Multiple Alternator Failure

[pjw]

My dynamo packed up last week and I decided to convert to an alternator using ESM's conversion kit. Fitted this as per instructions and all worked well - 14v at battery with the engine running.
24 hours and 30 miles later, I noticed the ignition light was not coming at start up (i.e. ignition on, engine not running). Further checks revealed the alternator had failed.
Got a replacement from ESM (who were excellent as always BTW) and all was well. Alas, after a further 24 hours - around 150 miles this time - the problem has recurred. Again, checked the supply to the alternator (12.5v) and at the battery with the engine running (also 12.5v) which seems to show the second alternator has failed.
I can't think of a fault on the car that would cause two alternators to fail. Have I simply been unlucky or am I missing something? Any suggestions welcome.

[SUE482]

Alternators are fairly robust and I don't think a car fault would cause a fail as you suggest. Could be a batch fault

[Jefftav]

I have had similar issues with starters and alternators, try another and see if it happens again. Do the obvious electrical cable checks again making sure connections are secure and clean + the fan belt is adjusted correctly and running freely.

[bmcecosse]

Broken wires? The car is converted to negative earth - I hope!

[pjw]

No, no broken wires (I've checked the continuity of everything right from the battery terminals to the alternator plug). It's the field windings that appear to have gone; normally the warning light circuit earths through there when the ignition is on and goes out when the alternator is running and field voltage comes up to somewhere around 12v. And if I run a wire from the plug to earth, I do get the light on. It all points to the alternator internals so I'm still assuming it's a bad batch of alternators.

Anyway, ESM are posting me a third unit (from another batch if they can find one) and sending the failed units back to their supplier. I've asked them to let me know what they find and I'll post the response when I get one.

[bmcecosse]

It's not the field wings on that wire - it's just a sensing connection. Some alternators work perfectly well without the connection, but of course then no red light to show Ign is on. You don't confirm the negative earth ? And - have you removed the old Voltage Regulator from the system ?

[pjw]

It's not the field wings on that wire - it's just a sensing connection. Some alternators work perfectly well without the connection, but of course then no red light to show Ign is on. You don't confirm the negative earth ? And - have you removed the old Voltage Regulator from the system ?

The brown and yellow wire - the one connected to the warning light - is indeed internally connected to the field windings, at least according to the schematic for the Lucas 18ACR which the new alternator is supposed to replace. I guess there may be other ways of connecting the internals (perhaps via the regulator and maybe that's blown??) but it would still need to earth through the field windings eventually. In theory, you do need this connection to provide an initial excitation (lovely phrase!) but in practice it's rarely necessary.

Sensing, on the other hand, is done on the standard gauge brown wire which is connected to the solenoid terminal and hence to the battery. It runs in parallel to the thicker brown wire which carries the alternator output. You do get some older systems where these two share a common connection at the alternator end but that can cause problems when there is a high current out of the alternator resulting in a voltage drop across the wire up to the solenoid.

It is negative earth (surely it wouldn't have worked in the first place otherwise) and the voltage regulator is still in situ but only to connect the various battery fed wires on the A terminals; the F & D terminals are no longer required and hence disconnected.

[bmcecosse]

Sounds ok then, Certainly my 18ACR runs fine without the small wire connection - it broke off and ran for ages without until I got round to remaking the connection, showing correct charging on the ammeter - balancing lights etc when required. It seems unlikely all these alternators are faulty...but let's see how long the 3rd one works ! Well done ESM for sticking with you....but bad news indeed if they are providing faulty units.

[pjw]

Just to close this one, ESM sent me a third alternator which they took trouble to ensure was from a different batch; it was possibly a different supplier as I noticed a couple of cosmetic differences. After 4 months, it continues to work fine.

Either I was particularly unlucky with the first two or there's a batch of dodgy alternators around. I have to say thanks to ESM who couldn't have been more helpful - although, not totally surprisingly, I never did hear if their supplier had identified the problem.

[chesney]

We had the same problem. We used to run 4 Minors, all with alternators, so quite infuriating when they give in after a short length of time, exactly how you describe!
I bought my last one from Accuspark on eBay and (touch wood) it seems ok.

[PAULJ]

Sounds ok then, Certainly my 18ACR runs fine without the small wire connection - it broke off and ran for ages without until I got round to remaking the connection, showing correct charging on the ammeter - balancing lights etc when required. It seems unlikely all these alternators are faulty...but let's see how long the 3rd one works ! Well done ESM for sticking with you....but bad news indeed if they are providing faulty units.

Older alternators can become magnetised over time and will charge perfectly well without the field winding connected.
Makes you wonder why ESM were so quick to just send another one. If it was a batch problem which it sounds like it is. you cant be the only one.

[bmcecosse]

I'm not convinced it's a 'field wire' at all - I honestly think it's just there to operate the 'IGN' light!! Magnetisation will occur right away - age won't affect it.

[PAULJ]

There is no fixed magnets in an alternator. No magnet, no power
http://alternatorparts.com/understandin ... ators.html
and this
http://en.wikipedia.org/wiki/Alternator_(automotive)

[katy]

There is no fixed magnets in an alternator

True, but there is residual magnetism in the core of the rotor and that's all it needs.
Ever have a screwdriver or other tool that small parts would stick to? Not a fixed magnet but still having some magnetism.

[bmcecosse]