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Electrical Question

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drdata Avatar
drdata Charles Duffy
Healdsburg, CA, USA   USA
Automotive electronics (let alone any electronics) have never been my "long suit"...

When driving my 1950 TD (negative earth), the ammeter pretty much stays at "0" or just slightly to the right (positive). When I turn on the side lights, the needle jumps to the first mark on the positive side, I guess that would be about "5" If I turn on the headlights and/or the driving lights, the needle jumps to "15" positive and stays there (tested it this afternoon). How do I interpret this? Does this mean the lights are drawing 15 amps? If so, will this kill the charge on the battery? Is the generator not up to snuff (otherwise starts and runs very well)?

I usually do not drive the TD at night but eventually, I am sure I will. I want to make sure I will make it home and not be stuck at the side of the road with a dead battery.

Thanks, in advance, for the interpretations.

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TD4834 Avatar
TD4834 Bill Chasser
Sacramento, CA, USA   USA
It means the dynamo and regulator are doing their job. As you put a current draw against the battery such as honking the horn or turning on lights the regulator allows a greater charging rate. Similarly with the key on and the engine not running if you turn your lights on the amp meter will point to the negative side. The charging rate with the lights off should near zero once the battery has recharged from using the starter. I would say. No worries

peter14222 Avatar
peter14222 Peter Gilvarry
Buffalo, NY, USA   USA
If you turn on the lights when the engine is not running it should go to the negative side, when you are driving as Bill says it will go to the positive side. If it moves to the positive side as you increase RPM from idle you are good to go.

Remember to keep the "Lucas Smoke" contained.

Peter

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drdata Avatar
drdata Charles Duffy
Healdsburg, CA, USA   USA
Thanks for the explanations. Nice to know that the old girl is running properly.

Robert H Gold Member Robert Harvey
Wichita, Kansas, USA   USA
1951 MG TD "Millicent Grace (Millie)"
Hhmmm...I am not so sure. What should happen: When the engine is running at about any speed above 1600-2000 rpm, and the battery fully charged, the ammeter should be just barely positive. At running speed (about 2400 and up) if you turn on the sidelights, the ammeter should jump negative for just an instant, and then back to just barely positive. Same thing when you turn on the headlights. You should only get 15A positive charge if your battery is depleted, and under that circumstance the charge will be higher with the lights off. If I were going to guess what is going on based only on your description of symptoms, I would say that when your car was changed from positive ground to negative ground, they didn't change the wires on the ammeter, and your generator / control box is not working at all, What you are seeing is a 15A discharge that looks like a charge 'cause the ammeter is working backward. Do this test: with the car not running, turn on the headlights. That will give you a large discharge (10-15A depending on your lights). The ammeter should move in the negative (discharge) direction. If it goes to the positive direction, you need to reverse the wires on the back of the ammeter. The control box (regulator) controls the voltage output from the generator to the battery. The regulator should be set to about 15-1/2 volts. If the battery is discharged, the battery voltage will be below this, and you will get a high current flow to charge the battery, assuming the generator is OK and running fast enough to deliver it. As the battery becomes fully charged, the battery voltage comes up to approach what is coming out of the control box, and not much current flows. So at this point with the battery fully charged, the ammeter will read just above zero. Here is another test--again with a good, charged battery, engine at idle, lights on, Should see a discharge (negative) of several amps (8-12). Let it sit at idle for a minute or so, and then increase engine speed (or just drive away). As the engine speed comes up you should see the ammeter go positive several amps (5-10) and then as you cruise along it will gradually fall back to just above zero. This happens because the battery is slightly discharged by idling with the lights on (generator can't keep up), and then fairly quickly becomes fully charged again as you operate at cruising speed. If it doesn't work like this, something is not correct. Hope this helps more than it confuses. There is a pretty good section of testing this system in the workshop manual, and also there is a Lucas technical course available online that explains it even better.

http://mgaguru.com/mgtech/books/lts_otcc.htm

MGTF1500 Ardeche France Thierry SUCHIER
TOURNON SUR RHONE, Rhône-Alpes Auvergne, France   FRA
In reply to # 3494943 by peter14222 If you turn on the lights when the engine is not running it should go to the negative side, when you are driving as Bill says it will go to the positive side. If it moves to the positive side as you increase RPM from idle you are good to go.

Remember to keep the "Lucas Smoke" contained.

Peter

That is exactly what is happening on mine.
Which also reassures me.
Thank you for the clarification.
Thierry

shoalsgary Avatar
shoalsgary Gary Simpson
Killen, Alabama, USA   USA
1954 MG TF "Junior"
1954 MG TF
I'm going to go with a one wire alternator in a generator case and tach drive. I'd go with an alternator alone if I could convert my tach to electronic.

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bobs77vet Avatar
bobs77vet bob K.
northern Va, Va, USA   USA
In reply to # 3508458 by shoalsgary I'm going to go with a one wire alternator in a generator case and tach drive. I'd go with an alternator alone if I could convert my tach to electronic.

FWIW if I spent that kind of money on an alternator I would go with a feed back loop alternator. madelectric has good write-ups on this

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