Help with my JD755 crawler loader

[xXThunderGodXx]

we have a jd755 crawler loader that wont crank. we were running it one day and the next day it wouldn't crank. the starter engaged and bumped the motor a tiny bit (just enough to make the fan jump) so we yanked the starter and had it rebuilt. got it back in and same story. So we dropped it for a while and asked around and a fellow we know said it may be the hydrostatic system may be causing the motor to be stuck. the only reason im buying this is one-I'm trying to be optimistic and two- if the motor was seized i wouldn't think it would even budge let alone make the fan belts move. and it doesnt help that im not too familiar with hydrostatic drive systems. i spent most of yesterday crawling all over and under the machine poking and prodding and found a rod sticking out from between the splitter box and the bellhousing and i am assuming this is disengaging the splitter from the flywheel but couldn't get it to budge. we acquired this machine for a fairly low price because the fellow who owned it before bought it for a single job and had no other use for it. I am not even sure what year it is and have the serial number 2592521. Aside from the tracks getting close to the point of just scrapping them the only problems we could find were just a multitude of small leaks in the hydro systems (both drive and boom) the motor is pretty tight with only minor leaks and some minor leaking on two of the injector lines at the pump. we were gonna fix all these problems once we figured out what the problem is with trying to get it started. it was used in our quarry for clearing castoff and rubble for maybe 20 hours making sure all fluid levels were maintained throughout. we wanted to see if the thing had the guts for the job before we put the work into buttoning up the system.
what im really getting at is, is there someway to disengage the drive system aside from unbolting the two prop shafts from the splitter box so we can see if it is a motor or drive problem im not afraid of rebuilding the motor if it is the problem but i find the possibility of having to work on the hydro-drive daunting especially if it requires work to either of the drive units.ANY info would be helpful, Thanks.

 

[kebo]

You might try posting this in the construction equipment forum too. Good luck with it.

 

[jenkinsph]

First I would measure the battery voltage at the starter terminal to ground while cranking. No mention of this in your post so thought that should be first. Check for problems with connections as this is likely and the cheapest thing you can fix. I used to have an 850 dozer and would not jump to conclusions that the hydrostatic unit is the problem.

You may be locked up with fuel or water on a cylinder or cylinders too. Might need to pull the injectors to see if it will turn over.

Did you call the seller to see if he has had this problem before?

 

[xXThunderGodXx]

I Forgot to mention the loader sat for about a year before i started messing with it yesterday. and contacting the seller is a no go. if it was a locked cylinder wouldnt it have bled off by now? and amperage to the starter is the same as prior to the problem. what i dont get is why i cant get the manual disconnect to work. almost all the linkage for it is a.w.o.l. and another thing is how does one distinguish the different models (a,b,c,d) the stickers just say JD755. no letter designation but those are just stickers. or would the drive system and most other components be the same?

 

[jenkinsph]

I had an 850 when they first came out about thirty years ago, so I am only familiar with the oldest in this series.

Have to ask you again though, did you check the voltage at the starter terminal to ground while cranking? Checking across the battery terminals doesn't accomplish the same thing. I am trying to understand what led you to have the starter removed and rebuilt. It could be as simple as a bad ground connection so need to make sure the battery and cables are not the issue.

It doesn't make sense to me that you could operate the machine for 20 hours and have no transmission problems then have the transmission lockup the motor when it stopped.

I have seen diesels appear to lock up with fuel or water on top of the pistons and cause major damage.

 

[jenkinsph]

I should add that if you have a 755 that should be the early model which is a little over 100 hp. Later models would be the 755B, 755C and have some changes such as electronic controlled hydro on the 755C and newer. The newer models have more power too, about 175 hp and weight about 8000 lbs more. So yes there are some major differences between the older and newer units.

 

[xXThunderGodXx]

only reason i said the same is because there was no change to the batteries from on day to the next and they were new batteries and new lines with a clean ground and never measured it. will try removing the injectors and see how that flies. and measure at the starter. I'm still wondering why i cant operate the clutch at the flywheel.

 

[xXThunderGodXx]

I should add that if you have a 755 that should be the early model which is a little over 100 hp. Later models would be the 755B, 755C and have some changes such as electronic controlled hydro on the 755C and newer. The newer models have more power too, about 175 hp and weight about 8000 lbs more. So yes there are some major differences between the older and newer units.

so maybe a 755 or 755A?

 

[jenkinsph]

Don't know that I have ever seen a 755A designation, Have seen the 755, 755B and 755C.


I keep pressing the voltage issue as I would think your unit is 24v and consequently has several connections that could cause the failure. With problems like yours I never assume anything, rather start at the beginning and eliminate the possibilities. I should also mention that a very short burst and then nothing is common to having a bad connection too.

I am going back in time:laughing: does your crawler have an inching pedal?

 

[xXThunderGodXx]

i have a sinking feeling it is indeed 24v. its got a honking huge wire running to the starter and huge ground strap from frame to motor and the reason i asked if it may be a 755 or 755 a is that i am looking at parts diagrams on the JD site and there are the 2 options. 755 and 755A there are quite a few similarities. from all parts descriptions i am leaning on their schems. for the 755. anyway it made mention of a 24 volt starter but im just wondering if it was just an option or the standard. would it hurt the system to re-arrange the cables for 24 volt and just give it a couple of bumps? the batteries have been out since the shut down so i cant remember if they were installed correctly or not but it did start before and the batteries weren't tampered with between running and not turning over. did you have a manual disconnect on yours? and if so how hard of a pull was it?


Update: Just checked some sources out, it is indeed 24. gonna pull the injectors and hook up the batteries properly and give it a shot. be back in a few hours with an update