Noise Radiated From The Engine Compartment; Primary Ignition Noise; Secondary Ignition Noise; A-Line (Power Feed Wire To Radio) - Jeep 1979 CJ-5 Technical & Service Manual

RADIO SOUND SYSTEMS 3V-7
of the CVR or a O.l-mfd, radio-type capacitor directly
across the CVR terminals.
(5) If the source is found to be a wire, reroute the
wire, or wrap a piece of screen around the wire or wire
harness and attach one or more ground leads to the wire
screen. It also may be possible to screen off the area
found to be radiating noise. Be sure to ground the
screen.
(6) If the noise is found to be an electric motor,
install a 0.25-mfd coaxial (feed-through) capacitor in
series with the motor.
Noise Radiated from the Engine Compartment
These noises can be separated into three areas:
• Primary Ignition Noise
• Secondary Ignition Noise
• Alternator Whine (Antenna)
Primary Ignition Noise
This type of noise generally affects the AM band. The
noise usually appears as:
• Frequency varying with engine rpm
• Loudness varying with engine rpm
• Stops instantly when the ignition is turned to the
Off position and turned to Accessory position
The first two classifications are usually the result of
poor grounds on the coil capacitors or a wire routing
problem. Cleaning the grounds or rerouting the wires
may solve the problem.
An extra long antenna lead-in may be prepared as
shown in figure 3V-12, and used as a hotspot probe.
Remove the ignition coil and its mounting bracket.
Clean the paint off the bracket and the engine block,
then assemble tightly. In many cases, this helps reduce
the amount of interference radiated from the ignition
system. Also, the installation of a hood bonding strap or
device will help reduce interference radiated fom the
ignition system. Be sure to check the coil polarity.
In some rare cases, extra suppression may be required
if the vehicle is operating in fringe areas. For those
special cases perform the following steps:
(1) Install a 0.1-mfd coaxial capacitor as close as
possible to the coil battery terminal, not the distributor
terminal. Do not use an ordinary bypass capacitor.
(2) Install a 0.005-mfd, 1000-volt ceramic disc ca-
pacitor at the coil distributor terminal.
(3) Install a 0.5-mfd coaxial capacitor at the alter-
nator output terminal. Be sure it is rated to handle the
maximum alternator voltage.
Secondary Ignition Noise
Secondary ignition noise will always affect FM and, if
severe enough, may also affect AM. Normally one of two
conditions will be found in the radio.
• Ignition noise all across FM band (and possible on
AM)
• Ignition noise (loud) in between stations but not on a
strong station
When these conditions exist in the radio, the problem
is more than likely the result of:
• Distributor cap carbon ball eroded, or cracked or
loose cap
• Rotor with a burned carbon contact spot
• Secondary wire not seated in the coil or distributor
• Defective coil
• An oil film on some of the lead terminals
• Copper core secondary wiring
• Defective or improper spark plugs
If a wire was found not seated, remove the wire and
check for carboned end. It is not advisable to repair an
end terminal on carbon core wire; replace the entire
cable.
If the noise in question sounds like one or two cylin-
ders and definitely not all of them, then the problem is
after the coil. Using the fabricated noise probe, which
plugs into the radio, have someone sit in the vehicle and
listen to the radio. Move 1 the probe from plug to plug.
The person in the vehicle should notice an appreciable
increase in the plug noise when the defective plug is
reached.
It is a good idea to install resistor spark plugs when
experiencing spark plug noise. If the vehicle has copper
core secondary wiring, these wires should be replaced
with original equipment, carbon core resistor wires.
Alternator Whine (Antenna)
Alternator whine can be described as an annoying,
high-pitched whistle, or a siren-type sound that in-
creases and decreases with engine rpm.
Methods of eliminating alternator whine and engine
interference noise:
(1) Install front fender ground straps.
(2) Install hood bonding strap or device.
(3) Run offending wire t h r o u g h a shielded
(grounded) cable.
(4) Clean slip rings and be sure the brushes are
making good contact.
A-Line (Power Feed Wire to Radio)
If disconnecting the antenna did not eliminate radio
noise, the noise is probably on the A-line.
Motor noise on the A-line is ususally the result of
voltage spikes on this line being so large that the input
filter circuit in the radio cannot handle them. There are
two ways to handle this problem:
(1) Locate the cause of line noise and eliminate it.
(2) Add external filters to reduce the spikes to a
point where the radio filter can handle the spikes.
A grounded capacitor touched to all hot electrical con-
nections will often identify the offenders (fig. 3V-13).
The fabricated antenna probe (fig. 3V-12) also can be
used to find hotspots.