Battery switch on negative cable- replace with solid cable to ground?

I would be frankly amazed if that were the source of your electrical problem, but it's easy enough to test: jumper cable from negative battery post to any solid metal in front, see if the problem goes away.

If that is the problem, best to replace it with a better quick disconnect instead of a solid wire - you don't want the unfused wires to starter and alternator to be hot when you're working with the alternator belt so the disconnect is very handy.

 

On Ferraris my friends and I mostly DIY guys routinely harden as much Ferrari electricals as possible including the kind of electrical cut off you describe. If you feel a need to be able to cut power without unbolting your negative terminal I would use high quality electrical cutoffs we use in boats for marine environment and racecars. Note that many racecar cutoffs have extra terminals and circuitry you don't need because we need to completely cit power so that the engine dies when power cut and not run on alternator power. That's a requirement you do not have as a simple cutoff. So you can buy a racecar 6 terminal cutoff or simple 2 terminal cutoff or use slick relay based cutoffs with a hidden interior switch that can be used as one extra layer of theft deterrence. A thief can get in your car turn key and get nothing like dead battery unless he knows where you unmarked switch is hiding. That unmarked switch can even be keyed for added security.

 

I can understnd this could die.
i bought two master cutoff made in China, install one, save extra spare. in one nut rotation the internals failed disconnect so must used spare.

 

Voltage Drop testing can quickly identify/pinpoint any issue with grounds/cables, etc.

FWIW, if you are not familiar with it, voltage drop testing isn't about checking battery voltage at the battery and then checking to see what the voltage is further down the line. It checks the amount of drop along ONE side of the current path (+ or -) at a time. Unlike testing system voltage, a V drop test can detect faulty cables/connections that show good "voltage" (and would show no/minimal/acceptable resistance on a multimeter) but can't pass sufficient current to do the job. Starter circuits are famous for this behavior. Watch some on-line videos or read descriptions re voltage drop testing.

 

Since Ferrari started to use those switches and having been in a couple of Ferraris largest dealers I am aware of one case where one needed replacement.

 

Strong Install can destroy it . I did

The posts are jamb nuts. Take care. I rotated too hard and felt perhaps internals move, then cutoff failed. replaced with 2nd Chinese unit and was very careful not to over tight the jamb nuts, no internal mmove and now works 100%

 

You can break anything by mishandling it.. Does not make it a bad part.

 

So true! I have a friend in New York who's opinion is to tighten any fitting to as tight as you can get it and if he happens to have the 1/2" drive socket on the 1/8" bolt, the result is not hard to predict! But he's a superb carrier of heavy items!

 

I started my path with my car and having a similar experience. I had multiple issues that caused the drop.

Dirty and corroded grounds.
loose battery cables (but they looked tight...)
The brushes wore down in my alternator - it needed servicing. I did the regulator too...
At first there was a unconnected wire hiding in my "umbilical chord" between the alternator and the starter
The plastic base for the terminals on the starter deteriorated and crumbled away, causing a loose connection.

There are tons of places in the middle it could be....

 

I would never recommend solid wire for any electrical connection in a vehicle.