LM2 not charging

[Dukedesmo]

1979, LM2 - on the last couple of rides, I have noticed that the charging voltage was initially low, basically just battery voltage so not charging. On both occasions after a few minutes it 'corrected' itself and reverted to charging correctly so I wondered if it was just a loose connection to the voltmeter?


Yesterday, I took the bike out and the voltage remained low, it didn't correct itself so after about 10 miles, I came home so as not to run out of battery power and need recovery.


As far as I can tell, assuming the voltmeter and (Gammatronix) charge monitoring light are correct, then there is basically no charge coming from the alternator/regular/rectifier with only around 12v showing constantly (fluctuates between 11.8 and 12.2) and it actually drops as revs increase - I presume due to more firing pulses taking more power which is then not being replenished?


So I need to find the cause, I presume the likely culprits are alternator and/or voltage regulator/rectifier.


To assist in this, does anyone know what sort of voltage I should see from the alternator when running?


I assume it will be a fairly high AC voltage that is then regulated/rectified to the correct DC for charging.


Any advice/tips would be greatly appreciated.

[PeteS]

I think this has a charging system similar to that eras BMWs. If so check the connectors on the diode board to make sure they are clean and tight. Same on the alternator. Usually if the rotor opens up it never charges, just stay low until the battery is dead.

Pete

[blackcat]

Depending on how old the rotor is, I'd start with checking that part for continuity.

[guzzisteve]

You should really look in a service manual, it gives you all the info you are asking. Much better explained than I can tell you. It could be any of the components & need to test each one separately. 

https://guzzitek.org/gb/ma_us_uk/1000/1000SP_G5_Atelier(GB).pdf

Page 126

[PeteS]

These are pretty simple systems. It has a 3 phase alternator the feeds a board that rectifies the AC and regulates the DC. Remove the alternator cover and you can easily check the rotor and stator connections. Guzzi, unlike BMW put their diode board away from the hot engine so problems there are not as common. Unlikely to have melted solder points like BMWs diode boards before they started using high temp solder.

Pete

[wirespokes]

First thing I'd check is the alternator brushes. They wear and towards the end-of-life make contact intermittently. Then they're short enough the lead hangs up in the holder and that's that. Usually it's the inner one that wears most.

I've forgotten what new length is - something like 17mm. And 12, I believe is gone. But you should know looking at them.

BMW airheads have the exact same Bosch system and this is the site with the data, and more, you'll ever need.

https://bmwmotorcycletech.info/trbleshootALT.htm

[Dukedesmo]

Thanks for the advice, I've not looked at it yet but as far as I know it's all original from 1979, other than the voltmeter and charging light which I fitted when I replaced the LM2 dash with LM1 style instruments and I'm confident that the voltage readings are correct.

Bike is low mileage (less than 20k) so, hopefully not worn out, I'll get on it in the next few days and update with findings. 

[Tkelly]

Probably a rectifier,they do fail.

[wirespokes]

I resurrected a bike that left me stranded close to home. Dead battery. The alternator slip rings had gotten coated with something that acted like an insulator. Once cleaned off, bright and shiny again, all was good.

[John A]

Probably a rectifier,they do fail.

On these it’s usually the rotor in my experience. Either the brushes or the rotor itself.

[Dukedesmo]

To add, I've just noticed that although the 'Gen' light appeared to out when running as it should be. I've just started it up in the garage out of sunlight and can see that it is lit but only very dimly when the engine is running. In fact it only lights dimly even when the engine is stopped despite the battery being charged and it doesn't change from that whilst running.

[Dukedesmo]

Bosch alternator;




Brushes look good, plenty of length in them and check through with a meter but there is no reading (Ohms) between the slip rings even after cleaning. Plus, there's no voltage output at all from the alternator, the voltage readings I was seeing whilst riding are just battery voltage.

[guzzisteve]

They fail way more than the alt windings, same w/diode board.
To check rotor, slide a matchbook cardboard between both brushes & rotor slip rings, then check Ohm's between slip rings.
For the diode board look at back to see if any burnt spots.

[Dukedesmo]

No reading (open circuit) between slip rings, suggests the windings are done?


Rewind or replace?

[PeteS]

I doubt you want to rewind them. Not sure who does it now but you can get rewound ones that are potted and less likely to break. I used to carry a spare plus a spare stator when that went. You will need modified bolt or use a blank stud seen in this video.

https://www.youtube.com/watch?v=dlNHGxR4UM0

Pete

[blackcat]

I have gone through two rotors on my 81 CX with the last time being a rather weird fail as the claws on the rotor moved to the point that they were touching. The other fail was a cracked brush holder that took me forever to solve. With that fail the battery was charging but not completely and only after sitting in front of the bike watching the brushes did I finally spot the crack in the plastic. At that time the brush holders were not sold separately so I had to buy the whole alternator.

[Dukedesmo]

Found a local company that specialise in rewinding alternators and have done many of these so, I've sent it off to them.


Thanks all for the help.     

[wirespokes]

The main problem with the rotors is centrifugal forces. Changes in inertia, and rather sudden changes possible from a performance engine wreak havoc on the windings. A lot of times the wires break near where they attach. That's one of the plus points of the permanent magnet Ducati alternator.

[blackcat]

The main problem with the rotors is centrifugal forces. Changes in inertia, and rather sudden changes possible from a performance engine wreak havoc on the windings. A lot of times the wires break near where they attach. That's one of the plus points of the permanent magnet Ducati alternator.

At that time my bike was mostly used as a short ride commuter and the rotation of the claws may have happened after I installed a lightened flywheel.

[Dukedesmo]

Indeed, that a winding rotates + brush and slipring setup seems to me to be asking for trouble.


On my Ducatis the only moving part is the magnet so much less likely to fail - that said the rotor on my LM2 is 46 years old so I can't really complain...

[PeteS]

Motorad Elektrik used to put epoxy or a similar coating on the winding near the slip rings to help keep them in place. Many years ago that was what we did with slot car motors too.
Checking their website now I am not sure he still does rewinds any more.

Pete

[wirespokes]

Motorad Elektrik used to put epoxy or a similar coating on the winding near the slip rings to help keep them in place. Many years ago that was what we did with slot car motors too.
Checking their website now I am not sure he still does rewinds any more.

Pete

I've ridden airheads many thousands of miles and don't recall a rotor ever going bad. But my spare did help others out.

I think Motorad Elektrik just sources them from China now. When he first started out he guaranteed them. There were racers who regularly returned them. Rick kept working on ways to solve the issue and eventually they were very robust and rarely failed.

[Moparnut72]

I bought one of the very first R75/5s when they first came out. My rotor failed at a fairly low mileage. The dealer said it was quite common and the factory started using epoxy to solve the problem. Mine was still going strong at 100K miles. It also started having weak charging at at higher mileage. The brushes were worn down and hanging up by the wires. I think that was caused too much accessory lighting drain. Overall it was a good bike but I did expect some issues being one of the first in the US. I was putting so much mileage on it in the first few months of getting it that BMW considered me as kind of a beta tester.
kk