Moto Guzzi Le Mans Mk 1 1978 850cc

[Bixxer Bob]

So, today's little game is the charging circuit.  Rotor is down on resistance but an acquaintance  suggested re-flashing the rotor windings (this thing doesn't have permanent magnets) before we part with cash.  I read how this works on mains electricity gennys but can't make the leap to what I have on the bench.  It seems that if the rotor loses all residual magnetism (this one has) they need a jolt to wake them up.

Anyone come across this?

[akendall1966]

Sorry BB being a bit thick the rotor is an electro magnet?

if  so how is  it excited slip rings or induction?

[Bixxer Bob]

Slip rings.  I understand the physics ie how they work, I just haven't come across re-flashing before.  Here's what it says on a Guzzi forum:

The Right Way to Polarize Guzzi Generators

In this Tip, Gary Cheek, longtime contributor to the Internet loopframe list, tells the right way to polarize the generators on the V700, Ambassador, and Eldorado:

You need to know the difference in the method of polarizing betweenan INTERNAL-ground generator and an EXTERNAL-ground generator. To determine which system you are dealing with look at the brush holder that is connected to ground. An internally grounded generator will also have ONE of the field leads connected to it; an externally grounded gene will have no other wire connected to the grounded terminal.  With an INTERNAL ground (such as the generators used on Guzzis) you polarize by DISCONNECTING the FIELD lead and MOMENTARILY touching it to the BAT terminal on the regulator. With an EXTERNAL ground you just momentarily connect a jumper wire from the BAT to the GEN or ARM terminals of the regulator.

I have seen many situations where every thing else looks okay, but the generator was not working, and that was remedied by re-polarizing. Folks take the generator apart to replace brushes and after reassembling fail to re-polarize. Do it by the book, and avoid simple problems!

[cosmo]

It is true, GENERATORS need polarising.

However, you have an ALTERNATOR. You risk real and permanent damage if you try a procedure NOT recommended for your motorcycle (you did state Ambassador, Eldorado, V700). Do NOT try this under ANY circumstances!!

Good news: the parts on your bike are the same as those on a BMW. So you have another avenue to try for parts. Years ago, I found BMW parts were much cheaper than MG. I also found out that the early (I had a 1975 850T) bikes did not have ventilation, so added the venting ring from a later bike. It goes between the cover and engine case, spaces the cover out about 10mm. Yours should have this ring, but check.

There are also companies that repair/rebuild these parts. I've used rebuilt to good effect, but t'was so long ago I have no contact info.

Alternators, BTW, are "plug and play", you do not need do any polarising or other such mumbo-jumbo.

Good luck, those are great bikes!! I'm just a wee tall for them (knees want to be where rocker covers are...)

Cosmo

[akendall1966]

If it is an alternator and has brushes / slip rings surely the battery energises the rotor coils to bootstrap the process, can't see why you need any residual magnetism.

If the rotor and stator coils are good, brushes are good and you can verify power at the rotor connections to energise the rotor coils you should get AC out of the stator. If you got reasonable AC but no DC your rectifier regulator is stuffed.

[Bixxer Bob]

Thanks guys, all input gratefully received. I emailed Newtronics yesterday (they make the retro-fitted reg / rec). I'm still awaiting a response but it looks like the Ducati dealer was off the mark. This thing should just work, but I'll go through it on Monday a step at a time and let you know how I get on.

[Sin_Tiger]

That's the conclusion I came to but the Web quote confused me and without a circuit diagram I decided to keep my mouth shut for fear of confusing the issue more.

[NeilD]

its obviously knackered... I'll take the bike off your hands for £50     

new (pattern) rotors are around £50 from Gutsibits or Motobins

[deepsouthtiger]

Pics, I need pics of that fine Goose to really be of any help........o.k. I won't be much help......I just wanted to see the bike.

[NeilD]

oh and by the way, i think with the newtronics unit the charging light wont work..

[Bixxer Bob]

Cheers Neil. I can't even remember a charge light (seeing it again tomorrow).  The whole thing was re-wired by an "expert" without reference to a diagram.  Can't be too harsh though, until it stopped charging everything was fine and it does run. Need to check slip rings are getting 12v and then see if there's ac coming out of the three connectors.  Resistance across all three segments of the stator are equal and about right to the manual so it is pointing to the low resistance in the rotor windings.  All the talk of 'flashing' seems to have been a red herring.

[NeilD]

agree that 'flashing' is something i remember from my days of messing around with old cars that ran generator systems... however, it is also mentioned in 'the bible' known as Guzziology as something that has cured otherwise mysterious charging problems on the bosch alternator system... the author doesnt know how or why but connecting a jumper cable to the battery positive and momentarily touching it on the D+ terminal works... ??  I sometimes struggle with christmas tree lights so dont ask me 

a summary of some of his tests/checks/tips..

rotor windings - measure at slip rings resistance around 3.5 ohms

rectifier grounding - standard earth point is behind rubber bumper on the frame so doesnt always tighten down properly

stator windings - approx 0.5 ohm resistance between pairs of the large spade connectors

stator grounding - some models have a black wire connected to the D- terminal, others rely on the mounting screws to complete ground. if the latter then you can add on and ground it to the frame.

if you end up buying a rotor, the one for a BMW is slightly cheaper but needs to be for a /6 model as other BMWs look the same but are slightly smaller circumference.  To remove the rotor you'll need a hardened piece of rod 5mm diameter and 2" long (i cut down an allen key).. remove bolt insert 'tool' insert bolt and the rotor should pop off as you tighten it...

one of the main problems with guzzi wiring is that firstly theres to much of it    (the power for the starter seems to go around the whole bike before getting to the starter but there is a mod for that!), the connectors can be of poor quality and some of the earth return paths are a bit iffy (on mine i've added extra earths back to points on the frame thence back to the battery rather than hoping it will travel on its own through rust, paint and muck)..

oh by the way, if you want 'interesting' wiring you should see my Ducati.. previous owner did away with the original fuse box which is stuck behind the front wheel (good idea) but all the replacement wiring is the same colour - black! (not so clever!)

[Bixxer Bob]

Can you post me a link please Neil,  I found the site but not the thread. Cheers!

[Bixxer Bob]

Here's a pic of the menagerie

[Chris Canning]

Whats the red and black bike behind the 916/748 Duc