Charging problem - Help Please. : MGB & GT Forum : MG Experience Forums : The MG Experience

Charging problem - Help Please. #1
oily-hands Owen Frankland Stockton on Tees, Cleveland, United Kingdom 1937 MG TA "Numbum" 1971 MG MGB GT "The Bruise" 2002 MG ZR	Posted: June 21, 2012 07:14AM Registered: 3 years ago Posts: 5,149

The 71 GT with stock Lucas alternator has a charging problem. The ignition light doesn't come on when switching on, but comes on when started and doesn't go out until about 3000rpm.

Checks so far.

Got 12v at the alternator on the heavy brown wire.

With ignition on 12v at the alternator on the brown/yellow.

Between the brown at the alternator and ground, using 20v scale on digital meter, I get increasing voltage until it goes off scale as the engine is revved.

The ammeter shows discharge when electrics operated but no charge at any revs.

A new regualtor was fitted a few months ago, but that doesn't mean it can't have failed.

Alternators are a bit modern for me so I'm a bit stumped.

Thanks for any help.

Member of The International Society of Luddites (Unrepentant Chapter).

Take the time to understand what a part does and how it does it, then you'll have a better understanding of how to fix it when it goes wrong. Beats the scattergun approach every time.

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Re: Charging problem - Help Please. #2
GILMGA Gil Dupre Chattanooga, TN, USA 1962 MG MGA 1974 MG MGB GT	Posted: June 21, 2012 07:35AM Registered: 6 years ago Posts: 7,300

Go to the MGE library and read UNDERSTANDING ALTERNATORS

Gil

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Re: Charging problem - Help Please. #3
tbarker7815 Tom . Knoxville, TN, USA 1979 MG MGB Limited Edition (LE)	Posted: June 21, 2012 08:15AM Registered: 6 years ago Posts: 2,867

I'm thinking regulator is shot.

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Re: Charging problem - Help Please. #4
Wray Wray Lemke ., South Carolina, USA 1956 MG MGA 1958 MG Magnette ZB Varitone 1973 MG MGB 1974 MG MGB GT 1974 MG MGB GT	Posted: June 21, 2012 08:27AM Registered: 11 years ago Posts: 10,982

Tom x2. Time for a new/rebuilt alt.

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Re: Charging problem - Help Please. #5
tomkatb Larry Baygents Dayton, Ohio, USA 1963 MG MGB	Posted: June 21, 2012 09:51AM Registered: 8 years ago Posts: 3,370

I vote with Wray

However, if it were me I would.

Pull and clean all battery connections.

Clean alternator connections.

Put a charger on the battery overnight.

Test the battery the next day. Voltage should be 12.7 or higher with the car not running. Running should be a minimum of 13.5. A reading of 12 indicates a bad battery. I have minimal faith in ammeters.

If the light is till on get a new alternator. I would assume you can get one easily.

L.W.(Larry)Baygents
63B
77 Spit

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Re: Charging problem - Help Please. #6
JohnTF John F Northeastern Ohio, USA 1970 MG MGB 1970 MG MGB	Posted: June 21, 2012 10:28AM Registered: 1 year ago Posts: 843

Owen, see you are in the other Cleveland.

I would read up a bit, but the local parts stores here will bench test alternators for free, that is an option.

If you have a good relationship with your local mechanic, he may test it for you in place.

My friend John in Cleethorpes says Brit mechanics are friendly and have seen B's before. ;-)

What's the condition of the battery? Does it take a good slow charge from your bench charger?

Am afraid long gone are the days when an alternator was $25, so you could throw parts at a problem -- as my garage says, "there are no more $50 guesses" left.

I put a "police" alternator in my Ford -- it was about $5 more and brightened up things, I have heard of MG police cars. ;-)

Regards, John

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Re: Charging problem - Help Please. #7
gunna Thomas Gunnery Derry NH, USA 1976 MG MGB	Posted: June 21, 2012 02:24PM Registered: 4 years ago Posts: 309

You can get an alternator at autozone with a life time warranty. I have replaced two so far no charge after the first one.

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Re: Charging problem - Help Please. #8
RearWing Al Mann Georgetown, KY, USA 1965 Triumph Spitfire MkII 1971 MG MGB 1973 Jensen Healey	Posted: June 21, 2012 02:30PM Registered: 3 years ago Posts: 111

In reply to # 2099864 by gunna You can get an alternator at autozone with a life time warranty. I have replaced two so far no charge after the first one.

What make of alternator, Thomas?

