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The M37 transfer will normally run a bit warm at highway cruise speeds. Have you checked the oil level in both units?

Is your speedo reading correctly? If low you may be pushing her harder than you think.

There are several on line data computers that will get you to a shaft speed to see just how fast you are now turning the transfer case gears.

Charles Talbert has come up with a very good solution to the warm tranfer issue. Check out his web site www.mseriesrebuilds.com

You said the T/case is almost hot to the touch; if you can hold your hand on it after 10 miles of driving at 45 MPH; that is certainly within the average range. Outside temp in which you are operating also plays a significant role. It will run significantly cooler if using straight 40 weight Royal Purple synthetic oil. 90 weight is thick and will actually creat a degree of drag which = heat. That type of oil (gear lube) just won't flow as free as a synthetic will, thus it will not dissipate heat and protect against friction as well. We have also done a t/case cooling system that you may wish to check out.

Our website address is: www.mseriesrebuild.com

Joe you might try the Lucas brand synthetic 40 wt that should be found at most part stores and if they don't have it they will most likely be able to get it for you,and you might ask your NAPA store about the Royal purple they can order that also!
hb

The above posts indicate how far lubricant technology has advanced since our trucks were new, and I'm sure that the modern stuff is much better than the old. You guys have done an admirable job of explaining how to solve the heat issue, but nobody seems to have tried explaining WHY the t-case runs so much hotter than the trans or diffs. Anyone want to take a stab at it?

The most probable reason why they run hot is the large amount of gears running in contact all the time. The oil capacity is not that great and I usually add another 1/2 litre or more to mine and that seems to help. The higher road speeds we use them for today does not help either. So in general we have lots of gears loaded and turning together to get the power out to the rear axle. When you run a centered rear axle and take the drive off of the e-brake flange the case runs cooler as the extra gears are now turning but not under load. Loaded gears = friction and heat. If the case could hold more oil without blowing it out the vents that would help as well. The M37 cases are also tucked up under the cab higher and recieve the hotter air from the engine blowing on them as well.Just a thought...I could be right or wrong. Terry

Edit: Terry beat me to it on a few points. I didn't think about the hot air under the cab, that could be a big factor too.

I'll take a stab at it. I think the three largest contributing factors are...
1-the offset rear power output
2-the relatively small oil capacity
3-the heavy cast case

I'll expand on each point as best I can.

The offset rear power output forces the power to be transfered through the gears of the transfer case during normal 2wd operation. Since the gears are subject to large amounts of force on the contacting faces of the teeth there is a lot of friction as the mesh and thus a lot of heat generated where the gears meet. This also leads to increased load on the bearings and more heat from them as well. If you look at the transmission, top gear, where you spend most of the time cruising, is direct drive, the power is transfered through the input shaft and straight out the output shaft without moving through the lay gear at all.

The oil capacity of the transfer case is another major factor. It only holds about 2 1/2 quarts while the transmission holds 3 or more depending on which one you have and the axles are something close to 3 quarts each if I remember right (I don't have manuals on me at the moment). Since the oil is one of the principle means of heat transfer to the case of the transfer case having a smaller amount means that it will reach higher temperatures because it will have less time to cool along the sides of the case before it is carried up by the gears and heated up again. The low capacity issue is compounded by the larger amount of heat generated by the offset rear power output.

The case itself is a big factor too, being almost entirely of cast construction is does not transfer heat to the outside air nearly as well as the thinner axle housings or the cover plates on the bottom of the transmission. Think about how long it takes to heat up a cast iron skillet compared to a stainless steel pot. With a relatively small amount of hot oil transferring the heat to a case that does not transfer heat well itself you can see how the entire system would retain a lot of heat energy.

Collin

Okay. Thank you. I get it, now. The skillet analogy was a good one.