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FYI my preference is for more detail rather than less...
So here it goes...

The Bendix/solenoid had slowly been dying in my '68 W300 with a 225 slant six. 2 years ago I purchased a rebuilt starter from Napa. Upon installation the starter did not function. I behaved as if I had a dead battery-it would just turn the motor over real slow. I ultimately ended up swapping the solenoid from the new starter onto the motor of the old starter. The truck started just fine. I chalked it up to a low quality rebuild and tossed the new Napa starter into my pile of cores. Fast forward to this weekend when my new solenoid/old starter began to behave the same... slow turning, lots of clicking...no truck starting.
I have several trucks, I took the battery from my W300 and placed it into a D100 and the D100 started fine. It's not the battery
I pulled a starter relay from a parts truck and it behaved the same. I doubt it's the relay.
The battery voltage is the same from the battery terminal to the terminal on the relay, to the terminal on the starter.
There is less than 5 ohms resistance from the negative terminal of the battery to the case of the starter.
I can bypass the key/ignition system and the truck will still not turn over-it behaves the same-lots of clicking and no starting.
I pulled the (new) solenoid, dissembled it, took some sand paper to the contacts and put it back together. Verifying that when it's engaged the circuit that directs 12 volts to the starter motor is complete.

I'm at a loss. What am I missing? It's not a GM starter that has to be shimmed, it pilots off of the nose. What else can I check? What other starter can I use?
I've spent the weekend prepping the truck for a trip and this happens at the last minute. No one has a new starter available until the end of the week, which is too late.

Adam wrote: ↑

Sun Feb 10, 2019 9:34 pm

There is less than 5 ohms resistance from the negative terminal of the battery to the case of the starter.

I can't tell you if this is the problem, but I can tell you that in the real world just because you have low resistance at the current levels of a meter doesn't mean you have low resistance for a high current path. You can't hurt yourself by assuring you have a good ground.

If I were to guess, it sounds like you may have a bad battery cable, one that has low resistance in a few of its fibers but they aren't all connected solidly so that it can't pass the current the starter needs. I'd play musical cables to see what gives.

All the cables are relatively new,~3 years or so. I did swap the lead that goes from battery to the relay

Good thing it's a /6, as the starter is easy to get to!

When you took the starter out, did you test it in a vise with jumper cables?

How about the line from the battery to the starter? Do you have a high current ground cable attached directly to the block? Do the connections have star washers?

Have you replaced or bypassed the #10 wire from the relay to the solenoid? To bypass that, jump the two lugs on the starter in situ.

If that doesn't work, I'm guessing it might be that there's something mechanical limiting the throw of the solenoid such that the disk doesn't make good contact across the internal contacts to the starter motor.

I have not pulled the starter from the truck. The star washers are a good suggestion, but If I didn't have a good connection, I think I'd see that in my voltage and resistance readings. Unknown to me at the time, there is a really good local rebuilder-I'm going to bring him both and have them redone. I'm also going to bypass the relay and get a battery cable that goes straight to the lug on the solenoid. I'll make a smaller cable to connect the battery to the relay for the trucks other power requirements.

How many amps is the starter pulling when it is turning slowly? Sounds like the windings have broken down internally and the starter motor itself is shot.
All the test and component replacement suggestions that have been made so far would be indicated as the problem if the starter is drawing low amps.
U need an amprobe meter to measure the current.
Another way to tell is by feeling how hot the starter gets when its cranking slowly.
You're in LA and u can't find a starter in stock anywhere? That's INSANE!

Adam wrote: ↑

Mon Feb 11, 2019 12:45 pm

I think I'd see that in my voltage and resistance readings.

Imagine a battery cable with only two strads of wire fully connected and the rest were loose and caked in oxides. That cable would show low resistance to a VOM. The same is true of any crappy connection that easily passes a small current.

Electrical contacts are complicated. They produce micro-welds for most current flow, with other electrons "tunneling" through very thin resistive materials.

Adam wrote: ↑

Mon Feb 11, 2019 12:45 pm

Unknown to me at the time, there is a really good local rebuilder-I'm going to bring him both and have them redone. I'm also going to bypass the relay and get a battery cable that goes straight to the lug on the solenoid. I'll make a smaller cable to connect the battery to the relay for the trucks other power requirements.

Hopefully he tests them first. Deep memory (we're talking nearly 40 years ago) has me recalling that the field windings came out of the motor housing as a rigid assembly with bolted connections in and out. I'm sure you looked at the armature, brushes, and such.

I vote starter itself. Seems like they fail often these days.