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Discussion Starter #1
Suddenly got the battery voltage light while riding. Blew the 40A fuse, melted things pretty badly. Not sure what caused it yet. I haven't done anything with the battery in quite some time and have been riding regularly. Anyway, I wanted to make sure everything around the relay is ok before I start trying to chase what else could be the issue.

Can the diode be tested in place? I get about a 0.55V reading doing a forward diode test. Testing the diode in reverse, I expected an OL reading, but I got 2.77V.

Is it because it's still connected to the other wiring that could be giving an unexpected reading when testing the diode in reverse, or is that evidence that the diode is shot?
 

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It is probably because you are testing it in situ.

I would take the solenoid out of the bike and clean all of the connections and inspect the wiring. This is a known weak spot in all MVs. The wiring is probably brittle and failed to ground at some point. Also, the spade connector female side terminals probably could use some “snugging up” to reduce resistance at those connectors.

Using advanced search will reveal many “solenoid” threads.
 
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?

Check between the battery positive terminal and the battery terminal on the solenoid. Then I'd disconnect the battery positive cable and go looking for the short
Connect an Ohm meter to the positive wire and ground, turn the key on, hopefully you'll see continuity
Pull the fuses one at a time
Good Luck:grin2:
 

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Discussion Starter #4
Check between the battery positive terminal and the battery terminal on the solenoid.
I thought of a theory during my hour train commute -- the positive terminal on my battery is partly exposed. I have a Shorai LFX19A4-BS12 and probably should have gotten a new terminal cover when I installed it since the stock one doesn't cover it all the way. I keep a helmet lock and small cable lock in the tail. It's possible either of those could have shifted down and caused a short. I didn't consider that when I pulled them out of the tail to start looking at things

Right now I'm focused on verifying the integrity of the diode and solenoid to verify whether it was the cause or just a symptom. To do a reverse test on the diode I think I need to disconnect whatever is on the other side of those two wires so that there's no way for the voltage from my multimeter to be doing a "back door" trip through those cables instead of the diode. Too bad the electrical diagrams I have are all so low res it's nearly impossible to see the wiring color labels

One might rightfully ask, "Why not cut the diode out to test it and just solder it back in if it's good?" Because I'm lazy and there's no AC power in my garage. Easy to solder it back in, PITA to figure out how to get power to the garage to run the soldering iron. Also wouldn't be fun to pull the whole harness just to test/repair that diode
 

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Discussion Starter #5
It is probably because you are testing it in situ.

I would take the solenoid out of the bike and clean all of the connections and inspect the wiring. This is a known weak spot in all MVs. The wiring is probably brittle and failed to ground at some point. Also, the spade connector female side terminals probably could use some “snugging up” to reduce resistance at those connectors.

Using advanced search will reveal many “solenoid” threads.
The solenoid threads are the only reason I had any idea there's a diode in the first place!

Everything looks surprisingly clean. I'm having trouble getting one of the spade connectors out of the terminal because of the way the terminal deformed from the heat. I'm trying to source a replacement white terminal so I can cut the old one off and check the condition of that 4th spade connector
 

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A short between your battery positive terminal and ground will just make big sparks and possibly set your bike on fire...no fuses involved !!!

Your loose and overheated as a result solenoid connector is the probable cause of your fuse blowing.

Replacement connectors are here:

Motorcycle 6.3mm - 250 Connector and Terminals

Here is a recent thread related to that connector:

https://www.mvagusta.net/forum/153-general-maintenance-tech-issues/242665-damaged-starter-relay-connector-parts-overheated-melted.html
 

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I noticed that water likes to find its way to the 40amp fuse housing. That and the loose connections will mean trouble.
 

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Discussion Starter #8
A short between your battery positive terminal and ground will just make big sparks and possibly set your bike on fire...no fuses involved !!!

Your loose and overheated as a result solenoid connector is the probable cause of your fuse blowing.

Replacement connectors are here:

Motorcycle 6.3mm - 250 Connector and Terminals

Here is a recent thread related to that connector:

https://www.mvagusta.net/forum/153-general-maintenance-tech-issues/242665-damaged-starter-relay-connector-parts-overheated-melted.html
Thanks for the tips. I'm getting a replacement connector from Pete (pittmeister), he's taking the loaner blue SPU and conveniently had some spare connectors I needed. Once I get it I'll button everything up and make sure the spades are tightened. I'm going to do a round of checking for a short like Noel suggested, see if that turns anything up before I start it up and see what happens.

Bike had started fine, but charging light came on almost immediately and it was reading at 11-12V across the battery terminals regardless of engine speed. Is it possible a failing charging system could have blown the 40A fuse?
 

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Only if it was shorted out....
 
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Discussion Starter #10 (Edited)
back to the diode

Getting this thread back on track: yes, the diode can be tested without removing it. Unplug the SPU, ECU, and dash. Might not need all three, but that's where the orange/black and blue/yellow wires go, and since I already have my tank off, I disconnected them all.

To pursue additional charging system/start relay questions, I'll post on a more appropriate existing thread (edit: posted HERE)
 
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