MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGA - Should I Be Worried?

I'm looking for advice from the collective wisdom of the BBS once again.
I have an ammeter fitted due to the hole in the dash from the PO and it has always registered a few amps of positive charge after starting and then settled down to around neutral charge after a few minutes of driving, with no lamps or accessories on (dayytime).
However, the past couple of times I have fired up, again in the day with no lights or accessories on, the ammeter has shown a high current draw when the engine revs past about 2000 rpm. I mean virtually pegged over beyond 30 amps. This goes on for perhaps 20 to 30 minutes of driving and then seems to gradually return back to the null mark.
Its really unusual for me with this car which I've owned for 37 years. New generator and regulator within the past 500 miles. Any ideas? Thanks.
Tom Heath

Tom,
If you can borrow a clamp on ammeter, put it around the battery cable and see what happens.
Maybe your voltage regulator is sticking?
Mike Parker

Tom, does your charge warning light extinguish when the engine is running? Is your dynamo getting hot? Could be an internal short in one of your batteries, or maybe faulty dynamo. Does your battery seem in a good state of charge, ie good engine cranking and lights bright, getting brighter as you go beyond tickover? Needs getting to the bottom of if it is a genuine fault, 30 amps is going to be getting something warm somewhere, and the last thing you want is a fire!
Lindsay Sampford

A failed regulator relay in the control box will make the generator overcharge dramatically, boiling water out of the battery. At higher engine speed it WILL cause internal meltdown of the armature, solder joint failure, breakup of the commutator segments and carbon brushes, and sometimes failure of the rear bushing (which in turn makes lots of screeching noise as the armature drags on the field coils).

I have had terrible luck with replacement control boxes failing prematurely in recent years. When a $40 control box fails it can take out a $100 generator. I finally got so frustrated with the bad control boxes that I installed an alternator with internal regulator.
Barney Gaylord

Hey Barney, I'd forgotten about that one, and I shouldn't have. Many, many years ago on a winter's night I was leaving the lights and warmth of north London and heading for home on a 1958 Ariel Red Hunter m/c. Suddenly my lights started to get brighter and brighter, and, just as I was starting to think "that's good, now I can see where I'm going" (6 volt lights) ping!! all the lights went out (engine was ok, it had a magneto). The regulator coil had gone disconnected and the volts just went up and up!
Lindsay Sampford

Like the others said, it is most likely either the regulator or else a shorted cell in the battery. Either way, it needs to be fixed because it can damage other parts. You need a volt meter to find the problem.

First thing is to measure the battery voltage with everything turned off. If it is around 12.5 volts it is probably not causing this problem. If it is less than 11 volts and will still turn the starter, then it most likely has a shorted cell and you need to replace it before it damages the generator.

If the battery voltage with everything off is around 12.5 volts then you need to check the charging voltage. When the ampmeter shows charging current, the battery voltage should be less than 16 volts. The regulator should stop the charging current when the battery voltage is somewhere around 15.5 volts. The actual voltage varies with temperature and the limits are listed in section N of the shop manual.


Jeff Schultz

Thank so much for your advice. I will start tracking down the problem based on your suggestions. I guess that I will first try the battery as it is of dubious age and then look to the regulator which is a recent aftermarket piece. I still have the old (1959 vintage ) regulator so maybe that is still serviceable. I only replaced it because of the new generator. Thanks so much to all of you who responded. You are a goldmine of information and may have helped me prevent a more serious problem. I'll let you know what the outcome is.
Tom Heath

This thread was discussed between 23/05/2009 and 24/05/2009

MG MGA index

This thread is from the archive. The Live MG MGA BBS is active now.