Jim Grant's Tech Tips
'94
Chevy K3500 Pickup; Chunks falling Out
Q:
This
is a '94 Chevy K3500 pickup truck with 47k miles on it. The front 4WD
gears were recently replaced. It is believed they failed because of lack
of lubrication. The grease cup is hard to reach & likely was not checked
for some time. A few days after the front end was rebuilt there was a
humungus knocking when shifted into and out of 4WD. A chunk of the aluminum
housing larger than my fist was knocked out. The repairman believes it
was because the rear tires (4 dualies) are ± 96 inches in circumference:
they are Goodyear WorkHorses LT215-85R16. The 2 front tires are ±
91 inches in circumference: they are Goodyear Wrangler AT LT 225-75R16.
Does that make sense? Or, perhaps was there something lacking in the rebuilding
of the gears?
A:
You've
performed about the biggest fauxpas one can do with a four wheel drive
vehicle. What was that? Mixing tires sizes! You never ever operate a four
wheel drive vehicle with tires that have different outer circumferences!
In doing so the weak link will show itself in no time at all and result
in driveline destruction. Any car or truck with four wheel, part time
or all wheel drive requires that the tires all be of the same outer circumference,
no mixing allowed. In theory, when four wheel drive is engaged power goes
equally to all four wheels. If the front wheels turn one revolution, the
rear wheels turn one revolution. But if one wheel travels 96" in
one revolution and the other only 91" how do you make up the difference
in travel? In four wheel drive it just doesn't happen. The driveline,
transfer case, front and rear differential will get all bound and wound
up because one set of wheels is traveling a different distance than the
other set. This all leads up to the type of failure you're describing.
After the destruction has been repaired buy six new tires that are all
the same size. Do not mix old worn with new, which is also a no-no with
four wheel drive and all wheel drive vehicles.
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