Short circuit!

So after a nice ride, I stopped for a coffee and when I got back to the bike, thumbing the starter button elicited a big fat nothing. Turned out the 30A fuse that also is for the headlight and tailight was blown, but replacing it just blew the new fuse. So there is a short somewhere. (I was able to bump start the bike with some help pushing, but don’t ask how I got home in the dark with no lights. )

Anyway I pulled off the starter and disconnected the taillight and it’s still shorted. Any ideas where it might be?

Follow the circuit diagram for the circuit in question, working back from the fuse. Here you need to be methodical, and do things one at a time. Look for wires rubbing against the frame too.

Thanks for the advice. I was wondering if there is some known common cause of this but it’s probably just something random. I checked the circuit diagram and the thing that could blow this fuse with the ignition switch off is the alternator or connection to it. I didn’t find anything in the wiring so far, but as it turns out I was just about to do an engine-out major service to fix an oil leak, and also take the frame off to get it powder-coated. So while I have it stripped I should be able to locate the source of the problem. Hopefully I won’t need to replace the alternator, but at least with the engine out it should be easy to do.

No end of excitement with this bike!

And here I thought a short circuit had 2 left hand corners and 1 right.

Old Electricians Trick
To find a short circuit get yourself an old headlight bulb, solder a couple of wires on it.
Stuff the wires in the fuse holder, the bulb will probably provide enough current to power up the circuit without lighting although it will power the headlight to about half brightness.
Now you have the circuit in question not powered by a fuse but powered by a lamp.
Start poking around the wiring, if you create the short again the lamp will flash allowing you to quickly zero in on the cause.
Caution, don't use this method for troubleshooting the ECU wiring, electronics doesn't like low Voltage.

Great idea! I’ll try it!

OK that technique helped me discover that it’s definitely the alternator that’s shorted or internally. The positive terminal on the alternator is directly connected to ground.

I guess I need a new alternator?

An alternator should last the life of the bike

I remember Pete Roper posting something about the connector on the alternator being "T" shaped
Only the top -- of the t is supposed to be connected.
Hopefully he will pick up on this.

You could open it up to see what's wrong.

I just took off the tank to take a closer look. The alternator must have been properly connected, everything was working perfectly for many miles and charging properly, until I stopped for a coffee and went to start the bike again.

When I disconnected the main positive battery connection from the screw at the side of the alternator, the short was gone. There is also another smaller connector plugged into the back with a T connector that has a single wire going to it.

I wonder if it’s possible to take the back of the alternator off without removing the entire thing?

I wonder what could have caused this unusual problem with it? There were no warning signs at all of anything wrong with the electrical system.

Kiwi_Roy wrote:

An alternator should last the life of the bike

I remember Pete Roper posting something about the connector on the alternator being "T" shaped
Only the top -- of the t is supposed to be connected.
Hopefully he will pick up on this.

You could open it up to see what's wrong.

That's what i thought but mine packed up at 2,500 miles from new!!!!!

We tried everything else before we finally replaced the alternator with a new one and its been fine for the last 3 years and 20,000 miles

GuZeee wrote:

I just took off the tank to take a closer look. The alternator must have been properly connected, everything was working perfectly for many miles and charging properly, until I stopped for a coffee and went to start the bike again.

When I disconnected the main positive battery connection from the screw at the side of the alternator, the short was gone. There is also another smaller connector plugged into the back with a T connector that has a single wire going to it.

I wonder if it’s possible to take the back of the alternator off without removing the entire thing?

I wonder what could have caused this unusual problem with it? There were no warning signs at all of anything wrong with the electrical system.

Although this is a different problem to mine, that stopped charging, one day it would work and the next day it would just stop dead.

So, Kiwi_Roy, I'd like to share in your optimism in this key component of the GRiSO's electrical system:

Kiwi_Roy wrote:

An alternator should last the life of the bike

...but then I noticed your tagline:

Kiwi_Roy wrote:

Moto Guzzi, Making Electricians out of riders since 1929

I ordered a new alternator, and figured I'll change the belt while I'm in there as well. Just $145 for the alternator from AF1 racing, let's hope this one fares better than the original part.

I believe the unit is alternator, rectifier and regulator all rolled into one just the same as most automobiles, when's the last time you changed one out on your car?

Please pull the old one apart and see if you can determine what might have happened.
At least they are inexpensive.

I hate to think we should have to carry a spare

Yes, when I say alternator I am referring to the unit that generates power, which includes the alternator and electronics. Moto Guzzi calls it a generator, which makes me think of old-style DC generators with brushes, but I guess it's reasonable to call a rectified alternator a generator since DC is what comes out in the end!

I will take the old one apart and see what went wrong. I expect it is something in the electronics, since it was working fine until it shorted to ground. If it's fixable, I'll replace it with the new one and keep the old one as a spare.

This thing here:

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As to the question "when's the last time you changed one out on your car?" I don't think that has any implications in regards to this bike. I've never had to change the valve tappets in my car or fixed a leaking rear main flange that was assembled incorrectly from the factory either!

It is kind of crammed in there I must say
Subject to more heat than a car alternator too

I will be interested to see how difficult it is to change the belt and at what milage you think that should be done

Could you please post photos of what you find
Thanks
Roy

My mileage is nowhere near time to change the belt, I just figured that while I am ordering the alternator for a few more bucks I can get a belt, and it should be easy to replace at the same time. Purely a question of opportunity, not need.

Think the belt is a 60,000 km change interval.

Manual says 30K miles on the GRiSO so it's like what? 45K kilometers?
Either way, I'd do it at that mark and the belt is cheap from Gates...

This is the guts of the alternator. The rectifier and regulator seems to be completely integrated. I’m just going to replace the whole unit.

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There must be a pair of brushes in there perhaps below the metal plate (regulator I think)
Is there any sign of overheating? Do the bearings feel ok?
There seems to be more diodes than you would see in a regular auto unit.
Is there a manufacturer / model No
A nice compact self contained unit, I wonder if it would fit on a loop?

Thanks for the picture
Roy

Brushes:

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Me thinks it has an unusual bearing arrangement, most alternators have a bearing outboard of the slip-rings.
It's hard to see how you can get at the bearings in this one, a throwaway design eh!

Mechanically it seems fine. Maybe a diode decided to call it quits for some reason, who knows. Still waiting for the new one to arrive, hopefully it fits!

Shiny new alternator arrived today!

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