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Alternator diagnosis


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Having been dogged by sluggish started for a while, I started with a bit of basic fault finding on my V8 90.


Engine - serpentine 3.9

Alternator A127I-100 Magneti Marelli

Alt bearings were replaced by me last year, so it spins smooth as a smooth thing.

Batteries (matched pair of Numax)are only a couple of years old, so assumed OK.

All terminals secure at battery and alternator.

Charge light on dash comes on with ign on, so bulb is OK.

Earth links checked from engine to chassis. Removed bolts and removed any signs of corrosion and cleaned surfaces (great things are powerfiles).

Ditto for battery to chassis cable.

Starter motor swapped for another known good one (just because I had one in the garage).

Charged main battery and out with the multimeter:

Battery with engine off 12.5V

Battery with engine ticking over, climbed to about 13.1V

Switched on dipped headlights, volts drop to 12.8

Rev engine to fast idle, no increase in voltage.

Turned on all lights, spots, blower and brake lights, elec fans, volts down to 12.1

No detectable change in engine speed when switching everything on. I would normally expect a small slow down under heavy electrical load.

At this point I decided the alternator wasn't working correctly, although the volts don't seem to have gone low enough to let the charge light come on.

Alternator removed and inspected, dried crud blown out from the casing fins .

Brushes removed and found to have plenty of meat left on them, springs all good too.

Next off with the regulator. I've kinda assumed this must be the part at fault already, but the back of it looks like this, with a slightly melted bit.


All parts reassembled and no improvement, there wasn't that much crud in there after all.

My reason for questioning this is that I know the diodes can fail too. I don't know which is more likely to occur, and I'm not sure what would cause them to fail. But I am leaning towards the whole regulator idea, as it still seems to chuck out some charge, just not enough.

So my question is - Does my diagnosis hold water? Does anyone disagree, or have any other checks to suggest?


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Voltages are screwed you ought to have 14V-14.5V out of a decent alternator and a fully charged battery left to settle overnight will give 12.6-12.8 with no loads applied.

Yes I'd say the sizzled bit might be suspect, new reg will be cheaper than a new alt anyway!

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A dead diode will normally result in the charge light being on all the time as the current is allowed to travel back up the wrong way - this is exactly what happened to mine a couple of months ago

Not necessarily. They can fail open circuit, in which case the alternator will still work...but will not chuck out much charge at all.

I'd say that's likely what's happened in this case.

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Well I have ordered a new regulator unit from these guys, it's about the only replaceable part available for this model of alternator. Damn things just ain't designed to be fixed any more :angry:

It's a lot cheaper than a complete new unit, fingers crossed that it turns up by Friday, and that it is in fact what is needed. I'll report back on this.

@ Redneck - I had similar on my first car, a mk1 Fiesta. 18V and very bright lights, they didn't blow in my case, must have been lucky with that. Not so lucky was the boiled battery that quickly lost interest in holding any charge at all.

Solution - a quick trip to my local auto electric repair man in his garden shed. A couple of days and £20 later I had a completely repaired and fully serviced alternator. No such possibilities any more unfortunately :(

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New regulator unit arrived yesterday, complete with brush pack.


The small wire rod holds the brushes back during assembly.

Assembled it then looks like this


All put back together and fitted to the truck, and success! 14.2V at the battery whatever the electrical load. All is well, or so I thought.

Then I went for a 15min drive to meet a mate at the pub to chew over anything Landrover for a couple of hours, everything looking good so far. Then when I went to drive home, I noticed a red glow from the dash :o . Well there was enough in the battery for the drive home, so I wasn't too worried.

This morning I went out to start the Landy, no battery light, (except for when the engine was off) and all seemed OK, the dash voltmeter was looking healthy.... and then it didn't :angry:

As quick take at all apart followed, to make sure I'd tightened everything up OK. I couldn't see anything amiss, so reassembled things for another test.

Now I have a battery light on constantly, but when the engine speed is raised above idle there's plenty (upto 14) of volts. The dash voltmeter rises and falls with engine speed. So it looks like there is a charge being output, but maybe not regulated. This of course gives me concern :unsure:

How can it produce a charge but the battery light still be on?

Can anyone shed any light on this situation please?

Oh and before anyone suggests it, yes, I did remove the brush retaining pin :lol:

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Have you connected the small wire securely? This could give the symptoms you describe.

All wires connected OK and double checked. Still no joy.

I did however find another old alternator among my treasure, which I have cannibalised, as it had the same type of regulator. I swapped the brushes for my old ones as there was more life left in them, and put my alternator back together with this regulator.

For the moment all seems well. I shall be off for a drive shortly to see if things stay good this time round.

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