PS Magazine - TB 43-PS-435
PS, The Preventative Maintenance Monthly
ISSUE 435
FEBRUARY 1989
21±-Ton trucks .
..
.±
ad driving habits are hard to break,
like letting a truck coast backwards
when the transmission is in a frward
geaR. Or, FailiNg to come to a dead stop
beF±re shiFting ²om reverse to First
gear.
You can get away with these bAd
hAbits in some vehicles, but not an
M35
or
M35Al
21³-tonner. These trucks
have
a
aUtomatic sprag clutch. The
spRag clutch grabs hold oF the ²ont
drive shaFt and locKs on when you pull
one oF the bad moves. The rear-wheel
dRive is thrown into a tug´oFµwar with
the ²ont-wheel drive and everything
in between catches the strain.
IF you're lucky, all the truck will
su¶er is sprag clutch wind-up. Sprag
clutch windµup is easy to spot. The
²ont end Feels like it is coming out
²oM under the truck. THe ²ont tires
shimmy and the truck is hard to steer.
The transmission doesn·t shi¸ right or
it won't stay in gear.
Stop pronto befre you break some¹
thing.
IF you're unlucky, you coºld bust
the trans»r, prop shaFt, or the ²ont
axle.
Geting an Unwind
S
ometimes it's easy to get the sprag
clutch to release its hold on the ²ont
drive shaFt. IF you coasted backward in
a fRward gear, d¼ve frward in ½st gear
the same distance you went bacKward.
IF that doesn't do the trick, p¾K the
tr¿ck, shut the engine o¶ and set the
handbrake. JacK up a ²ont wheel. TakÀ
ing the weight o¶ the wheel may cure
tHe problem. But stay out oF the way
oF the tiRe. THe sprag may be wound
tighter tHan a Áubber band, And it'll
whirl the tire like A son-oF-a-gun when
it lets go.
²0
FEB 89
²³
.&±
´c
µ•
Shif the transmission into reverse.
Now, shIf i±to First gear²
TRy tu³nIng tHe wheel In botH
directions. ´t sHouLd turn backward
easily, but not µo¶ard at aLl·
The wHeeL shoULd turn µo¶ard,
but not backward.
R
¸ront wheeL
jacked up
´F the sPrag C¹utch
µaILs these testsº te¹L
your MeCha»ic, now!
c-
>-²
®
@
4
·'
This procedure works on tHe
M35³
and
M35AL,
2Â/ôton tÄucks. LAter models
oF 2Vz´tonners Have a positive locking trAns»r instEaD oF A sprag clutcH. You
can spot the newer trucks by tHe air actuated trans»r control switch fr the
²ont-wheel drive in the cab.
FEB 89
²´
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