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Alternator diagnosis please


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Saturday night the Ninety's alternator light came on suddenly and without warning, and has stayed on. *!:@ and %!$! Just in time to stop me going to Donington on Sunday :angry:

The unit is a Magnetti Marelli 100A from a serpentine V8 Disco like this


I have checked everything I can think of like wires fallen off/broken, battery terminals tight, drive belt intact and alternator turning, brushes OK, charge lead is secure at alternator and starter motor ends, multimeter definitely shows no sign of charging.

I am guessing that some kind of rectifier pack is to blame but I cannot find any reference to replacement items.

Does anyone either have a better idea of what's gone wrong, and or know what is replaceable and where I can get it from.

Thanks in advance.

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do you still have power going into the alt (small wire)

take it you've pulled the brush pack out (3 small bolts)

Yep, had the thing apart and took the brushes out, they had plenty of life left in them. Springs and wires loooked fine too.

Will check the power to the alt later.


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This alternator is not the Lucas type as shown in the thread Paintman refers to but the later rather less-serviceable type badged as Magnetti Marelli.

Ign light was only on when ign key or and when engine running. Definitely no charging at 11.8V with engine running.

12V was present at small wire with ign on, when grounded to engine block the batt light went out, as you would expect.

I took the whole thing off, again, checked all the connections, again, nothing obviously amiss. Put it all back together and lo and behold, it works! :)

So it would appear to be some king of wiring gremlin, which is a bad thing, 'cos it'll no doubt crop up again at another inconvenient moment. :angry:

Oh well, at least it's working now, so I'm happy for now.

Thanks to all.

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take the wires off the terminals and check for corrosion on the connectors, the belt may need changing but it still usualy flickers on and off if it is wearing out, it may be worth you looking for a secound hand replacement if you can get one cheap, old local shops are also good a fixing them if you take it off or they can diagnose to see if it is the problem

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  • 3 weeks later...

Apologies for jumping in on this thread. I have the a '97 3.9 Serp V8 motor in my 90 and need a bit of help with the alternator - same as above I think.

With ign on, the warning light does not come on.

When cranking the engine, the warning does come on - warning light goes out again once the engine is running.

Finally, you can often see the lights flickering.

"Think" I've wired the alternator up the right way round, but unsure where to start diagnosing.



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