. Also the one in use looks like a later fitment with black tape insulation

I do wonder if it is live, and if the 'burn' marks are where it has worn through the insulation and has been arcing to the bolt on the alternator body

Just guesswork mind

Richard

Edited January 19, 2018 by RLWP

[bizzard]

It might be suffering from yellow jaundice.

[WotEver]

59 minutes ago, Punk Stig said:

definitely led and not incandescent bulb- they're built into the pcb

Then as previously said, look for a large resistor. It may be burned out, which can be easily checked with a multimeter. 

[RLWP]

3 hours ago, bizzard said:

It might be suffering from yellow jaundice.

[WotEver]