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MG MGB Technical - Drop in battery voltage

Hopefully not too long of a topic.

Some of you may remember the thread from last year regarding the replacement of three starter motors over the period of a few months. After the last "failure" of a starter, I decided to put the B away for the winter in very early December. In late February I decided to connect the battery up and see if it would fire, and to my surprise it did. Since then I haven't taken it out very often, just for a quick spin at the weekends.

I have been following the voltage carefully to see if there is any change/drop after driving. I have suspected in the past that perhaps I have a faulty alternator. I checked the battery before heading out and it read 12.7, and just drove it locally for a few miles over a weekend. During this driving time I did not see the alternator light come on, not even very faintly. The next time I decided to start the B up, the battery sounded very weak, the starter wouldn't turn the engine and then the dreaded clonk from the solenoid, nothing. I checked the battery and it read 12.05, which I thought was sufficient to turn the starter. I Charged the battery again and it read 12.8. Last weekend I decided to take it out for a few miles, just down the road and back. The alternator light came on and stayed on for about a quarter of a mile. After that it never came on while driving. I didn't see any other issues. Stopped for while and then headed the 2 miles home. The first thing I did when I got back was to check the voltage again and this time it had dropped to about 12.5. I drove about 5 miles that day.

Is this normal? or is the alternator not doing what it is supposed to or something else? Apart from the brief period when the light came on I did not see anything which would indicate a faulty alternator.

I am off work all next week and I want to get my B back on the road for some decent driving

GG Ginty


To check your alternator / battery combination, measure the battery terminal voltage, with the engine running at fast idle, it should be in the range of 14 to 15 volts if all is OK.
12.7 volts on a fully charged battery seems a bit low, I would have expected something like 13 - 13.3 volts. If you have or can beg, borrow or steal a hydrometer measure the state of charge with it, cell by cell.

I have had dead batteries' volts measure OK until a load is put on them. Try measuring the voltage with the engine stopped and the headlights on.

Herb Adler

As Herb notes, the voltage is low for your battery and would seem to indicate that it is either not holding a charge or it is not being charged. Note that a bad charging system can damage the battery.

First, fully charge your battery. Then measure the battery voltage on the battery terminals. Then, measure battery voltage on the terminal clamps. Any drop in voltage indicates a bad/dirty/corroded connection. If found, clean the terminals and inside of the clamps and test again.

Second, start the engine and measure system voltage. This can be done on the front of the fuse box where the white wire circuit connects. Then, measure the system voltage at the battery terminal clamps. It should be the same as at the fuse box. If not, there is a problem with the battery cables/ground cables.

A fully charged battery should have at least 12.5 volts and may be as high as 13+ volts. System voltage, with the engine running, should be between 13.5 and 14.5 volts, preferably near the higher end.

Let us know what you find and we can recommend further tests if these do not indicate where the problem is. If you do not have a wiring diagram of your car, they are available on the internet and are a good thing to have when doing more than this basic trouble shooting.

Les Bengtson


I had problems getting onto this site yesterday so I couldn't respond. I am off work all next week and it is the priority to get Wigan back on the road properly. I will take a look then and get back to you

GG Ginty

Start by replacing the long battery lead from your battery to your starter. It's probably over 35 years old. I'll bet that helps with your charging problems. Starter motors are probably the most heavy duty, durable components on your car, even Lucas starters. You might think about changing the negative battery lead also (if you MG is negative ground.)

It helps if you start by indicating which model and year your MG is and the mileage.
Safety Fast.
R Murray

Hi GG,
"I had problems getting onto this site yesterday so I couldn't respond"

Same condition yesterday , like if england had sunk.
IMHO, check for connectivity default too between alternator and batteries. The use of a battery conditionner during winter keeps them up, cold temperatures really affects dramatically unused ones .

12.8v is about right for a fully charged battery once it has been allowed to 'settle' after charging. Immediately after charging it will be higher than that, and with the engine running it should always be in excess of 12.8v, up to a maximum of 14.7v for an alternator. measuring the voltage at different places will give you difefrent voltages in practice, even though in theory they should all be the same. Alternator brown, solenoid battery cable stud, fusebox brown, fusebox white, fusebox green will give progressively lower voltages, and the more current being drawn i.e. the more things turned on/flatter the battery the more difference you will see.

The other thing is that while parked there may be a drain on the battery, which is when the battery voltage looks fine immediately after a run, but is weak the next day or later. The best way to determine that is to disconnect the battery ground strap and connect an analogue meter on its 12v scale in its place, with everything switched off, doors and boot lid closed etc. With a dynamo and no additional electronic equipment such as clocks, radios etc. you should see nothing on the meter. With an alternator you will probably see a few volts registered, which is just the reverse leakage of the alternator diodes and can be ignored. If you see 12v there is a drain, which could be due to a radio etc,. which should be insignificant, or could be a fault and significant. The first thing to do would be to remove the fuses for any additional kit and see what difference that makes. If still 12v then remove the purple fuse, if still 12v then you will have to start breaking down the brown circuit at ignition switch, lighting switch, starter relay, ignition relay and finally the solenoid until the 12v drops to zero, the fault or drain lies between the last two places disconnected.
Paul Hunt 2010

I had a similar problem and starter consumption on Chev with the 454 cu in engine. Problem was inside the battery. Some times it had full voltage some times it didn't. A different battery and no more problem but only after 5 starters..
Kelvin Hawkins

I bought one of those voltmeters which plugs into the cig lighter and has the red yellow and green lcd's.

I am still at a loss as to the problem. Plug the meter in and the voltage read 12.6v. Cranked the engine and the voltage dropped to about 10.5v As soon as the engine fired and off down the road, the voltage jumped up to 13.9v. After a couple hundred yards or so it was steady at 14.08v. It stayed at this for a while and then the voltage dropped to 12.6. I was under the impression that the ignition warning light comes on when the alterator is not putting out???. Then it jumped back up to over 14v again for a while. Then the ignition light came on and the voltage dropped to 12.4v. Reve'd the engine, light went out and the voltage read a steady 14.10v for the next few miles.

Tested the volage this morning and the battery read 12.55v.

GG Ginty

I assume the ignition warning light comes on when you first turn on the ignition and goes out when the engine is started and revved?

Everything looks fine except the voltage dropping back to 12.6v, *if* the engine was still running at 1000rpm or higher. That shows the alternator charge is no longer reaching the system. If it is the output wire going open-circuit that wouldn't show on the ignition warning light, except for a dim glow at night that gets progressively brighter as more electrical load is switched on.

If the engine slows enough the alternator will stop charging until it is revved again, but that does show as the warning light coming on at full brightness. Normally this only happens if the revs are dropped to about 600rpm, but can be higher if you have the wrong i.e. a larger alt pulley.
Paul Hunt 2010

PS. If you take a jumper lead from the brown in the alternator plug and connect *that* to your voltmeter, and *that* shows the voltage dropping to 12.6v when the engine is still running at 1000rpm or higher, then there is an internal fault in the alternator, which sounds like a loose connection.
Paul Hunt 2010

This thread was discussed between 13/04/2010 and 26/04/2010

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