help JD6420 4w/d IvT, won't drive in 2 wheel drive unless in differential lock

[uxboy]

Hi I am new to this I hope you can help.
We have a 2004 John Deere 6420 4 wheel drive farm tractor equipped with IVT .
Our problem is the tractor won't move in 2 wheel drive without engaging the differential lock.
This unit was working fine no heavy work. We were just moving hay bales. Drove forward dropped bale went to back away no drive.
If you can shed any light on this we would appreciate it.
Thanks,
Uxboy

 

[Tx Jim]

Sounds As if pinion gears in differential are not operating as designed or rear axle is broken/stripped. Has tractor ever been operated extremely low on hyd oil?

 

[jd110]

Driveshaft #21 in diagram could be sheared off, Bolt #18 could be broken or come loose which would allow axle to slide out, or if you have rack and pinion axle as pictured, key in hub could be sheared off, allowing axle to spin in hub. This is in addition to differential problem described by TxJim.

 

[RickB]

Driveshaft #21 in diagram could be sheared off.

This possibility gets my vote.

 

[Zebrafive]

I am thinking/voting bolt 18 or shaft 21. I would jack the rear off the ground and with diff lock engaged see which rear wheel the brake stops and take that side apart.

 

[RickB]

I am thinking/voting bolt 18 or shaft 21. I would jack the rear off the ground and with diff lock engaged see which rear wheel the brake stops and take that side apart.

If bolt #18 was broken the outer axle would move outward, the axle seal would leak and the wheel would wobble. No apparent leak, no wheel movement (if true) all but ensures an inner half shaft failure. Jacking and checking braking as you suggest is the best way to identify which side failed, assuming the failure is between the differential carrier and brake disc.