So, maybe I could have saved myself some time if I had just replaced the battery........ OR..... maybe there's something I still need to address.
I appreciate each and every comment... it has all been helpful, and I have learned quite a lot. Maybe the most important thing I have learned is to not overthink things.
I ordered a new Battery Isolator controller, because that's what the mechanic said was making the surging. Well, it was, but I think the reason it was doing that is because the battery was on its last leg.
The question remains, was the battery drug down to the abyss "because" of what and how it was treated while at the shop? After all, it was there for about 2 1/2 weeks, they have quite a few employees, and I have no clue who did what. It's a pretty good shop, I think, but it's not a Class 1 Pro shop, for sure. But I don't think they are hacks either.
A couple of posts referred to the alternator. I'm wondering if it might have gone down like this:
A. The alternator is on its last leg... it was sucking the life out of the battery.
B. When the battery started losing it's life, it put the Isolator Controller into some kind of "protective" mode, and it went sideways... temporarily.
C. I installed a new battery, and it fired up instantly... no surging from the controller, everything works as it should.
I didn't replace anything else. I bought a new breaker, but it wouldn't connect up to the terminals unless I changed the connectors to mate up with the new breaker... so I left it alone. I haven't replaced the Isolator Controller because it hasn't arrived yet... might have spent $90 that I didn't need to, but I did it on the advice of the mechanic.
Now I'm thinking that all it really needs is..... possibly..... a new alternator. But I called the mechanic and he said he didn't think so.
I'm just puzzled as to why the battery isolator controller went into def-con mode if it was just a battery on it's last leg?