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PROBLEM Descriptions and Probable Solutions
Excessive AC ripple
Possible causes include
•Old, weak, or badly sulfated battery
•Defective alternator rectifier diode
•Corroded battery terminal connections
•Poor engine ground strap or connection
Recharge or replace the battery or clean and tighten the
connections. A preliminary test of the rectifier diodes and
regulator can be made with an accurate voltmeter. When the
engine is running at a fast idle, at about 1000 RPM, there should
be between 13.4 and 14.2 volts at the battery terminals. Turn on
headlights and other high current electrical accessories and
note any large drop in voltage that may be due to a defective
diode. Have any suspicious alternators checked by a qualified
mechanic.
If there is no defect found in the system, it may be just noisier
than average. Installing an external line filter on the FM
radio may reduce the interference to an acceptable level.
Alternator whine heard
through the radio cable
•Check for any inadvertent grounding of any interconnecting
cable shields to vehicle ground. For example, if you routed cable
through a hole cut in the vehicle without the use of a grommet,
the rough edge of a hole may have cut the protective jacket and
the cable shield is grounded.
•Be certain that the lead is connected to only the microphone
low terminal of the radio, if such exists, and not to chassis
ground.
•Little or no change in the presence of whine when the external
power source is used indicates the problem may be associated
with the radio itself. Many mobile radios, particularly those with
remote control heads, utilize power leads for the receiver and
low-level transmitter circuits separate from the main heavy-duty
leads for the transmitter power output amplifier. Noise on the
audio output that is hardly discernible when the cabin speaker is
used can become overwhelming when heard through a noise-
reducing headset. The headset reduces outside noise by over
90% and places the earpiece very close to the ear making the
noise and/or interference seem much louder. Generally a filter
placed in the lead supplying power for the audio stages of the
radio will reduce the amount of whine to an acceptable level. On
most Motorola radios the green power lead from the control
head should be filtered.
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