328 Starting -- or not: Does this sound familiar?

Okay, this is odd:

Friday, I went to drive the 328 to the office: key on, lights indicator, buzzer, etc.
Turn to "start" and everything goes off.

So I put it on the 2A Schumacher over the weekend.

Tues AM, I turn the key, and it fires right up. Alt light goes out.
Drive it to the gas station, fill it up --- and it won't start again: same thing: everything goes dark with the key in "start".
Lights back on otherwise.

Put it on the 2A maintainer for the day, and check it in the afternoon:
Battery reads 12.5V, 11.5V with headlights: still goes black (and 1.5V) trying to start.
So I take the MT34 from the house inverter, and run it in parallel with the MT34R.
Same thing.
I do the disconnect and run off the external battery: same thing.

I haven't been able to check the alternator voltage, because I can't get it to start.

Everything looks okay until I try to start, then voltage goes to nothing.
And comes back to 12.5V with the car "on" but not "start".

I put the external MT34 back on the inverter and started the charge cycle.
It reads at 75% charge.

Last time I had something like this, it was a loose plus connection at the battery.
But it's tight now.

Does this ring any bells for anyone?

I don't know if this is a shorted starter or a bad cable somewhere underneath.

 

Voltage at the battery posts.

When I turn the headlights on, they're fine ... until I try to start -- then everything goes dark.
It's almost like the starter is a dead short.

Except ...
I forgot to mention one more data point:
At the gas station, when it wouldn't start, they put one of those "buddy packs" across the battery posts, and it started.

But at home, when I ran a second MT34 across the MT34R using jumper cables, no difference from the one battery alone.
(And, yes, I got the polarity right. )

I disconnected the ground strap and put the 10A charger on the battery until it read "charged", then hooked the battery back to the car.
12.75 V at the battery ... except in position III (start).

The car ran fine, a couple weeks ago.
The Interstate battery is from 2008, so it's got a few years on it.
But it doesn't seem to be a battery issue.

 

I thought of that.
I disconnected the ground strap to disconnect the battery in the car, then tried starting with an external MT-34 (from my house backup inverter).
(Jumper cables to the plus post in the car and to the chassis for neg)
No difference with either battery, or with both in parallel.

 

I had the meter clipped to a nail pressed into the clamp.
I removed the clamp on terminals, and there's corrosion ... again.

I also suspect that jumper cables light enough to fit in this car aren't hefty enough to help. I've never been able to get jumper cables to work.

So, after seeing that the plus lead is a $256 part that runs all the way to the back, I'm thinking about just putting a new connector on the cable.
But the factory arrangement is weird. There's like a "0" gauge wire to the fuse panel with a "marine" loop terminal that bolts to the factory battery clamp.
I was looking at converting both cables to marine, and using a post adapter, but I don't think a marine connector will handle that big honkin' (3/0?) cable to the starter.

I haven't replaced the starter in the 15 years I've owned this thing, so I suspect it takes a boatload of juice to spin it. (No pun intended: marine connector/boatload. )

So I'm looking at a big honkin' replacement connector (that I have on hand, after building my home inverter), but I'm trying to figure how to connect that second cable, without just twisting the wires together.

Anyone done a cable clamp replacement?

 

Update: there was a smidge of corrosion showing on the plus lead.

I've tried cleaning the connectors. I managed to open the plus "C" clamp, but I can't get the bolt loose.
The replacement cable is like $266 and runs to the back of the car.
Maybe when I have the car in for service.

The neg lead was easier to remove from the battery.
The "C" clamp is all the way closed, and I can't pry it open at all.
Oddly enough, that lead, that just runs to the disconnect at the front --
-- is also about $240. Huh?

The weird part is that the parts catalog shows the braid ground clamping to the car, and the battery lead as the loose connector.
On my car, the battery lead is bolted to the chassis, and the braid lead is the loose connector.

Anyone else have the battery disconnect that way?

I'd look at a compression fitting replacement clamp, but the NAPA stores here don't carry it, and it can't be mailed (lead hazmat).

I don't want to use a cheap wire pinch clamp connector on the 4/0 gauge ground wire, but I may not have a lot of options, short of putting the car in for service, and waiting for a slow delivery.

Alas, I missed the few days of good weather we had. DC doesn't get that many.

Since I plan to have the car serviced this winter, I just need to get through the rest of the season.
Then I might opt for new battery cables.