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MG MGB Technical - Failed alternator?

Driving home last night from a job interview, hence best suit etc., my alternator seemed to fail. Ignition light came on and ammeter indicated a large drain. I quick prod under the bonnet failed to identify any loose connections. However, since I was nearly home I pressed on and 45 minutes later was parked on my driveway. Question is how long should a good battery ('72 MGB converted to single 12V new last year) hold out when running the engine and powering dipped headlights? Or is this a 'how long is a piece of string' question? Also, and before rushing out and buying a new alternator, is there anything I can check?
cheers
steve
Steve Coulson

Steve,
Most of us have probably run home on the battery alone at some time, but to do so successfully with your headlights on is pretty daring, and quite an accomplishment! I'd say that you did well to make it home; you have a good battery. But, I would think that your headlights must have been pretty dim by the time you got there! A big amp draw like that might indicate an internal short in the alternator; take it down and have it checked out by an auto electric shop before you invest in a new one.

Best, Joe
Joe Ullman

Sidelights (also on) will draw 4 x 6 watts = 24 watts
Headlights will draw 55 x 2 = 110 watts
Ignition coil will draw 40 watts

Total 174 watts or 174 /12 = 14.5 amps

A fully charged 55 amp hour battery will in theory deliver 14.5 amps for 55 / 14.5 = 3.8 hours, but in practice I would only count on half this because you use indicators and brake lights as well - so a couple of hours would be reasonable.
Chris Betson

Steve. Chris's theory is supported by my observations. With a fully charged battery, you will be able to go a couple of hours even at night. As to checks, first thing to do is fully charge the battery. While it is charging, make sure the plug is fitted correctly into the back of the alternator and that the connectors are firm in the plug and engaging their tabs in the alternator. Sometimes, the molded plug gets worn and the connectors do not make full contact and come loose from the alternator.

When the battery is charged, use a volt meter to measure the voltage. It should be about 12.5 V. Start the engine and run it up to about 1,500 rpm and measure the battery voltage again. Anything above 12.5 Volts shows the alternator is charging, 12.5 V or lower shows it is not. Charging voltage should be 13.5 to 14.5 Volts. If above 12.5, but below 13.5, it is charging, but the alternator is on its way out. If you have the warning light and it is showing 13.5 volts or more, you need to have the amp rate checked. Most of us do not have the equipment to do this and have to go to a professional to have it done. Some parts stores will bench check an alternator for free. Some mechanics will test it on the car for a low fee or, for a regular customer, for free. Les
Les Bengtson

Thanks guys,
I checked the voltage last night. It seems to be 12.4V with nothing switched on, 12.1 with the engine running and 11.9 with headlamps on. I've 'fiddled' with the mutiplug at the back of the alternator. It was dirty but cleaning the contacts has made no difference. I guess it is time for a new alternator. I was a bit curious though as may previous alternator failures have been more 'catastrohic' rather than just a reduced power output. Also this alternator is only two years old. Mind you, the quality of the part might be a bit suspect. It is itself a replacement under warrenty. The first replacement lasted 6 months before a diode blew allowing the battery to discharge through the alternator over night.
Thanks for your help,
steve
Steve Coulson

FWIW

Early this year my alternator died just one day before I was to take my MGBGT from Tennessee to Michigan for a V8 transplant. As I was replacing the engine, I saw no need to spring for a new alternator, so I charged the battery up, stuck a fully charged battery from my TR6 in the back seat as a bcakup, and headed out for Michigan. It got dark about two hours from Detroit, so I disconnected all lighting except one headlight and one tail light and continued on. We stopped in a motel in Detroit and spent the night. Next morning, the engine started up with no problem, and we drove the remaining 100-150 miles to our final destination, still on the MGB battery. The engine died in front of my friend's house in Wayland, and the battery was stone cold dead - not even a click out of it as I tried to start the car again.

I jumpered to the TR6 battery, restarted the engine, and drove the last 150 feet up the driveway to my friends garage. I left the TR6 battery jumpers in place for a couple of days (plain forgetfullness, didn't intend to), then removed it and brought it back home with me. A few weeks later, my friend tried to start the car to move it into his shop to start work on it, and the engine started up on the first try, still on the original MGB battery that had been dead just a few weeks before. The recharge from the good TR6 battery was enough.

Moral of this story? There isn't any, just thought it might be of interest. Not that it makes any difference, but both batteries were Sears Diehards, both 3-4 years old.
Dan Masters

Steve,

There are a couple of things you can try -

1. Take the alternator off the car and remove the black plastic cover - underneath there is a central cover which carries the brushes - take them out and check for wear - it may be that the brushes are not making contact with the slip rings.

2. If that is OK then try gently lifting each of the tags soldered to the diodes - a blown diode will allow the tag and wire to be bent up away from the black round bit of the diode.

3. If brushes & diodes are OK then it is probably the regulator pack - not serviceable - but can be replaced - take the alt to an auto electicrian for testing.

Chris Betson

"The recharge from the good TR6 battery was enough."

There are 'jump leads' available in the UK at least that just plug into each cars cigar lighter. You wait, and an LED tells you when sufficient charge should have been transferred to try starting.
Paul Hunt

Thanks for all your input. I fitted an exchange alternator this morning (£23) and all seems well. Fingers crossed.
steve
Steve Coulson

This thread was discussed between 30/10/2002 and 02/11/2002

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