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MG MGB Technical - quick Alt question

I have a 79b and the battery has finally quit. But im trying to tune the carb and I have another good battery running from outside the car hooked up to the battery in the car via jumper cables. Intresting I know ! Now my question is: When I unhook the good battery via the jumper cables the car dies out. Is the alternator shot now too ? I was using a 60amp car charge jump to turn her over before I switched to the battery configuration and when it started It stayed running when using the charger. Any idea's ? I just forgot how to check if the alternator is good etc. Thanks.
JRB Mr

I don't think it is a shot alternator. The battery is part of the electrical circuit and the old battery may be interupting continuity. Hence car dies when good battery is removed. A running car still requires a battery to be wired in the curcuit to run. Just need to buy a new battery. A simple check for the alternator is to measure the voltage while running. It should be about 14.5 volts.
andrew.horrocks

Actually, once the car is started, it should be running on the alternator. If the in car battery had a dead short within it, the short would apply with both the battery charger attached and with the external battery attached. The same result, in both cases--the internally shorted out battery would quickly drain the second battery or cause the circuit breaker to blow in the battery charger.

My first thought is dead alternator which lead to a progressively discharged battery.

Disconnect the battery from the car's electrical system and put it on a battery charger for about 24 hours. If using a variable rate charger, charge it until it is charging at the two amp rate, indicating a fully charged battery. Measure the battery voltage. It should be between 12.5 and 13.5 V when fully charged.

Hook up the battery to the car's electrical system. Turn the ignition switch to the run position and look for the ignition warning light to glow strongly in the center of the dash. If not, that is part of your problem.

Try to start the car using the original battery. If the battery is good, the starter should crank over and the engine should start.

If the car starts, use a volt meter to measure the voltage at the battery cable clamps. If the alternator is good, you should show about 14.5V. If you are only seeing your original battery voltage, the alternator is not charging the battery and the engine is running off the battery.

This will get you started and, when we have more information, can focus on the numerous problems which could cause the situation you describe.

Les
Les Bengtson

Hi all.

JRB - As Les says, a failed battery is sometimes a symptom of a failed alternator.

Another possibility : some high current chargers are a bit 'savage' and may produce enough voltage to kill an alternator under some circumstances.
If your old battery has failed open circuit / high resistance (as they often do) it is possible that the 60A charger has damaged your alternator. If the battery had been good it would (hopefully) have provided a 'sink' for any excessive voltage from the charger.

I suggest that you follow Les's guidance, but I suspect that you will find that your old battery will not take charge (maybe you already know this).
If your old battery won't start the car continue with Les's test sequence with the jump leads from the other car connected (other car not running).

Andrew - not quite right, as Les says the car should continue to run (once started and the ignition light has gone out) with the battery disconnected but this is not a good idea as it may damage the alternator.

Les - in my experience 12V batteries never go fully short circuit, they are usually unserviceable when one cell has gone short (two at most) and a charger will usually tolerate a battery in this condition.

Don
Don

You should never disconnect the battery with the engine running on an alternator-equipped car as the alternator can then produce some very high voltage spikes, unless it is of a more recent design with over-voltage protection. As well as damaging the alternator it can also blow bulbs and other electric circuits on the car. The battery has a very large capacity to soak up these spikes.
Paul Hunt 2

This thread was discussed between 24/05/2006 and 26/05/2006

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