Stumped - Starting Issue with 1940 D30 GRD233

I'm stumped. I bought this truck almost a year ago. At the time, the truck started right up with no problems. At some point in its life it had been converted to a 12v system but still had the original 6v starter. It wasn't an issue. Over a few months of driving him on the weekends I noticed that he turned over slower and slower. After of few weeks of no activity I tried to start him up and got one slow turn and then nothing.

At that point I had the battery tested - came back "all good". I looked over the wiring - everything seemed ok (and it had worked before so nothing changed). I thought it was the starter. I took the starter off, tested it on the ground, took it apart and gave it a good cleaning, put it back together and tested it on the ground again. The starter would zip right up. Although, if I were really picky I'd say that it took a little longer than I'd like for it to get up to operating speed.

I put the starter back in the truck and got absolutely no motion of the starter at all. I decided to see if I could jump start it, because by that time the battery had drained a little bit. Jump starting it with another car got it to work!

I took the starter off again and took it to a pro. His conclusion was that the starter was good. I put the starter back in today and the starter doesn't turn at all. I tried playing with the wiring but ultimately concluded that the wiring worked before and nothing had changed over the last month or two. I tried jumping it again, like before, and nothing happens at all. I don't even know what to look for at this point. Any pointers?

My guess is you have excessive resistance in the starter circuit. The correct way to diagnois is measure the voltage at the battery terminals (the lead posts) and compare to that at the starter itself while loaded (cranking).
This is why I am down on most 12 volt change outs. rather then fix the truck, someone puts a bandaid on the problem and sooner or later it comes back to bit ya.
A real common place for problems is in the ground side of the circuit. IHC ran the ground to the frame then relied on a braided strap to ground the block. This save a little on copper cable but made for problems down the road. Not only does it add 2 more connections but the braided strap degrades and adds resistance.
As I said before, the correct way is to check each connection for resistance (voltage drop) and repair. I prefer to run the ground cable right to the starter, it takes a little more copper, but you are placing the ground right at the highest load (by 10 time anything else), and letting the the lighter loads ground thru the strap and frame.
If I can ever figure out how to make and place a recording on this site, I will let you hear what a good 6 volt system sounds like when cranking. many will not be able to tell it from a 12 volt system.

I'll check to see what the voltage drop is during cranking first thing in the morning. Although, I somewhat by-passed this at one point. I ran jumper cables from the positive terminal on the battery straight to the positive post on the starter and ran the other cable between the starter body straight to the negative terminal on the battery. Same result today.

I don't know if I can wait till morning. I think I'll go check the voltage across the battery and then again at the starter while cranking right now! I'll be back to report soon. Thanks for your help cornbinder89.

Jrod:
One other point I'd like to add is about the battery. Saying it's good depends on how it was tested. I had a similar problem with real slow turning over. Battery was charged OK (by specific gravity check) and the headlights were nice and bright (that might be about 10 amps draw), but the starter groaned. Took me awhile to figure out there was a poor internal connection in the battery that couldn't handle the high current draw. New battery was tried and problem solved.

Voltage across the battery: 11.96v
Voltage taken at the positive terminal on the starter was practically the same: 11.92v
Voltage taken at the positive terminal while attempting to crank: about 0.4-0.5v
Voltage taken at the same terminal while using a jumper cable from the lower bolt between the starter/engine block and the negative post on the battery: 0.6-0.7v

@d2crazy: I was thinking the same thing today so I tried using the battery from my car but it yielded the same result.

Have I overlooked anything else (very likely)? I'm still not convinced that the starter is working properly. Maybe it ran well on a test stand with no load, but maybe it doesn't have the torque to turn over the big green diamond.

And to give you all a little taste of of we're working on, here's the truck (and no that's not me, that's my sister...)! Overall, he's in pretty good shape for being 70+ years old.

the battery and load voltage readings are all low, cranking voltage is almost non existent. SO either you are not taking readings directly from the lead posts on the battery or the batteries is not good, or the volt meter is not any good.
The reason to take readings right at the lead posts (and not the battery cable ends) is to eliminate as many connections as possible that may be faulty. If the battery truly drops to less then a volt under load then it is undoubtly bad, but your no load readings are also low, so either the meter is off (check on another vehicle that you are not having problems with) or the battery is discharged or internally failed.
I'm a firm believer in a carbon pile load test on a battery for testing. I know some newer tester use an internal resistance test in place of a carbon load. What you want to know is how the battery preforms under a load and only a carbon pile (or other load) will truly test, other tests are looking at other criteria and making assumptions based on that.
If the battery is good, and the cranking load is enough to drop it instantly to less then a volt, then everything should be real hot, as you are pulling well over 1000 amps, but I doubt it. Much more likely is a bad connection that will pass low current but not a large draw, and you are not measuring the battery voltage but voltage after a connection. this is assuming a truly good a proved battery.
How was the battery tested? A hydrometer will show a bad cell, but not other faults, a load test will fail a bad battery without pointing to the exact cause, but if you know the battery is fully charged before the test, if it can't handle a load, that is all you need to know, replace it.

I attached a quick picture to show how I was getting the voltages last night. Is that not how you would do it?

I tried the same thing with a different battery. The battery read 12.8v, then the same test while trying to turn the engine over resulted in a voltage across the starter (per the picture) of about 1v. And nothing seems to be getting hot. I'm still not convinced that the starter is "good".

I tried bypassing the foot switch on top of the starter, too. But no improvement.

I'm not sure which method the auto parts store used to test the battery. I'll be going there today for a few other things so I'll find out.

Yes, that is how I would measure at the starter, at the battery right on the lead posts. It points to a bad connection between the starter and battery and not the starter itself. Again, as I stated in the 1st reply, I would suspect the grounding. I would run a new ground straight from the battery to one of the mounting bolts on the starter. 90% of the time, that is what is at fault.
It was a problem in the 30's and was still a problem on my '89.
Also make sure the lead posts are clean and bright, same for the inside of the battery terminal. If oxidized, it will have high resistance and no current will flow.

As a test, I've run my jumper cable from the lower bolt on the starter straight to the negative post on the battery and the positive cable straight from the positive post on the battery to the top of the starter. I've disconnected all other connections from the battery. This should mimic the test that was performed when the starter was off of the truck, right? Still nothing. I've tried it with the battery that's on the truck and a battery that I know works (from my other vehicle) Doesn't that take all connections out of the equation (assuming my jumper cables are good)? All of the connections are relatively clean.