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Nuts & Bolts: Overheating Transfer Case

Posted in How To: Transmission Drivetrain on October 23, 2017
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Some time ago I took my Dodge Ram into the Central Kalahari in Botswana, an area which has miles of thick, loose sand and temperatures reaching 45 degrees Centigrade.

After driving for quite some time in low range with the central diff locked, I stopped to clear away the dry grass which had collected underneath the truck, in order to prevent it from catching fire, a common hazard in these parts. I happened to touch the transfer case with the back of my hand and it was so hot that my skin blistered. This got me thinking as to why it should get so hot, and the conclusion I came to was that because of the difference in front-to-rear tire pressures, the rolling circumferences were also different, causing the transfer case to work really hard and heat up in the process.

In my case the rear carries a lot more weight than the front, which in turn results in a fair difference in the rolling circumference between front and rear wheels. With the central diff lock in the open position it makes no difference, but with it locked the load in the transfer case increases dramatically because the aggressive BFG all-terrain tires cannot slip as they would on a gravel road.

If this is correct, surely it would be better to adjust the tire pressures so that the radius of all four wheels was as close as one can physically measure to be the same, rather than to reduce the manufacturer’s recommended pressures by an equal amount all round, or to set all four tires to the same reduced pressure. One can obviously not do this in loose sand, but if one were to do this with the fully loaded vehicle on a hard, flat surface it would not be difficult.

I am interested to learn if any of your readers have experienced this overheating problem, and I would appreciate having the benefit of your technical guys’ thoughts.

Jack
Via nuts@4wheeloffroad.com

Sounds like a cool adventure! First off, your Dodge shouldn’t have a transfer case with a center differential. Unless it’s some weird export-only thing, your truck should simply have 4-Hi and 4-Lo, which splits engine torque to the front and rear axles equally. Because it’s locked rather than having a differential, there really isn’t an opportunity for things to create additional heat. Any variation in tire size will result in slippage on loose surfaces, which is why four-wheel drive should only be used in soft surfaces. Running a locked transfer case on hard-pack surfaces and pavement is hard on equipment, and the stresses of doing so for prolonged periods of time can cause catastrophic failure of many component, including the transfer case itself. But of course, there’s no way that was the case in the loose, sandy conditions you describe.

You are correct that the weight of the rear causes the rear tires to squat a little compared to the front, resulting in a small difference in circumference. However, the difference would be so small that we doubt it would have any real impact on the transfer case, and certainly no more than running a new set of tires on the front with a worn set of the same brand and size tires on the rear.

We’d be willing to bet the heat of the transfer case was the result of running in low-range for an extended period in that environment. Remember that unlike engines, the transfer case has only airflow to dissipate its heat. If the undercarriage was choked with grass as you mentioned, what airflow the transfer case might have received could have been blocked.

Also, the operating temperature of transmissions and transfer cases is roughly equal to engines and easily hot enough to burn skin. For sure, 230 degrees F (110 C) might be the high side of normal, yet it’s completely normal in those conditions. If you’re concerned, drain the fluid and have a look. If it’s burned or heavily discolored, that could indicate a problem. The pickup for the case’s internal oil pump could be plugged, or there could be some other problem. If the fluid looks good, we’d change it and peel out. With the conditions you describe, we would recommend following severe-duty maintenance intervals, which dictate changing the fluid more often anyway.

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