Radio; Diagnosis - GMC G Service Manual

DESCRIPTION
For operation of the factory installed standard and
optional radios available, refer to the Owner's Manual or
radio supplement supplied with the vehicle.
The receiver is mounted using the front support tubes
surrounding the control shafts, and a rear support bracket.
The front support tubes are secured with nuts. The rear
support bracket is secured with a screw.
Since radio problems are most often repaired at autho­
rized warranty repair stations, the tendency is to remove
the set when a problem is reported, without any preliminary
diagnosis. This results in a large number of radios showing
up as "NO TROUBLE FOUND" units when received by the
warranty repair stations. Many times, when this is the case,
the trouble usually could have been corrected without
removal of the radio, such as noise complaints.
The inconvenience of driving without a radio, while the
set is being serviced at a warranty repair station, can fre­
quently be avoided if the following quick checks are used to
eliminate external radio system problems before removing
the radio for repair:
• Test the radios outside, with the hood down.
• Most noise can be found on weak "AM" stations near
the low frequency and end of the band.
• Ignition noise on FM indicates a possible
malfunctioning ignition system.

RADIO

The power, antenna, and speaker harnesses connect to
the rear of the receiver.
Depending on the system, the speakers can be mounted
at the center or end of the dash, in the side panels or in the
rear corners of the cab.
The antenna is mounted on the right front comer of the
vehicle.

DIAGNOSIS

• If a test antenna is used, ground the base to the car
body and DO NOT HOLD THE MAST.
• 90% of the noises enter via the antenna.
• Coated screws or bolts CAN act as a poor ground.
• Most hoses are conductive unless they have a white
stripe.
• When shielding the dash, wire, hose, etc., use foil or
screen and ground it.
• Weak or fading "AM" is often caused by improperly
adjusted antenna trimmer (when present).
Always determine the exact nature of the radio problem
as an aid to diagnosis. Knowing whether the condition is
intermittent or constant, whether it occurs with the engine
off or running, and whether it occurs with car stationary or
moving, will help to pinpoint the problem. Use Chart 1 (fig­
ure 19) to isolate radio problems, then proceed to the diag­
nostic charts (figures 20 through 28).
ACCESSORIES 9-15