Case 350 crawler starter problem

I'm trying to fire up a '76 Case 350 which hasn't been run in a few years (and the last time it wasn't by me, so I can't speak for its history or health).

When I hit the starter, the engine turns over for anywhere between half a second and 2 seconds, then the starter disengages from the flywheel and whirs up to high speed: if I hold the start button down it'll keep whirring away till released. Stop and try again and the behavior repeats. Starter sounds reasonably healthy with no odd thunks or grinds.

I've pulled the starter solenoid and it seems OK - there's resistance across the terminals where there should be and the core and high-current switch move freely.

I initially thought it was battery voltage drop, but driving the solenoid directly with a second battery doesn't help. And in any case, if the solenoid wasn't holding, the starter motor would stop when it dropped out.

It also doesn't make sense to be chipped teeth on the flywheel - I'd expect that to lead to nasty noises if it'd stopped in the wrong place last time, and clunking but riding through if the starter encountered it while already rotating. Ditto any other major mechanical fault with the rotating bits - I'd have expected to hear it.

I guess the next step is pull the starter and see if anything else becomes obvious, but the bottom bolt looks like a pig to get to and I don't want to waste hours swearing at it if I'm missing something simple. I know my way round cars but I'm a newbie with heavy equipment, so I may well be.

Any suggestions, anyone?

That sounds like the Bendix clutch is slipping if all your voltage checks are right,if all all the battery connections are clean and tight check the voltage of the battery,it should be 12 volts +,connect the meter at the main cable of the starter and a good clean ground and check the voltage,it should be the same as the battery,watch the meter and hit start,the reading should remain near the same,if not either the battery is failing or power is been lost between the starter and battery,if it's the same ok,connect the meter to the out terminal of the solenoid and hit start,note the reading it should remain near enough the same,if not the solenoid is suspect,if it remains near the same the solenoid is ok,make sure the machine is out of gear,with a jumper wire cross the solenoid battery cable with the light exciting wire to eliminate any fault that the neutral start could have,if it is still dropping out the Bendix is the main suspect,the Bendix as it is commonly called is the over run clutch fitted in an assembly with the drive gear,it works like the freewheel of a bicycle.
AJ

I'm pretty sure the solenoid is good. There's significant voltage drop at the battery posts when the starter engages, but I think that's just because I'm not using a beefy enough battery: the manual says it should have 2x70AH in parallel, but I'm using a single 60AH I've borrowed out of my SUV. (Having said that, despite what the manual says there's only battery cabling for one battery, so who knows). At any rate, I guess if the battery voltage is drooping that could cause the starter to rotate too slowly which might exacerbate a slipping clutch on the Bendix.

I'd have thought a slipping clutch wouldn't engage and then give up though, it'd just never hook up properly.

Another possibility (and this one's well out in "dumb question" territory) - when it turns over the radiator fan rotation is right for blowing not sucking. Is that correct in these? In a car it'd always be suck but I can see where blow would be better to clean dust, and you're not exactly fighting ram air pressure in a crawler. If it should be suck, that implies the engine is turning over backwards somehow, which won't help the Bendix.

Sounds like maybe I try a bigger battery, and if that don't work it's out with the starter.

Nearly all equipment have blow out fans,I now think by what you are saying that it is a battery issue,there is not enough punch in it,if the chassis is been used for ground make sure the cables are making good ground,a lot of stuff had two 12V batteries in parallel up to the eighties then a single 12V 128/130AH was used,what I would to get it started is put 24 volt to it for the first start,everything switched off connect the two 12V batteries in series and hit start,it should jump to life,if it does stop it and put it back to 12V and see you fare from there.
AJ