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MG MGB Technical - Battery Failure

Left the headlights on the B a couple of weeks ago and battery went fully dead. When we tried to jump it car would turn over but not start. Let the jumper set on the batter for 5 minutes and the car started fine.

Yesterday while driving, the radio goes out but the car continues to run. Come to a stopsign and the car dies. Not even a click when I turn the key.

Alternator light did not come on.

I was somewhat out in the country so figured I would buy a new battery and get it home. Checked the altenator and it is charging fine. Body ground for the negative terminal is clean. Terminals on the battery are not corroded.

I have not been able to check out the cells on the old battery (only 2 years old), so I don;t know if it just went bad.

It appears that you cannot run the MG if
1. the battery is disconnected
2. the batter is totally dead.

Are these correct observations

PS Car is a 67 converted to Neg ground, running a lucas alternator with one 12 volt battery.
BEC Cunha

The car will run without a battery or with a dead battery as long as the RPM is keept high enough to keep the charging system working although a shorted battery may kill it.
That being said you should never run an alternator equiped car without the battery connected. The battery supplies the charging reference voltage for the alternator as well as a load, without it the alternator can greatly exceed the rated voltage of the system. If your lucky you only blow light bulbs, unlucky and you can cook every piece of electrical equipment in the car including the alternator.
John H

Thanks John

That would explain why the car still continued to run until I got to the stop sign. It would also explain why when we jumped it, as soon as we took the cables off, the car would die. It was running at idle.

It would also appear that the battery powers the radio as it stopped working a good 10 miles before.
BEC Cunha

The radio is powered exactly the same as everything else. Either it failing first was a coincidence and some other problem, or it is a 'safety feature' whereby the radio cuts itself off when the voltage falls below a certain level, to stop the battery draining any further, i.e. if the radio is left on in the garage. Under normal circumstances this *may* stop the battery running down below the point at which the car will start, however I've never heard of that as a feature.

Likewise if the engine turns over with jump leads connected then the ignition is also getting voltage. However depending on the condidion of the donor battery and the jump leads and their connections, the system voltage may still be low, leading to slow cranking and insufficient ignition voltage hence not starting. But if the engine was cranking over at normal speed then there should have been normal voltage to the ignition and it should have started. i.e. if it didn't then there was some other problem.

The only issue with a completely flat battery, indeed one with less than half charge, concerns dynamo-equipped cars. Unlike an alternator, a dynamo control box needs a certain amount of voltage to be available from the battery before it will start charging. So if you jump-start a dynamo car with a completely flat battery it may not continue to charge unless you leave the jump leads connected long enough to at least half-charge the battery. Even though an alternator uses battery voltage to start it charging from about 900 rpm, used alternators at least tend to be self-exciting when revved to 3k or more, so with these you can disconnect the jump leads immediately it has started, rev the engine to 3k or more to start it charging, then it will continue to charge down to about 600 rpm as normal.

As John has said as long as the alternator continues to output a charge it will run the ignition and everything else. If the revs drop below 600 rpm, i.e. the point at which the warning light would normally come on, and if the battery is still completely flat, then the engine will stop. However even a few minutes running with a normal charge should put enough into the battery to enable it to power the ignition for a few moments, but it would only be needed *if* the revs dropped below 600 rpm. Unless you had headlights on as well and it was only a very short distance between starting and the stop sign, then I'd say something else is wrong.
Paul Hunt 2

"Yesterday while driving, the radio goes out but the car continues to run. Come to a stopsign and the car dies. Not even a click when I turn the key."

All of these indicate a bad connection in the N/W/G circuit. Likely before the G section, given "not even a click".
Radio quit from no juice.
Stoplight application used all the juice coming through N/W, stopping car - very common when Batt is very low and/or very bad connections in IGN feed.
This would be even more apparent when the Batt was near flat, since voltage is already low, and the bad connections just add up.

"Alternator light did not come on"
If this means when running, that would be normal, since Alt is working.
If after it stalled, again indicates no power from IGN switch to light.

