Chevrolet 70 Series 1970, Service Manual

Page: 219 / 841

REAR SUSPENSION 4-6
Figure 8—Tubing Connection Seal
any trouble within the sy stem . When checking the
w iring h arn ess for sh orts or open c ir c u its, exam ­
ine for broken insulation.
The control or ignition sw itch can be b est
te sted by substituting a new unit.
If the v eh icle sh ifts norm ally, but the speed­
om eter adapter fa ils to operate, make the te s t to
determ ine whether the adapter is getting current
in low range. If current is p resen t, replace the
adapter. When the trouble is traced to the shift
unit, it should be replaced.
AIR SHIFT UNIT
A ir shift unit should be inspected and tested
for proper operation and air leak s.
1. P la c e control sw itch at shift le v e r in "HI”
p osition to admit a ir to shift unit, then apply soap
su d s to entire unit. Any lo s s of air should be
corrected .
2. Move control sw itch through all shift po­
sitio n s to determ ine that shift unit i s operating
p rop erly. Should there be any indication of a
m alfunction, replace or repair the sh ift unit.
NOTE: When testin g the a ir shift unit, note
that air p r e ssu r e is presen t in both shift units
only when control sw itch is in "HI” , a lso that
there is no air in either shift unit when control
sw itch is in "LO” p osition. When control switch
used on th r e e -sp e e d shift m odels i s in "INT”
p osition , air is p resen t only in forward axle shift
unit and a ir i s exhausted from rearw ard axle
shift unit.
INTER-AXLE DIFFERENTIAL
CONTROL LEAKAGE TEST
AIR LEAKAGE TEST
1. Build up a ir p r e ssu r e in air sy stem to
m aximum p r essu r e lim it (100 to 105 lb s.). P lace
axle shift control le v e r in “ LOCK” position.
2. Coat a ll a ir line connections with soap
suds to check for leakage. No leakage in a ir line
connections i s p e r m issib le . Leakage can so m e ­
tim e s be co rrected by tightening the connection.
3. If th is fa ils to c o rr ect lea k a g e, rem ove
line having leaking connection. Slide tube nut back
on tube and rem ove a ll p a r tic le s of se a l ring from
tube and from inside of nut. Slip new se a l ring
over tube end. Insert tube into connector fitting
and sea t tube so lid ly in fitting (fig.
8
). Slide sea l
ring (do not roll) into fitting. Thread tube nut into
fitting and tighten slow ly and firm ly.
4. Coat entire surface of control valve and
check valve with soap suds to check for leakage.
Control valve may have som e leakage past
plunger. L ib erally coat control le v e r slo t in
control valve body (with dial plate rem oved).
Leakage should not exceed 1-in ch soap bubble in
one second. Leakage at control valve in let and
outlet connections is not p e r m issib le . If leakage
i s evident in any of the other u n its, faulty unit
should be replaced.
VACUUM LEAKAGE TESTS
1. T em p orarily in sta ll a " T ” fitting at shift
cham ber and connect a vacuum gauge betw een
cham ber and vacuum lin e. Tighten gauge con­
nection s firm ly.
2. Start engine and place d ifferen tial lock
control le v e r in "LOCK” position . Run engine
long enough to obtain maximum vacuum , then stop
engine and note rate of vacuum drop. If drop ex­
c e e d s 1 inch in 15 secon d s, leakage i s e x c e s s iv e .
3. If leakage i s indicated, coat all vacuum
lin e connections with hydraulic brake fluid. If leak
e x is t s , fluid w ill be drawn into connection with en­
gine idling and control le v e r in "LOCK” p osition.
4. R em ove lin e having leaking connection.
Slide tube nut back on tube and rem ove a ll p a rti­
c le s of sea l ring from tube and from in sid e of nut.
Slip new se a l ring over tube end. In sert tube into
fitting and sea t so lid ly . Slide se a l ring (do not
roll) into fitting (fig.
8
). Thread tube nut into
fitting and tighten slow ly and firm ly.
5. Repeat leakage te s t a s d irected in step s 1
and 2. If vacuum drop is s till e x c e s s iv e , leakage
i s in one of the shift control units. R em ove each
unit and overhaul or rep lace as d irected la ter
under individual headings.
REFER TO NEXT PAGE FOR REAR AXLE SERVICE
DIAGNOSIS
CHEVROLET SERIES 70-90 HEAVY DUTY TRUCK SHOP MANUAL