The Rear Up
Front
With four-wheel-drive and all-wheel-drive
vehicles being produced in such huge numbers, these trucks and
cars have become an important source of repair business for our
industry. Transmissions and transfer cases are now a huge part
of the repair and reman market. It is also obvious that these
designs use both front and rear differentials to transfer power
to both sets of drive axles. This has effectively doubled the
amount of differential work that will find its way to the
repair shop.
Although the principles are basic to all
differential work, the front differentials and drive axles have
some unique issues that we will look at here. For the purpose
of this article we are reviewing the front drive axles used in
GM K-series trucks. The common K trucks are graded by
gross-vehicle-weight capability. The K10-series 1/2-ton trucks
use an 8.25-inch ring gear with ratios of 3.42, 3.73 and
4.10-1. The 3/4-ton K20 trucks use the 8.25-inch ring gear with
ratios of 3.42, 3.73, 4.10 and 4.26-1, and the 1-ton K30 series
uses the 9.25-inch ring gear available in ratios of 3.73, 4.10,
4.26 and 5.13-1.
When you’re repairing one of these
vehicles, it is important to make sure that the front and rear
axle ratios match exactly and to make sure when you are
ordering parts for a front drive axle to specify that to your
parts vendor, because the front ring and pinion gears are cut
for reverse rotation.
The K-series trucks’ front-drive-axle
design uses an aluminum differential case that splits in half
for service and contains an electrically operated front-axle
disconnect. Proper operation has the front axle disconnected
from the drive axle in 2WD, and when the transfer case is
shifted to a four-wheel-drive mode, power flows through the 4WD
switch on the transfer case to energize the front-axle
actuator, which now locks the front axle to the drive axle in
the differential to provide four-wheel-drive traction. This is
a simple system, but there are many areas that if not
understood can create issues for the customer and the shop.
1. Whenever removing any axles or
driveshafts, match-mark the flanges so that they will be
reassembled at the original locations. The vehicle manufacturer
balances all these shafts and flanges during assembly, and
noise, vibration and harshness (NVH) will result from
reassembly in an out-of-balance position.
2. Shifting into 4WD mode will engage the
transfer case, and there will be a short delay before the front
axle is engaged. This delay is
normal, and cold temperatures can increase
the delay. When the system is in 4WD with the front axle
locked, the 4X4 status light should turn on.
3. If the vehicle is shut off in 4WD, the
front axle can disengage into 2WD. When the engine is
restarted, the unit will re-engage 4WD with the same short
delay.
4. These units can shift into 4W High
“on the fly” (while moving). These units also will
be hard to shift into 4WD while stationary because of lack of
motion in the meshing parts resulting from lack of spline or
gear match. Putting the vehicle into motion should make the
shift to 4W High easy.
5. If you shift into 4 High while parked,
the 4WD status light usually will not turn on until the vehicle
begins to move and gear mesh is obtained.
6. During the shift from 4WD to 2WD the 4X4
status light may remain on because of spline lock of the
front-axle components. Releasing the gas pedal and then going
back to throttle usually relaxes the components enough to
release. At slow or stopped speeds it may be necessary to stop
and reverse direction for a few feet to allow the components to
disengage.
7. Because the axles turn at different
rates during turns, it is NEVER advisable to drive these
vehicles in 4WD mode on dry pavement.
Another issue on the K-series front drive
axles that generates a great deal of confusion and
technical-hotline calls is the electric axle shift-actuator
motor. The 1988-90 models used a screw-type drive motor to
engage the front axle, which was equipped with a two-wire
connector and had a silver body tube. In 1991 the design was
changed to reduce engagement delay, and a black-bodied,
three-wire “thermal gas-charged actuator” was
introduced.
Unfortunately, the package included no
installation instructions. The following will give you the
proper way to hook up a three-wire system when replacing a
two-wire actuator. There are two brown wires and one black or
blue wire. The short brown wire goes to the positive side of
the transfer-case switch. The long brown wire goes to ignition
on, and the black or blue wire goes to ground.
Caution: If you wish to test the three-wire
black-bodied actuator by supplying 12 volts to it, do so only
when it is installed properly in the front-axle case. If you
put 12 volts to this unit while it is removed from the
front-axle housing, it will be ruined and no good things will
happen to anyone who is standing in front of it, and the guy
who is holding it usually will have to change his shorts.
These 4WD systems involve many parts. We
have the rear differential and axles, the rear driveshaft, the
transfer case, the front-axle driveshaft, and the front axle
and differential. NVH (noise vibration and harshness) problems
are always the most difficult to diagnose and a real cause of
customer complaints. A bad day generally starts with the
customer saying, “It wasn’t like that until you
worked on it.”
The chart below is a pretty comprehensive
factory guide to solving these problems, but you can prevent
many of them by rigidly following your own set of established
steps. Verify the customer’s complaint, with the customer
along if possible. Drive the vehicle thoroughly in all modes
and ranges if possible. Match-mark all flanges, yokes etc.
before disassembly. Remember that there are no unimportant
parts on the vehicle and that the manufacturer will not go to
the trouble of developing and attaching components to any
system just to make more work for you. Do not leave off any
parts that were removed. All weights, baffles, shields, clips,
grommets, bushings etc. must be in their proper location and in
good condition for the system to function correctly and quietly
and without vibrations being transferred into the cab.
The front differential’s ring and
pinion are the same as those of the rear axle except that they
rotate in the opposite direction. The same rules apply to
repairs such as replacing a ring and pinion or bearings. Pinion
depth and pinion- and ring-gear backlash need to be set
according to the factory specs as you would in any differential
repair. A couple of items not included in the factory
diagnostic guide for front differentials follow:
Low-speed knock generally is caused by worn
universal joints or play in the differential side gears caused
by an oversize side-gear counterbore.
Backlash clunk is defined as excessive
clunk on acceleration or deceleration. Common causes are excess
ring-and-pinion backlash, worn thrust washers, worn teeth on
side gears and pinions, excess clearance between case
counterbore and side gears, axle and side-gear spline wear,
worn case, and a bad or worn-out rear-axle pinion shaft.
These units are money makers if you invest
the time to learn how they work and establish good diagnostic
routines. Be sure to inspect all related parts carefully, as
they affect proper operation just as much as the transmission
and transfer case. 
©2006 Transmission Digest