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GMC
MOTOR
HOME
SERVICE
DESCRIPTION
All GMC Motor Homes have both bumpers
mounted to energy absorbers. (figure 1)
The energy absorber is a device which uses hy-
draulic fluid within the cylinder tube to absorb im-
pact energy and inert gas to restore the bumper to its
original position after a low speed collision. Right
and left energy absorbers are the same . (figure 1)
During impact (figure 2) the piston tube moves
back into the cylinder tube . Hydraulic fluid is forced
from the cylinder tube through the orifice around the
metering pin into the piston tube . The metering pin
controls the rate of fluid flow . Fluid pressure in the
piston tube against the floating piston moves it and
compresses the gas. After impact (figure 3), gas pres-
sure against the floating piston forces fluid back into
the cylinder tube and pushes the bumper back to its
original position .
Figure 1-Energy Absorbing Bumper
SECTION 14
BUMPERS
ENERGY ABSORBERS
LEAKAGE
DIAGNOSIS
OCTOBER, 1973
A trace of oil on the piston tube is normal due to
grease packed in the seal area during manufacturing.
If oil is dripping from the unit it should be replaced .
DAMAGE
Inspect the bumper bracket, frame bracket, piston
tube and cylinder tube for evidence of visible distor-
tion . Scuff marks on the piston tube are normal . If
there is obvious damage to the unit it should be
replaced .
ON VEHICLE TEST
This test involves compressing EACH unit sepa-
rately 3/8" or more and observing that the bumper
returns to its normal position.
1 . Turn off ignition, transmission in park, park-
ing and service brakes set.
2. Use a barrier such as a pillar, wall, post, etc.
3. Align a pressure device, such as a hydraulic
jack, with the energy absorber . Make sure it is posi-
tioned squarely with the bumper so it will not slip .
4. Apply pressure to compress the unit 3/8" . Use
a 6" scale to determine travel . Release pressure and
note if the bumper returns to its normal position .
5. If either unit fails to return to its normal posi-
tion, replace it .
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