GMC 1976 ZEO 6083 Maintenance Manual page 656

DESCRIPTION
All vehicles have both bumpers mounted to en-
ergy 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
Figure 1-Energy Absorbing Bumper
SECTION 14
BUMPERS
ENERGY ABSORBERS
the cylinder tube and pushes the bumper back to its
original position .
LEAKAGE
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
DIAGNOSIS
BUMPERS 14- 1
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 .
BENCH TEST
The bench test may be used to pre-test service
14