I'm thinking one of the prime candidates for my next failure (98
'burb, 180+k) is the rear end.
The story I get is that they don't give much warning: maybe a
little tire chirping on the turns and then "bam!"....
I'm thinking that, as long as there aren't parts laying on the
pavement, I should be able to put the vehicle in 4wd "Auto" and
limp home on the front diff without damaging any other systems.
Or am I just wishing?
Pete, sounds like you may have heard about the Grenade 80 (G80) optional
locking rear end?
They don't give much warning when they fail BUT depending on how they
fail you may not be able to move the vehicle without some work. I have
seen them break at the weight pivot and lock up tight. Pull the cover
(try to save the oil) then remove the broken parts so it can move. Then
toss it in 4X4 and drive home.
I have done this a few times. Best one was when a multi-piece drive
shaft under my BILs Subaru Justy failed outside Syracuse. Drove up there
and looked it over. Finally decided to beat a can over the output shaft
housing and drive it. He made it to my shop and went part hunting. Found
out that all the parts cost more than the car was worth!
He wanted to drive it so I did a few modifications. Took the front yoke
and cut it in the lathe so it was smooth all the way back. Drove it into
the trans with a rubber mallet to get it as tight as possible. Then made
up a cover for the housing just in case it slipped back. Locked the
transfer case into 4X4 and disconnected the interior lever.
He drove that car for 4 more years as a front wheel drive!
This is something that I've just come to realize: that the stress
on a locked-up 4wd system on pavement is from the difference in
rotation between front and rear wheels. I had always thought it
was between the two front wheels.
I'm guessing, then, that the "4wd Auto" setting has something to
do with de-coupling the front and rear systems.
Most of the torque wrap is due to the difference in rotational speeds
during turns. The reason is because all 4 tires need to turn at
different speeds. With an open differential on each end and in 2wd this
isn't a problem. The diff. allows the tires to rotate as needed.
When you toss it into 4wd the transfer case gets a LOT of stress if your
on surfaces with good traction. I have seen them break apart, drive
shafts torn up, failed U-Joints.
4wd Auto is a PIA system.
In the non-auto system the front axle has a simple collar which slided
across a set of splines to lock the right hand axle together. This takes
the place of the locking hubs used on other makes and earlier GM
products. The parts that usually fail are all outside that axle. The
actuator (vacuum, electric solenoid, wax pellet) or the cable are the
On the 4wd Autos the front axle is a LOT more complex.
What they did is install an electric clutch pack on the input shaft as
well as the interlock. When you select the auto position it uses signals
from the ABS and speed sensors to engage/disengage the clutch pack as
the ECM thinks it needs 4wd. The problem is people leave them in auto
4wd through most of the winter thinking it's better. However every time
a tire slips the clutches cycle and causes shocks in the system. I have
rebuilt a LOT of the front axles.
Other than the clutches the systems are a lot alike.
Now if you look at a Bravada you will find the same type system, EXCEPT
the interlock is missing on the right axle, and there is a viscous
coupling in the transfer case that allows the front and rear drives to
rotate and drive but still slip to prevent torque wrap. They also are
full time 4X4 so no selector switches or actuator to mess with. BUT you
do see increased tire wear normally.
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