FR Millmore

I have always found that if a car battery is dead due to some light being left on, that when attempting to boost the car using another cars battery (with that car running) that often the car being boosted won't turn over unless it has had a chance to charge up somewhat. Sometimes I would get a buzzing sound (the starter relay rapidly connecting/disconnecting) or not even a click. This has been with non-MGs, newish or older cars. I put it down to the dead battery being so dead that it absorbs all current coming into it such that very little (or none) makes it down the cables to the starter. Don't know if my theory is sound or not.

Not that it is likely helpful in this thread, but I once had my MGBs alternator fail, and the warning light did not come on. I knew that the battery was not being charged because the starter became sluggish with each start (I had been making several small trips that day, and noticed the deterioration in starter operation each time I operated it). After sorting through the electrical system, finding no draws, finding that the battery tested out fine, I took the alternator to be tested. It turned out to have a break in the windings and the tech said that explains why the light never lit. Had it rewound and all has been great for the 6 yrs since it happened.


Erick Vesterback

Not so much absorbing all the current, as not aiding the 'less then zero resistance' jump leads plus the limited capacity of batteries on many modern cars particulartly those with smaller engines. I just have a pair of hobbyists aluminium cables which are fine for jump-starting a car where the battery has gone just too flat to start or a little more. But when trying to start my son's BMW with completely flat battery even though I left them connected for 20 minutes with the donor engine running, checking that voltage *was* going to the BMW battery, it still wouldn't start. Eventually I called out the AA who connected their booster pack and it started straight away, partly because of the bigger capacity battery and partly because of the heavy-duty jump-leads.
Paul Hunt 2

Worth remmebering that batteries can fail with an internal short circuit as well. When this happens the car will cut out when running and defy attempts to jump start, I lost a car this way as it was rammed by a lorry while waiting, and waiting and then some more waiting, for the RAC van.
Stan Best

Thanks FR

I will check that out. Still have not been able to get the old battery checked to see if it developed a short or has bad cells.

An internal short would normally only affect one cell and so show itself as reduced voltage and capacity. If this happens while running the alternator will continue to run all the electrics as normal. I can't imagine what would cause a battery to go completely short-circuit, open-circuit yes. A complete short on a battery, from external causes at least, causes a massive discharge and potential battery explosion.
Paul Hunt 2

"I can't imagine what would cause a battery to go completely short-circuit,"
I can't either, and agree with everything Paul said here, but I just had a car that had exactly that. Batt was dead after winter layup, customer (correctly) installed a new batt, drove about 60 miles, and it stopped - no electrics. When it got to me, the new batt was dead shorted internally. It melted the regulator/rectifier, but did no other damage.

FR Millmore

Update on my problem

I checked the alternator and it was showing power to the battery. Light came on when key was turned to on.

Today. No alt light. Due to it being dark in the morning and when I get home, Used the lights for a 20 mile trip. Radio again died, car continued to run. By the time I made it home, Lights were very dim.

Appears I am getting periodic power from the alternator. Checked all wires for a loose connection.
All ok.


If I do need to replace the alternator, what is recommended. Running a Lucas two wire currently.

How did you check that "it was showing power to the battery"? What was the voltage and how did it vary with engine speed?

What does "Checked all wires for a loose connection." mean? Underdash? Solenoid? Battery cables?

If the alt light does not come on with the key, then the alt/reg is dead, there is no power coming from the key via the N/W circuit, or the NY wire is bad. If the alt bulb was bad, the light would never come on.

If this is an early car with the cockamamie alt conversion that "sort of" retains the OE control box, you'll have to find someone else to help.

FR Millmore

If the brown/yellow 'priming' circuit has an intermittent connection you have to rev the engine higher than normal to get the alternator to start charging. On both my cars this is around 3k (with a working circuit it should be about 900 rpm), after which it charges normally down to about 600 rpm, then stops charging, and has to be revved to 3k to start again.

When the warning light *doesn't* come on with the ignition you need to check the voltage on the brown and brown/yellow spades on the alternator disturbing them as little as possible, I suggest by unclipping the cap beforehand (but being careful not to ground any of the terminals it them exposes).

If you see battery voltage on both i.e. about 12v then the problem is inside the alternator, or just possibly with corroded/loose connections between the harness plug connectors and the alternator spades.

