Chevrolet 1980 Light Duty Truck Service Manual page 277

Fig. 3C-9--Gripping Methods
• a skip or lag at reversal near mid-stroke
When air adjustable shocks are being manually
operated, the air line must be disconnected at the shock
absorber.
BENCH CHECKS
The bench checks are recommended if the proper type
hoist is not available to perform the "on vehicle" tests, or
if there is still some doubt as to whether the shocks are
defective. In addition, the bench test allows a more thorough
visual inspection.
Bench check procedures are discussed for three general
types of shocks.
SPIRAL GROOVE RESERVOIR
If this type of shock has been stored or allowed to lay
in a horizontal position for any length of time, an air void
will develop in the pressure chamber of the shock absorber.
This air void if not purged, can cause a technician to
diagnose the shock as defective. To purge the air from the
pressure chamber, proceed as follows: (Refer to Figure SC-
)
10
(a) Holding the shock in its normal vertical position
(top end up), fully extend shock.
(b) Hold the top end of the shock down and fully
collapse the shock.
(c) Repeat Steps (a) and (b) at least five (5) times to
assure air is purged.
Bench Test Procedure
1. This is a comparison type test. If possible, obtain a
new or known good shock with same part number as shock
under test.
2. With shocks in vertical position (top end up), clamp
bottom mounts in vise. Do not clamp on reservoir tube or
mounting threads.
3. Manually pump each shock by hand at various rates
of speed and compare resistance of suspected shock with the
new one.
Rebound resistance (extending the shock) is normally
stronger than the compression resistance (approximately 2:-
1). However, resistance should be smooth and constant for
each stroking rate.
THAT W ILL FIT THRU
EXISTING HOLE
FRONT SUSPENSION
Fig. 3C-10--Position for Purging Air From Shocks
4. Observe or listen for the following conditions that
will indicate a defective shock:
• a skip or lag when reversing stroke at mid travel.
• seizing or binding condition except at extreme end
of either stroke.
• a noise, such as a grunt or squeal, after completing
one full stroke in both directions.
• a clicking type noise at fast reversal.
• fluid leakage.
5. To check for a loose piston, completely extend
shock to full rebound; then exert an extra hard pull. If a give
is felt, a loose piston is indicated and shock should be
replaced.
PLIACELL OR GENETRON
Pliacell and Genetron are some of the trade names used
to indicate a gas-filled cell in the shock reservoir. The
reservoirs of Pliacell and Genetron shocks are smooth,
compared to the spiral groove type. The cell takes the place
of air in the reservoir. Thus, aeration or foaming of the fluid
is eliminated, as air and fluid cannot mix.
Due to this feature, these shocks should be bench
checked in an inverted position (top end down). If, when
stroked, a lag is noticed, it means the gas-filled cell has been
ruptured, and the shock should be replaced. If no lag is
noticed, the remainder of the bench check is the same as
given in the Spiral Groove Reservoir, Section 1, Bench
Check Procedure.
AIR ADJUSTABLE SHOCKS
This type of shock contains an air chamber like the
spiral groove reservoir type, and must have the air purged
from the working chamber. See Section 1, Spiral Groove
Reservoir. After air has been purged from shock, proceed
as follows:
(a) Clamp lower shock mounting ring in vise in vertical
position with larger diameter tube at the top.
(b) Pump unit by hand at different rates of speed.
Smooth resistance should be felt through the length of the
stroke. Since the units are normally pressurized, the sound
of air bubbles or a gurgling noise is normal .
(c) The remainder of the bench check is the same as
given in the Spiral Groove Reservoir, Section 1, Bench
Check Procedure.
3C-9
LIG H T TRUCK SER V ICE M AN UAL