Alternator charge light

[webcraft]

Doesn't go out completely, even at cruising revs.

However, the instrument panel voltmeter shows 14+ volts when the engine is running.

Should I worry, and if so how can I check?

(Recommendations as to cheap web-purchasable multimeters that will do everything I am likely to need are also very welcome)

- Nick

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[tcm]

You say it doesn't go out "completely", which may mean that it goes out partly? If so, there's a good chance that it's a loose fanbelt - it is producing nearly enough charge for the battery, but not quite. With all the rain, praps the belt got wet? Whereas, if the rectifier diodes are bust, more likely to be on all the time and you wouldn't have said "completley", Holmes?

But praps that won't work. At least for one or two times, it's worth borrowing a multimeter. Anyone who has one just loves all the prodding and poking about, it seems. Buying the damn things just confirms that yes the such-and-such is indeed knackered as suspected, and you're already fifty quid+ down with blimmin multimeters. But you have to buy one eventually.

fanbelt first?

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[VicS]

I wouldn't like to even speculate on the problem of the warning light without investigation but <A target="_blank" HREF=http://www.maplin.co.uk>Maplin</A> do a small digital multimeter fo £4.99. Unfortunately you've just missed their special offer of two for £4.99 but they seem to repeat it from time to time.

The problem with the warning light reminds me of the time the light on the car glowed when the turn indicators flashed, small hours on the M4 and miles from home, but a check by an AA man I found in the next service area showed the alternator output to be normal. He suggested a bad connection somewhere like a fuse. He must have come across it before because thats what it was. Impossible explain without refernce to the wiring diagram tho'. The message is that the problem could be something quite obscure but if the alternator output is OK then dont panic.

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[aztec]

usually a bad connection at the plug. or verdigris on the terminals, or a fault in the diode pack. it'll need sorting in the long run...but if it ain't bust etc.

as to the multimeter, i found that the one from screwfix uses a small (12V i think) battery. when you forget to turn it off (user error, but you will) it dies and finding another battery locally (maplins do ,em) is difficult and 'spensive.

get a digital if you're using it on electronics, or anaalogue if on an engine etc.

hope you find the fault, steve.

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[vyv_cox]

I think you'll find that the cause is one of the diodes has broken down or failed. This produces a small ripple of AC in the DC supply. Voltage is registering at the correct value, ruling out the likelihood that there is something wrong with the connectors or belt. There is no major problem with this in the short term but on a long trip under engine it can cause batteries to heat very considerably. I once literally boiled mine and a friend had the same experience. Suggest you take the alternator to an automotive electrical specialist to be refurbished. Don't use your local non-specialist garage, they don't have the equipment to find the fault.

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[brianhumber]

Agree with Vyv, I had the same symptoms 2 years after it had to be run with a salt spray mist from a leaking split seam on the exhaust muffler - the salt ate through the PCB and diodes.
How old is your alternator and has it exprienced a similar dosing. If it is getting on in years I would suggest you cost out a replacement - is it worth changing just the diodes on an alternator with 4,000 hrs? the bearings and brushes also should be considered.

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[dickh]

You can also get DMM's on Ebay at the moment, I've just bought one at £3.99 + P&P, works well and looks robust - and it's bright yellow so you don't lose it!
I think they were on the boating section.


<hr width=100% size=1>dickh
I'd rather be sailing... /forums/images/icons/smile.gif

 

[stubate]

brushes are on the way out, change em, the light is in line from the alt to the battery, when the voltage going out equals the voltage going in it doesnt light, anything either way and it will glow bright or weak depending. the brushes making bad contact with the commutator cause this, they have worn too much to be making goood contact. they will still be making enough contact to be exciting it to produce voltage but wont for much longer.
stu

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[DepSol]

I had the same problem 6 weeks ago. Alternator was removed and sent to Lucas where the diodes were replaced and everything is ok now. Sometimes this problem is caused by the battery switch being worn and breaking down and therefore needs replacing or by the fact that someone has turned the battery switch off whilst engine is running.

Shouldn’t cost much if you remove it and take it to Lucas yourself.



<hr width=100% size=1>Dom

Now where is that Brendan hiding?

 

[webcraft]

Thanks everyone,

It will take some time to process all this info but my interpretation is:

Most likely a diode fault. Batteries may get overcharged, so keep an eye open for overheating, electrolyte loss etc.

What can I check with a multimeter to investigate further?

Lucas not an option, a local auto electrician might be.

However, one other explanation I was given has not been forthcoming here. Someone told me that if the batteries are absolutely fully charged and not under any significant load this could also cause this. Is this a possible cause.

- Nick


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[LeonF]

Nic I've been having a problem with expensive AGMs-- see previous posting. It seems that the fully charged batteries, may, when on low revs-idling- cause an impedance which the trickle charge seeks to overcome by upping voltage which causes voltage readings as high as 15.1V and sometimes causes the green light on the Adverc to flash signalling charging problems. Upping the revs or putting a small load on solves the problem inmmediately. I know your problem is slightly different, but there is a thing called a back EMF. Can anyone else shed some light?

<hr width=100% size=1>L.A.R.Ferguson

 

[Gunfleet]

If it's a bog standard Lucas (127 or whatever) why not get a second one as a spare? It's much the best arrangement and a lot less expensive than you'd thing (ask Adverc - they supply them). THen if the wretched thing fails while you're at sea or somewhere exotic, replacement is dead simple. Meanwhile, the one which is knackered can go to that nice auto electrician you found in the yellow pages and you still get to sail your boat.
Cheers
J

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[ccscott49]

I had a car that did exactly the same, I had it for ten years, it still did it, then my mate ran it for a further five years, until the floor fell out, it still did it! I'm not sure this is terminal, as long as your batteries charge and don't over/under charge, I wouldn't worry about it.

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[webcraft]

This is an encouraging post, cc

The moral being, make sure it's broke before you fix it . . .

JohnM's suggestion of carrying a spare alternator also sounds like a good idea if it's not too expensive.

Thanks everyone.

- Nick

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[andyball]

Some motorcycle regulators do that.....give a higher voltage at tickover than fast idle,worse with low loads.

I've always thought it a ( not uncommon) design flaw, might be worth trying a capacitor across the battery terminals or directly across alternator output: works wonders sometimes. Go for 25V or more and 10000uF or larger & carefully check voltage over full speed/load range. the capacitor will be electrolytic type, so must be connected right way round, or it'll go pop.

Had charger(mains) overvoltage problem recently, fine connected direct to batteries, hopeless via diode splitter,capacitor across output fixed it .


Going back to alternators & faintly glowing charge lamps....my friend's nauticat alternator just packed up after a few months of that, think I'd keep a spare .

<hr width=100% size=1><P ID="edit"><FONT SIZE=-1>Edited by andyball on 24/05/2003 20:40 (server time).</FONT></P>