My truck has a
Chevrolet 15,000# vacuum actuated 2-speed rear axle. During my initial test drive, I discovered
a problem that I believed was related to the 2-speed rear axle. Rear axle
operation was normal when in "Low"; however, when shifted to "High", it felt and sounded like the truck
was popping out of
gear. Shift it back to "Low" and everything worked fine. It should be
noted that the original differential was replaced by the previous owner (or
owner before him) for some undisclosed reason. The original differential was,
however, included with the purchase of the truck.
I decided to take the original
in an attempt to figure out why it needed to be replaced. This merely
involved taking the differential apart, cleaning the pieces and looking for
obvious signs of failure such as broken components, excessive wear and tear, thermal
The removal of all the components was fairly straightforward, though I sure am
glad I bought that 24 piece 3/4" socket set...everything is big on a C60.
Each piece was cleaned in my parts tank as it came off. I found the major
components (yoke, drive pinion gear, ring gear, shifter sleeve and yoke,
bearings and races) to actually be in great shape given their age and the fact
that it's a 2 ton truck.
Next I unbolted the ring gear from the Differential and Planet Support Case and
Ring Gear and split open the case to reveal the sun
and planetary gears.
The first sign of trouble came in the form of thermal damage to the Differential
and Planet Support Case and Ring Gear. Note the
thermally induced discoloration on the inside walls of the case. Additional thermal damage was found on the top and
base area of each of the four
differential pinion gear posts, again evidenced by the bluish thermally induced
was then disassembled piece by piece and each component was cleaned and
inspected. The thumbnails below detail the piece by piece disassembly of the
scoring was noted on the inside of the 2 halves of the Differential and
Planet Support Covers. Further inspection revealed scoring and galling of the
shafts and differential pinion thrust washers.
This truck has a 22,000 lb. GVWR. Inspection of the frame and undercarriage
prior to purchase revealed no obvious signs of abuse (cracks in the frame of
leaf spring hangers, sagging springs due to fatigue, frame twist, etc.).
Therefore, the thermal stresses must have been caused either by using an
improperly rated lubricant or by an insufficient quantity of lubricant rather
than severe overloading. In
addition to the obvious property of lubricity, gear oil also dissipates heat
from the metal components. The damage observed here suggests that there was an
inadequate quantity of gear oil in the differential case, thus leading to the
thermal damage on the inside of the differential.
Aside from the damage to the inside of the differential, the rest
of the 2 speed components are actually in great shape. It's unfortunate that improper maintenance
most likely led to the failure of this
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