Troubleshooting Off-Highway Transmissions

Today I’d like to look at a component people often overlook; the “transfer case.”

Most off-highway equipment operates in a dirt road environment. Because of this, transmission power must be forwarded to both the rear and front axles. This is accomplished by the transfer case. This Tip will focus on the transfer case, what it does, how it does it, typical fluids, performance issues and support that we can provide to improve both equipment performance and customer productivity.

So grab a drink, sit back and let’s learn together.

Transfer Case Basics

The transfer case is a device that splits power between the front and rear axles. It is attached at the rear of the transmission and is most often of two or three shaft design. (See figure 1).

 Transfer cases contain drive gears or chains, bearings, seals/gaskets, and drive shaft yokes. They may provide 1:1 gear ratio from the transmission or 2:1 if further transmission speed reduction is needed to improve vehicle traction.   Most transfer cases have a clutch that allows decoupling from the front axle when conditions warrant.


Transfer cases are important as they distribute power to where it is needed. Though small compared to other components, a problem with a transfer case impacts the entire drive train and may disable the vehicle. Typical problems include:

  • Lubricant Leakage: Often found at the housing gaskets/seals and bearing retainers. Leakage needs to be verified and repaired or it will lead to low oil levels and shifting problems.
  • Improper Engagement:
    • If a transfer case fails to engage, no power will be transmitted. This may be caused by stripped gearing/splines or broken chains, but before tearing into the transfer case, make sure the transmission is functioning as intended.
    • If the transfer case fails only in one direction, power to only one axle, the problem typically lies within the gearing of the unit.
  • Noise: Transfer case noise varies depending on the speed being made and road conditions. First and foremost, make sure the noise is coming from the transfer case and not another part of the drive train.
    • Whining: Most often worn bearings or low oil level.  
    • Grinding: Worn gears or drive chain.
  • Gear Disengagement: If this occurs there is typically a problem with the particular gear impacted. It could also be a worn clutch pack for the affected gear.
  • Hard Shifting: Bent shift lever or linkage. Readjust and lubricate. Shift forks inside the unit could be bent and in need of replacement.
  • Shudder: Occurs during acceleration. Problems with the gearing and worn clutches may be the primary cause.

Cost to repair or replace a heavy duty off-highway transfer case may be in excess of $2,000 for a rebuild kit (without gearing) plus labor. If you include 8 hours of downtime and lost productivity, the actual expense my exceed $120,000 in labor ($1,200 = 2 Men x $75 / Hr Labor Rate x 8 Hours) and lost productivity ($120,000 = 800 tons product impacted at $150/Ton). This number could be even higher if the repair parts are not on hand and cause further delays. These expenses can all be avoided through understanding how a transfer case operates and knowing the early warning signs of a problem before it becomes a major operational and productivity issue.

Mobil Transmission Fluids

Transmission fluids are used in most transfer case applications and are separate from the oils in the transmission. They provide engagement power and lubricate bearings, gears, chains, clutch packs and seals. In addition, transfer case lubricants (transmission lubricants) act as a heat transfer medium, and act as a lubricant for a control circuit if equipped. They must have an operating range from the lowest expected ambient temperature to over 150°C for extended periods. As such, transmission fluids for the transfer case must contain:

  • Superior base stocks
    • With a naturally high VI to provide a wide operating range and fluidity at temperature.
    • Natural oxidation stability.
  • Additives to provide oxidation stability for extended fluid life and to ensure system cleanliness for proper valve operation.
  • Material compatibility to preclude seal leakage and promote gasket and seal longevity.
  • Shear stability to maintain oil viscosity and prevent gear wear.
  • Anti-wear retention to protect gearing and clutch materials.
  • Fluid friction capabilities to efficiently transfer fluid forces, preventing slippage of oil immersed clutches.

The recommendation for transmission oils is governed by the Original Equipment Manufacturers (OEM) for both on and off-highway equipment. Mobil products that are used in these applications include:

  • Mobil Delvac Synthetic Transmission Series
  • Mobiltrans HD 30 and 50
  • Mobiltrans AST
  • Mobilfluid 424 and LT

For additional products or product specifications, please refer to the current Mobil Product Data Booklet or consult your local Mobil representative. Also check out our additional content on our new Mobil Lubricants North America LinkedIn page. Until next time!




  1. Figures 1 and 2 - NAVEDTRA 14050A, Chapter 8 “Troubleshooting Transmissions, Transfer Cases and Differentials, Section 2.0, figures 8-19 and 8-21.
  2. General Reference - NAVEDTRA 14050A, Chapter 8 “Troubleshooting Transmissions, Transfer Cases and Differentials, Section 2.0, pages 8-28 through 8-45.