D6D Engine oil getting in to transmission

Check/replace the seal on the transmission pump. When the seal/shaft leaks it will draw engine oil into the transmission system.

Hi vernier,
when fitted, the transmission pump seal lip--garter spring side--faces outwards.
It faces this way as the area behind the seal is connected to the pump suction port so as any pressure oil leaking past the pump bearings can drain away.

A seal facing with the lip innermost will draw oil from the engine.

The seal case also needs to fitted squarely in the bore and the case back face not distorted/dented by punch blows, this causes uneven pressure on the seal lip and if dented it will leak in a short number of hours of operation.
Check the old seal lip edge for even wear--a damaged case will show signs of flat spots at high contact pressure points caused by the distorted case on the seal lip edge.

We used to remove the garter spring and fit suitable sized piece of O Ring into the garter spring area and use a flat plate type tool to install the seal so as to not distort the case in any way or damage the sealing medium, refit the garter spring after fitting the seal.
Check the seal bore as many have been damaged by careless use of a chisel, or the like, to dig out the old seal. Smooth up any high spots and use a gasket sealer that sets solid when fitting the new seal to seal the damaged seal bore.
Hope this helps,
Cheers,
Eddie B.

I reckon it will be the rear main crankshaft seal, if my memory serves me correctly, the gear train which drives the transmission is between the flywheel and back seal and is lubed by transmission oil.

D6D 4X05963 was originally equipped with the early design crankshaft rear seal. Caterpillar introduced a new much improved seal design in I think the late 80’s/early 90’s. I hope Old Magnet will reply with the details. The new design does require a special seal installation tool.
If your recent engine overhaul was “in-frame” the rear main seal was probably not replaced.
To replace the rear crankshaft (main) seal, the torque converter must be removed.
I replaced these seals on two D6Ds (4X7045 and 4X7366) back then using only hand tools and a small chain hoist plus the special seal installation tool which I was able to rent at that time. I think a Service Manual for your D6D would be a big help, they are sometimes available on ebay or on the For Sale/Wanted page here.
When you determine that the rear seal is the source of your leak and you are making this repair, it is a great time to re-seal/overhaul the torque converter. Don’t forget to have a look at the torque converter scavenge pump and the U-joints too.
Craig

Thanks for the replies to everyone.......I do have a service or shop manual on this dozer. I will first check the transmission pump seal, if I find an obvious problem there and make the repair am I good to go or should I go after the rear main seal too? Just want to do it right the first time. I hate doing something twice (don't ask me how I know....LOL)
Thanks again,
Vern

Take a close look at your parts book Vernier, the transmission pump is driven from flywheel side of the crankshaft seal.

That's true. The pump auxiliary drive is lubed by the transmission system so drawing engine oil through the transmission pump seal does not apply here. Only leaves the crankshaft seal as the culprit.