Generator failed?

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#1 Post by **Clifford Pope** » Tue Mar 27, 2007 11:42 pm

The generator was working perfectly until yesterday, when the red light
suddenly came on. Tests show there is only 2 volts output, measured
between the joined F and D terminals and earth. 
According to my Autobooks handbook, that points to a fault in the field coil. 
The field coils have a resistance of 5 Ohms. According to the book, any
resistance much less than 6 indicates the insulation is breaking down.
Is 5 Ohms \"much less\" than 6? 
I have tried cleaning up the commutator, which looks OK and has continuity at each segment. The brushes are quite new. 
I have even tried re-energising the field coil, as per the method when I swapped to negative earth many years ago. 
However, the generator refuses to give more than 2 volts. 
 
Does that mean it has definitely failed ?- I cannot think of anything else feasible to check or do. 
I am now thinking of getting one of Chris Witor\'s alternator
conversions. The wiring looks pretty easy - has anyone any experience
of this conversion, or any other advice to offer? 
 
Many thanks, 
 
Cliff Pope

#2 Post by **Alec** » Wed Mar 28, 2007 1:54 am

Hello Cliff, the field coil measurement seems within normal readings given that accurate resistance measurement at low values tends to be inaccurate. The simple check for generator function is to connect the field and main terminals and apply 12 volts (belt disconected) and if it turns then it is probably OK. Certainly alternator conversions are quite easy, given that the Lucas alternators are quite inexpensive. Alec

#3 Post by **Clifford Pope** » Wed Mar 28, 2007 3:16 am

Thanks for the quick response Alec. 
I\'ve tried connecting a battery as advised. It twitches, but shows no
desire to turn. Probably some wiring inside is breaking down. 
I\'ll go for the alternator conversion. 
 
Thanks, 
 
Cliff

#4 Post by **Mike Stevens** » Wed Mar 28, 2007 5:58 pm

Hi Cliff,
Dynamos (generators) are so simple that it sounds like you have a fault in yours.  With the field and output shorted and the engine above tick-over, I would expect to see LOTS of volts.  Well in excess of 20.  If you have a field resistance of 5 ohms, then with your 2 V you should get nearly 0.5A field current.  That should be plenty to excite the field and produce output.  It sounds like there is a fault inside.  It might be interesting to measure the resistance of the main output.  I would expect it to be very low.
I have in the past fitted an alternator to a Mk1.  As long as you get all the relevant bits to mount it off the block, it should be no problem.  I removed the regulator box and re-wired there, but with the extra output of the alternator, it\'s worth fitting an extra wire from the 2nd output terminal (alternator) direct to the battery +ve.  Nice and short too as they are next to each other.
I have just done a dynamo - alternator swap on a Herald.  I suspect they are much the same, so if the dynamo is any use to you.....
Cheers, Mike.

#5 Post by **Clifford Pope** » Wed Mar 28, 2007 6:27 pm

Thanks Mike.
You have confirmed that the dynamo is shot - nice to know I haven\'t missed something simple (no, not the fan belt!)
I\'ve finished with dynamos (or generators - never known which to call them) now, and ordered an alternator conversion. I realise the potential for shortening or removing all the wiring to the regulator, but of course there is still the long cable to the ammeter and back. I presume it can cope with the alternator current - it\'s only a modest 34 amp one.

#6 Post by **Mike Stevens** » Wed Mar 28, 2007 6:49 pm

Hi Cliff,
It\'s so long since I had a Mk1 that I can\'t remember if it has an ammeter!  Maybe you\'ve added an aftermarket one?
The Lucas 17ACR is rated at 36A I think so in theory is too high for your ammeter.  However, the difference is small and I doubt that the alternator will be called upon to deliver that sort of current, not for long anyway.
I think fitting an alternator is a good idea.  The Lucas ones are simple, low cost and easy to repair.  Of all the ones I\'ve \'accumulated\' over the years, there\'s only been one that I couldn\'t repair as the field coil had gone open circuit.  (Back to field coils again!)  As we add more and more electonic \'gizmos\', we do need more power, especially at tickover.
Bear in mind that the car wiring will not be designed for the full output of an alternator.  Putting an additional short wire direct to the battery is a nice idea, but is complicated by having an ammeter in circuit.  Maybe this is why the change to \'battery condition meters\' came about!  Having wire maybe not quite thick enough should not really cause any problems.  It may get a bit warm, but the only knock-on effect that I can think of is that due to the extra volt-drop down the wire, the battery will take slightly longer to fully charge.
When you have fitted it all, do make sure that the ignition lamp bulb is OK and works.  Without the small current through this bulb, the alternator field gets no current and the alternator won\'t start.  I have a friend whose battery kept going flat and his garage couldn\'t find out what was wrong after replacing the alternator etc.  I idly asked him if the ignition warning lamp came on and it appeared that it had NOT been working for some time!  A salutary (and expensive) tale!
Cheers, Mike.

#7 Post by **Clifford Pope** » Wed Mar 28, 2007 7:45 pm

Yes, original ammeter; one of three gauges (with fuel and temperature) in a cluster of the same size and style as the speedometer.
I realised about the light. Of course, it\'s being tested quite a lot at the moment, with the dynamo not charging, but that would be one reason for it failing quite soon.

#8 Post by **Mike Stevens** » Thu Mar 29, 2007 6:04 pm

Hi Cliff,
Of course, it all comes back to me now.  After I fitted the alternator, I added this extra wire, but not too well as one day while driving the connector came off the back of the alternator and fell onto a brake pipe.  LOTS of current flowed and the poor ammeter had it\'s needle wrapped around the endstop!  There was also a wisp of smoke as the meter shunt got a bit hot.  The other effect was that the brake pipe developed a tiny hole where it had been blasted by the surge current and if you stood on the brake pedal it produced a fine jet of brake fluid that went a few feet in the air!
Good luck with the conversion.  I\'m sure it\'ll be for the best.
Cheers, Mike.

#9 Post by **Clifford Pope** » Fri Apr 06, 2007 2:11 am

I have just fitted the replacement alternator - a modest reconditioned
35 amp Lucas. The conversion was dead easy - literally a matter of
removing the control box and joining the two thick wires (D and B) and
the two thinner wires (F and WL). There aren\'t any others apart from E,
which is just discarded. 
I left the ammeter, and it works perfectly. Full charge is only just
over half scale, the same as with the dynamo, so presumably the ammeter
is a 50 amp one not a 25. 
The big difference of course is that it charges at much lower speeds. 
 
For the moment I am leaving the rather tortuous wiring from alternator
to old control box position and then back again, but perhaps in due
course it would repay shortening and re-routing. The current is not
significantly higher than previously.

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