Engine won't spin

My early 1950's D6 9U cable dozer ran beautifully until I tried to start it two days ago. The pony motor cranked the diesel engine as usual and the engine started to run, but needed more warming up to keep running. So I engaged the pony motor again. I heard a clunk and the the pony motor immediately stalled. Now the diesel engine will not spin. Any idea of the cause and how to fix?

Sir I always enjoy helping and dealing with these machines all my life I may be able to. I do the simplest things first, one of which is make sure it didn’t get put in gear and this time I would disengage the hand clutch. If this don’t help you are going to need to isolate the lockup. And it’s most likely the engine which could turn into a major job. Disassembly might be required and also professional assistance from qualified neighbor. Good luck Low country cat

Well it is not the hand clutch and I double checked that it is not in gear. Could it be the pony motor pinion? I cannot feel the usual positive click when I try to engage it. Maybe it is stuck engaged to the flywheel. However the pony motor clutch lever does move and when I try to engage it, the pony motor tries to spin the engine but bogs and starts to stall.

Read up on pony clutch and pinon adjustments. See what you can see in there. It maybe you need the pony clutch pinon assemble to come off as that is what last you touched before it jammed up. But see what you can first as Msjoy was suggesting. It could be something in the master clutch compartment.

Great suggestions! I will definitely do that and let you all know what I find out. (It will be at least next week before I can work on it, due to traveling.) Fortunately, I have two parts dozers (also D6 9U) in case anything needs to be replaced.

is there any weight on the pinion engagement lever? if there is no weight its engaged you will need to manually release it.

there is a inspection cover you remove to get access and using a screwdriver release the pinion latch. i dont know the exact process. i dont have a d6.

so the pony still runs fine on its own?

can you bar the main engine over?

is the decompressor engaged? is it working properly?

do you have a borescope, i would look inside the bell housing has something happened with the clutch or flywheel

what the coolant like is it discoloured does the water pump turn grab the fan and see if it does move on its own

Thank you for these suggestions! I will follow your advice when I get a a chance this week and report back. I do have the feeling that it has something to do with the pinion gear and/or starting motor clutch. The pony motor does run fine on its own, but the diesel engine does not seem to turn over at all--even when I try to turn it manually. I think the water pump is OK. And I do have a borescope and will try to look inside before taking too much apart.