Chevrolet 1977 light duty truck, Service Manual

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T O O L J - 2 3 5 2 0
Fig. 9-lR--Testing Windshield Antenna (Typical)
S ta tic Heard-Trim radio - check for spark plug wire
breakdow n, loose or im properly seated wire, or loose
or missing engine g round strap.
Check suppressors on voltage regulator, alternator,
and resistor on timing control solenoid.
Static Still
Present-Defective receiver.
W EA K R A D IO SIG N A L
Test w in d s h ie ld a n t e n n a as d escribed u n d e r
" A n te n n a T esting" in this section.
D IS T O R T E D T O N E
T u rn on radio, adjust for high volume and
m ax im u m bass. Check speaker by substituting a good
speaker.
No Distortion-
Replace speaker.
Distortion-Defective receiver-remove for servicing.
T E S T IN G W IN D S H IE L D A N T E N N A (FIG.
9-1 R )
All C-K model trucks with factory installed radios
are equipped with windshield antennas. To positively
id entify a n te n n a failure and elim inate the possibility o f
unnecessary w indshield replacement, W indshield A n ­
tenna Tester J-23520 should be used to determ ine
continuity o f the thin a n te n n a wire.
W hen a n te n n a failure is suspected, the following
checks should be m ade before replacing the windshield.
1. Check Tester J-23520 fo r operation on any
vehicle radio a n te n n a that is operatin g norm ally to test
for a weak or dead battery.
2. Check all a n te n n a connectings to insure that
a n te n n a is electrically coupled to the radio.
3. T urn ignition switch to accessory position, turn
radio " O N " , select AM band if receiver is A M / F M and
tune ra d io to an ofF station position.
4. Flold tester to a n te n n a beginning at the upper
corner o f antenna:
CAUTION: The plastic Shield m ust be on tester
at all times to avoid scratching windshield.
a. If a shrill sound is emitted through the
speaker w hen both a n te n n a wires are tested, a n te n n a is
operational.
b. If no sound is emitted through one or both
a n te n n a wires, move tester along the wire toward center
o f windshield and down toward radio.
c. If a shrill sound is picked up, find exact
location where the noise begins, this is the area o f the
defect. Replace windshield.
d. If no noise is heard over entire length o f
antenna, unplug a n te n n a lead at radio and touch tester
to a n te n n a socket in radio.
e. If radio now makes a shrill sound, check
connectors and a n te n n a lead for possible defect before
replacing windshield.
f. If no noise is emitted, radio, speaker, or fuse
is defective.
N O TE: M ake sure that a n te n n a tester is turned off
a fte r com pleting a n te n n a test.
ON-VEHICLE SERVICE
M A IN T E N A N C E A N D
ADJUSTMENTS
R A D IO IN S T A L L A T IO N P R E C A U T IO N S
Listed below are com m on causes o f inoperative
ra d io receivers or p oor reception on the A M scale upon
reinstallation a fte r repair.
• R ad io speakers not connected—this could cause
the o u tp u t transister to b urn out in the receiver.
• A n te n n a lead not plugged into the receiver or
windshield.
• A n te n n a trim m er not peaked.
T R IM M IN G R A D IO
I f diagnosis indicates the ra d io receiver must be
trim m ed, p e rform the following procedure:
1. G Models-Set a n te n n a mast at m ax im u m height.
2. Remove tuner control knob and bezel (right
h an d knob).
3. Place ignition key in the " A C C " position.
4. Turn volume control to m axim um volume.
5. Tune radio to a weak station (near 1400 KC) on
the AM scale.
6. Adjust a n te n n a trim m er screw ( ± 2 ° screw
rotation) until m axim um volume is received (fig. 9-2R).
7. T urn radio volume off.
8. Replace tuner control bezel and knob.
9. Turn ignition to "lock".