Al
Al Mann
Georgetucky, USA.
'71 MGB
'73 Jensen Healey Mk1
'65 Triumph Spitfire IV Mk2
'12 Mini Cooper

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Re: Charging problem - Help Please. #9
stuart sinclair honiton, United Kingdom 1979 MG MGB GT 1998 MG F "RITA"	Posted: June 21, 2012 02:34PM Registered: 1 year ago Posts: 615

I had loads of problems with mine last summer after drowning it on a few occasions with rad water the the AA had a go at drowning it so i got one from the BEEHive if you order it in the morning it will arrive the following day,.

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Re: Charging problem - Help Please. #10
ingoldsb Terry Ingoldsby Calgary, Alberta, Canada 1971 MG MGB	Posted: June 21, 2012 08:53PM Registered: 9 years ago Posts: 9,752

If the ignition lamp does not come on when the ignition is switched on, and the engine is not running then a possible cause is bad brushes. Another possible cause would be a bad field coil winding but I have yet to see that failure.

First of all, with the engine off, turn the ignition key on. Remove the plug from the alternator and momentarily ground the brown w/yellow wire. The ignition lamp should glow. If it does, but does not glow when plugged into the alternator, then something is wrong with the field coil circuit. Again, the first thing I would check would be the brushes and commutator - I've seen commutators wear right through!

Quote: Between the brown at the alternator and ground, using 20v scale on digital meter, I get increasing voltage until it goes off scale as the engine is revved.

The ammeter shows discharge when electrics operated but no charge at any revs.

These two readings are contradictory. I suspect that the AC output or proximity to magnetic fields is confusing your voltmeter. If you really had 20v DC on the alternator output (which is connected to the battery), then the ammeter would show very significant charging of the battery. What happens if you read the voltage at the battery (far from magnetic fields) when you rev the engine? My guess is that you will see nothing but battery voltage (12.6v).

Another thing to check is the status of the plug going into the alternator. I've seen cables that look physically attached, but when you inspect the connection internally, you find that the crimp has loosened with time. This can cause really weird behaviour due to partial connections. You can (carefully) remove the spade connectors from the plastic plug by depressing a little tang on the spade.

It is unusual (but not unheard of) for voltage regulators to fail. The much more common failure is with the rectifier pack. Of course, one failure can induce the other!

Some time ago I wrote up a fairly complete explanation of what the alternator lamp does. You can find it at
http://www.mgexperience.net/phorum/read.php?1,1759313,1759313#msg-1759313

Terry Ingoldsby
terry.ingoldsby@DCExperts.com

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Re: Charging problem - Help Please. #11
gunna Thomas Gunnery Derry NH, USA 1976 MG MGB	Posted: June 22, 2012 08:51AM Registered: 4 years ago Posts: 309

I will have to check what make it is when I get home, It's my wife's car and its hard to pry it out of her hands. She drives the thing daily to work and wherever and you should see the look I get when I tell her I am taking it to work. angry smiley

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Re: Charging problem - Help Please. #12
oily-hands Owen Frankland Stockton on Tees, Cleveland, United Kingdom 1937 MG TA "Numbum" 1971 MG MGB GT "The Bruise" 2002 MG ZR	Posted: June 22, 2012 09:57AM Registered: 3 years ago Posts: 5,149

Larry, battery & connections are fine.

John F. I can get it tested off the car but haven't got a contact for on car testing yet.

Thoams, what's Autozone when it's at home. grinning smiley

Thanks for the Beehive tip, Stuart

Terry, thanks for that. I was puzzled by the contradictory readings. I'll dig into it a bit deeper when it stops raining.

If it needs a new alternator, so be it, but being a Yorkshireman, if I can fix it for £20 instaed of spending £100, I'll be happy.

Member of The International Society of Luddites (Unrepentant Chapter).

Take the time to understand what a part does and how it does it, then you'll have a better understanding of how to fix it when it goes wrong. Beats the scattergun approach every time.

Ignition testing made easy.

Making your MGB handbrake work

My You Tube Channel

Life with an MG TA and an MGB GT in the UK.

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Re: Charging problem - Help Please. #13
ingoldsb Terry Ingoldsby Calgary, Alberta, Canada 1971 MG MGB	Posted: June 22, 2012 06:03PM Registered: 9 years ago Posts: 9,752

Quote: If it needs a new alternator, so be it, but being a Yorkshireman, if I can fix it for £20 instaed of spending £100, I'll be happy.

It is almost always cheaper to rebuild a Lucas alternator than to replace it. I've never seen a field or stator coil die on a 16ACR alternator. So, that leaves brushes, bearings, rectifier pack and regulator.

BTW - there were several regulators used (2 wire, 3 wire, 4 wire). They are almost all interchangeable if you know what to connect to what (and I can help you there). You are much closer to the source of parts than most of us - it should be really easy for you.

Terry Ingoldsby
terry.ingoldsby@DCExperts.com

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