Transmission-Transfer Case Fluid Connection

Hi,  I can't find answer in prior topics and am hoping for some advice.

The Willy's Mechanic's manual includes:   "All transfer cases should be serviced separately (from the transmission) even thought the lubricant circulates to the transmission."

My initial impression after reading that was that the fluid "flows freely" between the two units.   Yesterday I noticed that the transmission's fill hole is around 1.5" higher than the transfer case's fill hole and that the transmissions drain hole is around 2" higher than the transfer case's drain hole.

Based on my initial impression and based on "fluids always seek their own level" (yes, even Willy's follow the laws of physics, when not off-roading), that would mean that if I filled to the transmission's fill hole that the level would be above the transfer case's fill hole and that the transfer case would be overfilled.

My guess is that my "flows freely" initial impression is not accurate and that the  "....lubricant circulates to the transmission" via some pumping action that also prevents the fluid to seek a common level in the two units.

Can anyone share some detail regarding all of this?  I've held off draining and refilling the units until I've gotten some feedback from here.

Thanks as always!  Greg

Hey Sean, Thanks for the very helpful link.   If I accurately understand what you were saying in that link, the lowest open passageway between the TRANS and the TC is the one that is level with the TRANS' fill plug hole.   Since the TRANS' fill plug hole seems to be around 1.5" higher than the TC's fill plug hole, is it possible that if both units were initially filled to their respective fill plug hole level, that when the jeep is driven at a certain angle (be it  up or down hill or a side slope) that the TRANS' fluid could flow through the lowest open passageway and into the TC and when the jeep is back on level ground that the TRANS fluid level would then be below its own fill plug hole and that the TC's fluid level would be above its own fill plug hole level?  If yes, is that ok for continued operation?  If yes, why didn't they put the fill plug holes on both units at the same level (assuming that I am correct in stating that they are about 1.5" different heights)?     Also, what is the correct drain/fill sequence when doing both units?   After draining both does it matter which one is filled first?  

Not an expert either, but wasn't there an issue with some gaskets having and not having the prepunched hole for this passage?

Thanks for the detailed explanation and fill sequence. That is exactly what I needed.   Hopefully the cross-flow hole is slightly lower than the Transmission's fill plug hole; otherwise, if the jeep was not on perfectly level ground when filling,  then the fluid may never reach the cross-flow hole level.  Thanks again. Greg

I would like to second what md just posted.  This thread also answered my questions the very day I got to this point in my rebuild.  There is also a great thread on the correct tranny fluid to use.  Search "transmission fluid" if you have any questions, but don't go to your local parts store and think that you can just ask for some non-hypoid gear oil and get anywhere.  You'll get answers like - hy what? or yep any of that gear oil will work!  Ask them if their stuff will hurt yellow metal and you get - yellow what?   

Sorry for the dleay Sean;

Drained and replaced Transmission (TR) and Transfer Case (TC) over the weekend and made several measurements along the way so as to add to the  TR – TC fluid connection passageway data collection.  Used the Valvoline GL1 Gear Oil mentioned in one of my prior posts, above.   Some of the readings don’t make sense to me, but I’ll leave that up to someone else.   (Just to be sure I’m not misidentifying the units, when I reference the TR,  it is towards the driver’s side and the TC is towards the passenger side and the front and rear driveshafts go into the TC).

Some General Info:  
Serial number 26448,    3 on the tree.    Bought the jeep 6 weeks ago and it had been rarely driven in at least the prior 9 years.   Fluid was dripping from the underside of the emergency brake drum (for lack of a better word) at the back of the TC even before I bought the jeep and had continued to drip during the entire 6 weeks I’ve owned it including right up to doing the TR and TC drain and fill.   Gears grind a little bit (especially when warmed up) sometimes when going from 3rd to second,  and sometimes at a standstill going into first or reverse.   Regarding the measurements below:  The drain volumes should be extremely accurate (used a measuring cup).  The fill volumes should be very accurate (used the see-through side of a 5 quart oil container that had quart lines on the container).   All drain and fill completed with all 4 wheels on garage floor with a slight downward slope from front to rear of vehicle.

----Opened the TR fill plug and was filled to the plug hole (used 90 degree piece of wire) but no overflow came out.
----Opened the TC fill plug and 4 oz came out (color half way between milk chocolate and dark chocolate and seemed old).   Then checked the TR fill hole and its level was unchanged.   I noticed that the ABOVE MENTIONED LEAK UNDER THE EMBERGENCY BRAKE DRUM HAD NOW STOPPED (now 2 days and 30 miles later the leak is now basically non-existent – A NICE SURPRISE!).
----Opened up the TC drain plug and an additional 62 oz came out (I am sure of this number)
----Then checked TR fill hole and I don’t think the level had gone down,  but if it had, then was around 3/16”.   
---Opened TR drain plug and 52 oz came out.
---Installed TR drain plug but did NOT install TC drain plug, and then starting filling the TR.  After around 20 oz some started draining from the TC drain hole but only a couple of ounces came out and it stopped.   TR took very close to 48 oz before it started overflowing the TR fill hole.
----Installed the TC drain plug and filled the TC through the TC fill hole and it took 51 ounces.
---Checked the TR fill hole and it seemed to still be filled to the hole.  I tried adding more but it immediately started running out its fill hole.
-----------Since the TR’s fill hole is around 1.5 inches higher than the TC’s  fill hole and since both units were filled to their respective fill hole when I was done, it seems that the connecting passageway between the units must be as high or higher than the TR’s fill hole.   If this is true, I’m not sure of the connecting passageway’s function.

Here is a bit of information that Willys did publish....

After reading Bob's information, I think we may be overthinking this thing.  Just put 3 pints in the transmission, replace the fill plug and then put 3-1/2 pints in the transfer case and you are done.  The Willys publication said that in normal operation each case would be within 15 percent of its normal level - that's plus or minus 15 percent as I read it.  In other words, 15 percent overfill of the transmission is perfectly acceptable.