GMC 4000 Series Service Manual page 136

5. Turn chalk marks on both wheels so that
marks are level with axle and toward rear of ve­
hicle. Take measurement with toe-in gauge at
chalk marks; then turn both chalk marks to front
and level with axle and take another measurement.
If measurement at front exceeds rear dimension
by 1 /8 " or more, axle is bent to the rear. If the
measurement condition is the reverse, the axle is
bent forward.
ELECTRIC SHIFT MAINTENANCE
A N D DIAGNO SIS
The only general maintenance necessary on
the shift control system is periodic lubrication of
unit as described in LUBRICATION (SEC. 0). The
wiring connectors should be kept tight, and w ires to
the various units should be kept in good condition.
DIAGNOSING TROUBLE
If the electric shift fails to operate properly,
the system should be tested and the trouble diag­
nosed as described in following paragraphs. A test
light consisting of a 12-volt bulb with two w ires a
few feet long with small battery clips on the ends.
Refer to Wiring Diagram and test in following
sequence:
1. Disconnect two w ires from shift unit at
rear axle. Place engine control or ignition switch
in "ON" position, also place axle shift switch at
shift lever in "Hi" position.
2. Connect one lead of test light to ribbed wire
and opposite lead to ground. One of the following
conditions should exist.
a. Should light come on and stay on, the c ir ­
cuit is satisfactory and any trouble will probably
be found in the shift unit. Repeat check for "Lo"
position as directed in step 3 following.
b. If the light fails to come on, this indicates
that the circuit is open between the control switch
and the shift unit and further tests will be required.
c. If the light com es on but cycles, this indi­
cates that there is a short between the control or
ignition switch and shift unit, and further tests will
be required.
3. Remove test light lead from ribbed wire
and connect to smooth wire, also place axle shift
switch in "Lo" position. Observe conditions as
outlined in sub-paragraphs above.
4. Connect one lead of test light to control
switch side of circuit breaker and opposite lead
to ground. Should light come on, and stay on, the
circuit is satisfactory. However, if the light fails
to come on the circuit between the control switch
and circuit breaker is either open or shorted.
5. Connect one test lead to load side of circuit
breaker and opposite lead to ground. Should light
come on, and stay on, the circuit breaker is satis­
factory. However, if the light fails to come on the
G M C S E R V I C E M A N U A L
circuit breaker is faulty.
then test circuit through red wire with white stripe.
Reconnect both w ires to terminal.
terminal. Place axle shift lever switch in "Lo"
position and test circuit.
terminal. Place axle shift lever switch in "Hi"
position and test circuit.
any trouble within the system . When checking the
wiring harness for shorts or open circuits, exam­
ine for broken insulation.
tested by substituting a new unit.
ometer adapter fails to operate, make the test to
determine whether the adapter is getting current
in "Lo" range. If current is present, replace the
adapter. When the trouble is traced to the shift
unit, it should be replaced.
tem must be kept free of vacuum leaks. When vac­
uum leaks exist, rear axle shift mechanism will
operate sluggish and in some instances may not
shift axle. If this condition exists make tests to
determine location of leaks.
VACUUM LEAKAGE TEST
axle shift chamber and insert a vacuum gauge in
line, using a "Tee" fitting.
"Hi" or "Lo" depending upon location of vacuum
gauge. Run engine long enough to obtain maximum
vacuum, then stop engine and note rate of vacuum
drop on gauge. If drop exceeds 1 inch to 10 min­
utes, leakage is excessive.
REAR AXLE AND CONTROLS
6. At chassis junction, remove both w ires,
7. At ch assis junction, remove wire from
8. At chassis junction, remove wire from
The preceding tests should readily localize
The co n tro l o r ign ition sw itch can be b est
If the vehicle shifts normally, but the speed­
VACU U M SYSTEM MAINTENANCE
All lines and units in vacuum power shift sy s­
1. Disconnect flexible lines one at a time, at
2. Start engine and place axle shift button in
3. If leakage is indicated, coat all vacuum
Figure 7 — V a cu u m Tubing Connection Seal
Sec. 4A
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