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MG MGB Technical - Charging system woes. - 1968 MG


Went to fire up the 1968 MGC last week..reconnected the battery ground...tried to fire it was dead. (No is/was the Nissan 300zx, the '69 MGC, and the '78 MGB!)

I disconnected the alternator and put the charger on it.

Yes...Negative to negative and positive to positive.

Battery appeared to re-charge on trickle...

Went to reconnect alternator and ...SPARK!

"This ain't right, I say!"

Pulled the one wire off of the alternator that I had connected (brown).


I disconnected battery at ground (negative terminal).

Reconnected alternator.

Went to reconnect ground....SPARK at battery terminal.

"This ain't right, I say!"

Any clue as to what's going on here?

Thanks in advance

rick ingram

If the battery ground cable were disconnected and the battery still went flat then either the battery is knackered (excessive internal leakage) or you left it waaay too long.

You almost certainly did charge the battery correctly as connected incorrectly it would have blown the fuse of cooked the wires, unless you have a really massive charger.

You don't need to disconnect the alternator for 'normal' charging, and by that I mean up to 15 or 16v, only if you were using very high voltage for a rapid charge to get you going would it be sensible.

You can get sparking for a number of reasons, especially at the battery terminal, even an interior light is enough is to generate some. I wouldn't expect any from the alternator plug though, unless perhaps the ignition were on. A big flash is not right though, if in doubt I'd connect via a 30 amp fuse and if it blows then something is definitely wrong. If the ignition were off, the battery connected the right way round - check it with a voltmeter, unless you have a really odd-ball alternator it should always be +ve on the brown and -ve on the body/engine - and you get a heavy current draw when you reconnect the alternator then that indicates one or more alternator diodes are short-circuit. If you didn't disconnect the battery ground cable *immediately* you switched off the last time you used it, then this will have flattened the battery in a very short time.
Paul Hunt 2

Paul..thank you for your insight!

OK...The saga continues....

I swapped out the battery just in case it was a short in the battery itself.

I connected the alternator back up (black wire on top, brown beneath it) and reconnected the battery.

It SPARKED at the negative terminal.

For grins, (I would say tickles and grins, but I didn't want to get shocked), I disconnected the alternator and reconnected the ground to the battery. NO SPARK.

I verified that the ignition is indeed switched OFF.

I have a feeling that the alternator, or maybe the alternator control box, is at fault.

In this model of MG, what does the Alternator Control Box do as a function? Could the fault lie here? (My '69 MGC does NOT have this feature.)

Also noticed the one time I did get the C started (a few weeks ago), that the tach would jump to 7000 rmp or so until the car started, then show normal rpm.

Thanks again in advance..


rick ingram

Rick. The 1968 models used an alternator having an external, solid state, voltage regulator. This was the Lucas 16AC alternator. In 1969, the 16ACR alternator, having an internal regulator/control box, was fitted to all of the cars. On my 68, I replaced the 16AC alternator with a late model 18ACR, giving both a higher output and having an internal regulator. Fairly easy conversion and makes it easier when one has to replace the alternator. Late model alternators are available from a number of sources, including Checker and Auto Zone.

Les Bengtson

I have some similar issues. When ignition is off ign. warning light is on, car on or cranking it is off. Also with ign. off and ig. light disconnected there is a slight current draw in system, a few tenths of milliamp on a meter in line at batt. Any ideas with the warning light problem?
p.a. howard

p.a., again an alternator diode problem. Battery voltage is leaking back though at least one of the diodes, via the warning light (which is why it glows), and things like the coil, to ground. It will discharge the battery. There should only be microamps flowing with a good alternator, if you see more than this, with the warning light disconnected, then one of the other diodes is leaky as well. They are usually contained within a single pack i.e. changing the pack replaces all the diodes.

Because of the dangers of measuring current (an ammeter is effectively a short-circuit) I always use an analogue multi-meter on its 12v scale to look for a drain. Connect it in place of the battery ground strap, and if it registers full battery voltage you have a significant drain. If all you see is a few volts registered i.e. significantly less than 12v then this represents micro-amps on most meters, is normal, and can be ignored. It should drop to zero with the alternator unplugged. I say use an analogue meter as what a digital meter will display when connected like this depends on its internal electronics whereas moving-coil voltmeters are pretty-much all the same.
Paul Hunt 2

This thread was discussed between 12/03/2006 and 13/03/2006

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