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Alternator is useless


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It seems that every time I post on this forum it is about a part that I have broken. This time it is my alternator.

While adding various electrical items to my land rover, something has happened to the alternator. It no longer charges the batteries. and when the engine is running the battery light is on in the dash, as it should be when the alternator does not provide any charge.

Am I right that when an alternator spins and is not connected to a load it will fry itself? Fortunately it is connected to the battery as I have checked the continuity of the leads from the alternator to the starter motor and then to the battery. The voltage at all points is whatever voltage is left in the battery, and not the expected 13ish volts from a working alternator.

Has the cold weather finished the alternator off, or is there anything i can do to recover it? The alternator is from a Defender 200TDi.

Any advice or ideas would be very helpful to recover the alternator, however I am prepared to buy a new one if that is what it necessary.

Many Thanks


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If the alternator is working properly and connected to the battery, you should see about 14 volts at the battery with the engine running, if the battery is disconnected from the system, the alternator output voltage will rise to maybe 18 volts or so, it probably won't do any harm to the alternator but the rest of your electrics may fry and bulbs go pop!

I you're only seeing battery volts of 12v or so, with the alt connected and the battery light is on, then I would say you have a duff alternator.

It may be the diode pack that's given up the ghost, or it could just be the brushes that have worn out or become encrusted with crud, either way it's most likely repairable, or of course a replacement would be easier, then you can always repair the old one and keep it as a spare.

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Thank you for the reply crwoody. Purchasing a new unit and repairing the broken one sounds like a good idea. While it is snowy and cold i don't want to order a new one, not at least i can be sure it would arrive in time.

I have removed the alternator, and started to try do do some problem solving as to why it does not provide any voltage when running. The alternator is a lucas 127 55A unit.

Using a multimeter the diode on the back of the unit does not have any continuity in either direction. Can it be this simple that this diode has failed?

I would like to know please, if there are any ways to diagnose if the main diode pack has failed, or the alternator has burned out.


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You can buy a new regulator/brush pack for an A127 style alternator quite easily. any decent motor/truck factors should stock them, or I'm sure you can find one online.

There are posts in the tech archive on just replacing the brush pack, as well as stripping down and servicing the whole alternator.

see This post and this post

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