MG Car Repair: Dead battery/alternator, volt range, poor earth

Question
Hi Barrie,

I have a couple sets of questions, which may be related...

First, I own a 1968 MGB with a fresh motor.  Now that it has gotten colder outside (and in the garage) the battery now doesn't have enough juice to turn over the starter, but the car jumps easily.  After I run the car for 20 minutes the car will restart easily.  I have no idea how old the battery is; it may very well need replacing.  Are there tests I can do to rule in/rule out the alternator as a culprit?  I am handy with a voltmeter.  I also put a 12 volt test light between the negative cable and the battery, looking to find a short, and I did not find one.

Secondly, can you explain to me the meanings of the ignition warning light, in terms of when it's glowing brightly, dimly, off, etc.?

Thank you in advance for any information you can give me.

Regards,
Dan

Answer
Hi Dan
You say your car is a 68 and has an alternator.  That helps.  Up to 67 they had a dynamo.  

As a test, switch the headlamps on and then operate the starter.  Do the lights just dim a bit, or do they go out completely?  If they go out, I would suspect the batteries.  If not, then you have a problem in the starter circuit, such as a frayed engine earthing cable.  You say that you have a new motor.  Was the earthing cable re-connected correctly?  There should be a fat cable bolted around one of the engine mounting rubbers.  It is easy to forget, and the rubbers insulate the engine from the chassis, so there is a poor earth.

The ignition warning light shines when the demand on the battery exceeds the current that the alternator is putting in to the battery.  The more the demand, the brighter it shines.  Apart from at tickover, the red light should never be visible.  If it comes on, first check the fanbelt, which could be slipping.  After an engine replacement, a new fan belt may well need tightening.

To test the alternator, connect up your voltmeter to the brown wire at the fusebox.  Select the 20 volt range.  With the engine stopped it should read at least 13 volts.  Now start the engine.  At tickover, it should read maybe 14 volts, and when you increase the engine speed to 2,000 rpm it should read over 15 volts.  All these figures are approximate.

Batteries do get old and they tend to gradually get worse as they age.  They could well need replacement.  However, before you go to that expense, check the terminals.  If you look under the access plate behind the seats you should find two 6 volt batteries.  What type of battery terminals are fitted to them?  If they are the original lead cups with a small screw in the centre, then they could be the culprit.  I cut them off, reveal about an inch of thick copper wire using a sharp knife, and replace all 4 terminals with new ones.  Buy the type that clamp around the terminals.  Remember, the two battery terminals are different diameters, so you need 2 positive and 2 negative (or 4 universal ones).  Also, the right-most one (as you look at them) is an earth, so make sure the far end is bolted to a clean piece of metal, not a rusty or painted surface.

There are tests that can be done on batteries, but they need to be under heavy load.  A dead battery may still put out 6 or 7 volts as shown on your volt meter, but they die when asked to produce 300 amps.  Your meter cannot cope with that, but a good garage should be able to test them under load.

One final warning, a failing battery can damage an alternator.  I have seen several cases where someone has replaced a faulty alternator, and the new one has failed again, because of faulty batteries.