GMC 4000 Series Service Manual page 532

operating properly, repair or replace the speed­
ometer head. Replace the cable if kinked or broken.
If the speedometer head, adapter, and cable are
operating properly, replace the speedometer drive
gear.
To check speedometer for accuracy, use a test
machine equipped with a drive. Connect speedom­
eter head to drive and operate at a known 1000
rpm. The m iles-per-hour hand should register 60
mph and the odometer should register one m ile-
per-minute. If not, speedometer head is defective
and must be replaced.
SPEEDOMETER CABLE
REPLACEMENT OR LUBRICATION
1. Disconnect speedometer cable from speed­
ometer head.
2. Remove cable by pulling it out from speed­
ometer end of conduit.
NOTE: If cable is broken, it will be necessary
to remove lower portion of cable from transm is­
sion end of conduit.
3. Lubricate lower % of cable with lubricant
specified in LUBRICATION (SEC. 0), then push
cable into conduit. Connect upper end of cable to
speedometer head and road test vehicle to check
for proper speedometer operation.
CAUTION: DO NOT over-lubricate
the flexible cable or adapter. Excessive
lubrication will seriously affect speed­
ometer operation.
ELECTRIC HORN
An electric air-tone "S" type horn is used on
all vehicles covered by this manual. The horn is
carefully adjusted and inspected during manufac­
ture and should operate indefinitely without atten­
tion. The horn assembly should not be adjusted or
repaired. If horn becomes inoperative, it must be
replaced.
If horn fails to operate, use a jumper lead to
check the external circuit as follows:
1. Connect a jumper lead from No. 1 terminal
on horn relay to ground. If horn then operates,
trouble is in the horn control circuit. If horn does
not operate, remove jumper lead and proceed with
Step 2.
2. Momentarily connect a jumper lead between
No. 1 and No. 3 terminals on horn relay. If horn
operates, the relay is defective and must be re­
placed.
3. Connect jumper lead from horn bracket to
vehicle frame or grounded side of battery. Be sure
contact is made through paint on horn bracket and
frame.
NOTE: Since the horn circuit is internally
grounded through the horn mounting, a good ground
G M C S E R V I C E M A N U A L
CHASSIS ELECTRICAL A N D INSTRUMENTS
connection must be maintained between the horn
mounting bracket and its mating part.
the above checks, the trouble may be that horn con­
tacts are being held open by a foreign particle.
This can sometimes be corrected by energizing
the horn, then lightly tapping the horn to dislodge
the particle. If this fails to correct the condition,
replace the horn assem bly.
tals attached to the cab roof panel. Air pressure
to horns is controlled by a control valve mounted
on the inner hinge pillar panel at left side of cab.
A signal cord is used to manually activate the con­
trol valve.
tection valve. Refer to "AIR BRAKES" (SEC. 5B)
for information on the pressure protection valve.
ative, a service kit consisting of a spring, rubber
seat, and plunger may be installed after discon­
necting the air inlet line and removing end plug
from valve.
HORN RELAY
"Wiring Diagrams" booklet for wiring connections
at horn relay.
connected in series with relay operating coil at
relay terminal No. 2. When circuit through relay
operating coil is completed at the horn button, a
sm all amount of current flows from the battery
through the coil winding. With winding energized,
armature is attracted to core and points close.
Current from the battery (No. 1 terminal), then
C h a ssis Jun ctio n Block
___________
■
Figure 7 — Chassis Junction Block
(Tilt Cab M odels) (Typical)
If the horn fails to operate after completing
AIR HORN
Air horns (when used) are mounted on pedes­
Air pressure is supplied by the pressure pro­
If the air horn control valve becomes inoper­
RELAYS
NOTE: Refer to applicable wiring diagram in
The horn button in center of steering wheel is
Sec. 12
Page 521
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