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MG Midget and Sprite Technical - Alternator query

1275 rwa.
My alternator lght glows at tickover (900/1000revs) and does not totally go out until about 1800 revs (slight glow at 1500 if you shroud the gauge to view).

Voltage even @ tickover is about 14volts plus or minus a tad. Any ideas if this is a problem ?

The battery was flat today (not started for a couple of months - but disconnected) so charged for a few hours and still pretty low - thinking new battery on the horizon (few years old now).

Is the charge light likely to be related to a poor condition battery ? Earth's seem good.

Currently leaving on charge (not able to give it a good run today unfortunately) as need to ensure it starts next Sat as MOT time !

R.
richard boobier

not going to comment on alternator as more knowledgeable people will be along later but for battery don't give up on it a few hours is not enough

check electrolyte (distilled water) level in all 6 cells and then put the battery on slow charge over at least a night but better still until completely full, indoors if possible

clean up battery posts and battery clamps before reconnecting tightly and then protecting with your preference of protecting agent
Nigel Atkins

Re-reading my post I was questioning the charge light not the 14 volts - Oh the printed word !

R.
richard boobier

If I'm not mistaken, the charge light's energy comes from the difference between charge current and battery current. When the resistance in the wires / connections gets more over the years, the light starts to glow. As does mine. It has done that since I got the car, even with new battery.
Alex G Matla

Richard, once the light has gone out, does it then stay out, even at idling revs, until the next time the engine is started? If that is the case, I had the same on my V8 - on a cold start the light stayed on until the revs got up to about 2000, but once out it stayed out.

Apparently an alternator needs a load to kick start it into life, but once it is charging it keeps on going. I was advised to wire a resistor across the ignition lamp, and it works. The resistor gives the alternator an instant load and so it charges from the get-go. Once the light is out, no current flows so the resistor is not in circuit any more.
Mike Howlett

One simple problem could be the dash lights....

The ignition light has a small tube to isolate it from the dash lights.
Over time this can crumble and disintergrate leaving a small gap for light to enter and give the ignition light a glow.

But if it still happens with the lights off then it's some thing else
Onno K

Mike,

No the light comes on (glows - not as bright as when engine not running i.e full on) whenever the revs drop to idle and decreases in intensity until it goes out fully at about 1800 revs.

It's been doing it for some time but the flat battery has made me rethink about it.

I can recheck/remake all earths again next weekend - however I'm trying to understand the charging circuit a bit better first.

Seperate question - can the battery water auto fillers that fit to bottles still be obtained - its a pain trying to get to the battery to check levels.

R.
richard boobier

It is an open circuit earth side output diode in the alt. The most benign of alt faults - slightly low max charge (by about 1 amp) but not much else. You might hear some excess noise from the alt, if you can sort it from the general clatter. I've seen them last for years like this.

FRM
FR Millmore

Thanks FRM,

I'll try to pick up another Alt some time and swap it over and recheck.

There seems to be a fair bit of mystery to alternators and there workings - well at least to me when they play up a bit.

I'm O.K for fairly major issues, but this one is beyond my understanding of their workings.

R.

richard boobier

Richard -
Thinking (and reading) about it some more, there will also be output "poor at low speed" so if you are dawdling about with lights/heat/wipers etc. it could account for low battery. The 14V at low idle seems a bit high. I take "poor output" to mean low amps, hence the high voltage as it tries to charge the battery. The diodes are all part of the rectifier, so any bad diode means a new rectifier, and the voltage may result from the diode or a bad regulator. Open circuit diodes are generally separated from the rectifier plate, which they are a permanent part of; this could be from vibration but is usually from heat, from over voltage. You can sometimes see melted solder where the diodes attach, and I've thought of attempting to solder them back, but:

In my experience, no matter what I diagnose based on the WSM, the alternator guy always winds up replacing both the rectifier and the regulator, and can demonstrate that they are both bad.

"solid state" = inert object
"solid state electronics" = device to make your car an inert object

FRM
FR Millmore

As FRM said most likely a main diode. A blown exiter diode can also do it. Basically the diodes create a three phase rectifier if one diode is blown i.e. open circuit then only 2 phases are working so you need the engine reving quite a bit higher to get a reasonable output out of the alternator. End result less charge at lower revs. Main diode will give you a noisey voltage signal too. If you connect a multimeter across the batery instead on no AC voltage you should see a bit there. Not uncommon. You could just need a new regulator which includes the ex diodes or just the mains but these Lucas alts are getting on a bit so if it's apart you might as well do the lot. So prob a few dollars in fixing it if you have a secondhand one a cheaper option to get you going.
Greg H

>>Seperate question - can the battery water auto fillers that fit to bottles still be obtained - its a pain trying to get to the battery to check levels<<

I don't really know what you mean but once you have the battery and charging system in good condition then the battery level will remain constant unless you have a problem

getting to see inside is a bit of a pain but you're probably only going to check a few times a year

I have mirror glass out of an old door mirror I use with a torch

or next battery get one that has the little green ball indicator

usual story if you spend a bit of time at the start having the battery fully topped up and charged and all connections clean, secure and protected then if everything else is working as it should then there's very little if anything to do on the battery other than the occasional check
Nigel Atkins

>RB - can the battery water auto fillers that fit to bottles still be obtained

>>NA - I don't really know what you mean...

I do; put Battery filler bottle into ebay search box and you will find a couple....
David Smith

David,
right cheers, I can't remember ever seeing one but that's probably what's shown in the Driver's Handbook so I guess the valve works by pressing the tube on the battery plate to release the water

if you have one it's handy but unless you have a workshop you'd use it very, very few times ever nowadays and most times that you'd be topping up the battery it would be out of the car ready for charging

I guess I'm just too young to know about these things :)
Nigel Atkins

This thread was discussed between 12/02/2012 and 13/02/2012

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