If 12v on the brown and ground on the brown/yellow then the brown/yellow circuit is open-circuit back through the warning light bulb and the white wire to the connector by the steering column. Rev the engine to about 4k with the voltmeter on the brown and if the voltage increases to about 14v or so the alternator itself is probably OK.

If you see no voltage on the brown with the engine stopped and the ignition off the brown is open-circuit between the alternator and the solenoid. This is normally a direct connection, and is probably corrosion on the solenoid battery cable stud or brown spades. Don't just tighten it, disconnect the battery ground strap then remove and clean all spades/lugs and reassemble with coper grease or similar.
Paul Hunt 2

Thanks FR and Paul.

After the first incident, I put a volt meter on the battery and it showed 12 volts. With car running I am getting 13.6 at the battery. The alt light was coming on when the key was turned to on.

I cleaned the terminals on the alt, and on the starter solenoid. Also checked Connections on the battery and Neg ground to body. A

Will recheck per Pauls instructions this weekend.

FR. This is a conversion (gen to alt) but I am not using the old regulator.

I don't know the model, but this is one of the lower amp Lucas generators. What is a good replacement?

Will running a higher amp unit affect wiring or other systems?

You definitely should *not* use the original dynamo/generator control box with *any* alternator. The first alternators (16AC) did have an external voltage regulator *like* the dynamo/generator, but it was a very different unit. After that the 16ACR, 17ACR and 18ACR alternators had an internal regulator, signified by the 'R' in the designation. These should not be used in conjunction with any external voltage regulation unit, just connected to the solenoid and the warning light.

An alternator will only deliver the current required of it by the load - up to its maximum capoacity of course, then the voltage starts to drop. Unless you have added electrical components with a heavy demand the load on any alternator is going to be exactly the same and the standard wiring will be fine.

The standard Lucas alternators are fine for the standard loads, if they aren't i.e. you are getting a flat battery when running then there is something wrong somewhere which a bigger alternator may not fix. 13.6v is too low unless you have significant electrical load switched on or the battery is pretty flat, it should be closer to 14.5v. If you have bad connections/thin cabling anywhere between alternator and load (i.e. solenoid in your case) this can cause the volt drop you are seeing. Check the voltage on the alternator output spade, and if this is 14.5v when the battery shows 13.6v then you have problems with the cabling or connections. The two should be very close. When *cranking* you should ideally only see as low as 2 or 3 tenths of a volt lost in each 'leg' of the circuit between battery and starter, you are losing nearly a volt in one leg at a much lower current.

If you *have* added large electrical loads then they need to be supplied through their own heavy gauge cabling and fusing. Easiest to pick this up from the battery cable stud on the solenoid, but then the browns between solenoid and alternator need to be upgraded as well, and connected to the same stud and not the original spades.
Paul Hunt 2

Thanks Paul

Will get the tester out and do all the checking this weekend. It appears to be an intermittant short as some times when I turn the key to on, alt light comes on and other times it does not. Unsure until I check if it is alt. I suspect it is.

I have the 16ACR alt. While I do have the original Votage regulator still in the car, it is only there for looks. Have the alternator connected per the instructions on Bee Hive site. Has worked perfectly until I left the lights on.

Almost certainly *not* a short but an open-circuit. There is a big difference - clouds of smoke being one of them :o)
Paul Hunt 2

The 13.6V indicates that the alt is working, at least it was when you checked. It should be a bit higher, once the batt is fully charged - around 14 -14.5V. It takes a very long time to charge a low batt with the engine at 2500rpm and no load - at idle or loaded it might take forever.

Your symptoms still sound like bad connections in the N/W circuit. Most likely (by symptom) is the W cluster just after the IGN switch. This is the one that most commonly is worst, and the double connector(s) most likely of all, due to more heat. This is also the first point of the W feed to the warning light, which gives the excitation power Paul described.

FR Millmore

Pulled the Alt today and had it bench tested. It showed no output at all. Can't really explain why I got some power, but it is dead now.

FR. Will check those connections also.

I hates intermittents!

FR Millmore

This thread was discussed between 08/11/2007 and 18/11/2007

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