IF NOT THE STARTER.......

Then what??

Starter went bad ( or so I thought ) in my Magnum "Ruby". Started it several times last Sunday, moving it around to wax on it as I worked on it inside. I had moved it out of the garage part way and when I went to put it back in, nothing but click click click. Hmmm. So I know I have several good ones laying around, so I take the old one off, replace it. Now I did the screwdriver and ground test and it worked. OK, install it and click,click, click. Hmmm.

I had a new starter relay laying so I changed it. Click click click.

Battery is good. I had wife hold the key on and I wiggled all the wires on the battery, click click click. Each time the starter feels like it's engaging, just not turning.

What should I check now.

Not sure why you'd go to all the trouble of swapping out starters twice without checking the battery first.

Do a load test on the battery. easily done by just turning the headlights on and trying to start it. If they go out or dim a lot you have abad battery that may appear good until a heavy load (starter) is put on it. I encountered this the first time in my life about 6 weeks back. After chasing my tail for ~ 5 hours I found the battery was at fault. 10 minutes earlier it started the vehicle just fine. I have since read this is becoming more commn as battery MFR's cut costs on what's inside the battery.

I did, batter is / was good. All components work shows 12.9 volts

That is exactly what tripped me up. the V showed good unilt a load was placed on it and it would then drop like a rock. within 15 or so seconds it was reading 12.5 + V again.
Voltage and current are 2 different things
Do the load test with the headlights OR see what your meter reads when you try to start it. You could have a bad cable or two as well.

I checked the cables too. I'll check the battery on load.

I can't be critical, just changed the ignition switch in one of the Belvedere's, put a "known good battery" in and nothing. Convinced it was the switch tried another switch. No go. Then I started voltage checks and guess what, the "known good battery" wasn't good. It happens to all of us.

It doesn't show 12.9v during a load test. If it does you did it wrong.

When you checked the wiring did that include making sure you have good contact where the negative cable attaches to the body/engine block?

You bring up a good clue. One of my cars that I take out often during the fair weather months to cruises has had issues when I go to start it, it just clicks. Sometimes the starter would turn for a split second then I’d get the clicking.
I’d grab my battery terminal cleaner and give it a brief clean up, reconnect the battery cable, and then it would turn over and start fine.
I keep a battery ground cable disconnected on my vintage cars when they are parked in the garage. There’s no corrosion on the battery studs or clamps so I have never figured out why this happens. And since a quick scuff with the terminal/clamp cleaner resolved the problem, I just consider it a minor inconvenience.
This year just for the heck of it I put some grease on the battery stud after the first time the clicking happened, and haven’t been having the problem since. I don’t recall now what I used, white grease maybe.
So even if there is no corrosion visible, try cleaning the battery studs and clamp and see if that helps.

Try a single jumper cable from your cars battery negative terminal to a clean ground on the engine.Just a quick way to rule out or reveal a bad ground.

As I mentioned, this is something that I've read is becoming a more common thing supposedly due to cutting costs by the battery mfr's.
It sure bit me in the a-- even with an engineering and electrical background. I kept looking at the Voltage which I eventually discovered would drop like a rock under a heavy load(starter) and recover very quickly once the load was removed .
I've read of this on several different forums as well.

one thing that tripped me up for a long time on my charger.
the 10ga [?] wire going from the relay to the starter was bad internally.
this happened many, many [and many more] years ago, before i knew much about trouble shooting electrical problems.
sometimes it would start, sometimes just click, click, click.
if you used a screwdriver to jump it at starter, it would turn over.
somehow, i got the brainstorm of running an "extra" wire along side of the original...............problem solved !
just something else to check. that bit me, so i'm sure i can't be the only one.

Well, it was the battery cable!!! Not the end on the battery, but somewhere along the line. Failed a continuity test. Replaced it with an NOS one a fixed it.

Excellent, glad you got it fixed! Have to wonder how far down into the cable corrosion can work itself over time. It would be interesting to peel back an old one and see.

Thanks for the Update and

Me too!! So the 44 y/o starter lives on! Terminal looked clean and wiggled the wire while wife kept the key turned. Nuttin'

I had a wire from the pickup coil inside the distributor break internally (due to the vacuum advance movement I assume) Looked fine on the outside it was a fun one to find as it was very intermittent and the car would always start right back up. One if the things I learned to do over the years is flexing the wire in short sections along it's length. If you find a softer spot when flexing it, you likely have found the issue.
Another method is a pull test which in most cases will expose the broken area as the insulation is softer and will start stretching

glad the problem has been conquered !
a person would be surprised at just how far the "green stuff" will work it's way down the battery cable[s], with just a slight cut or split in the insulation.
this is one of the main reasons i detest those "clamp on" [temporary] ends. this allows moisture [condensation] a place to enter, causing the copper strands to oxidize.
the other reason i despise those ends, they look hoakie !

Thats not the first time I have had a bad battery cable that still looked ok but was bad. Corrosion is a funny thing.

Yup
Here too

Fixed it for ya LOL

way more fun [at least for me] than doing those "word games" in the books, magazines, or newspapers.