Will a Bad Alternator Cause Engine Noise in a Radio?
Most people have heard a high-pitched whining noise on a car radio that gets worse as the engine revs increase. The phenomena can also occur in home radios sometimes, when a car passes by. The most common cause is a malfunctioning alternator, a problem that certainly can cause noise in a radio.
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Rectifying Alternator Current
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Alternators create alternating current, hence their name. Car batteries cannot accept or store alternating current, so it must be changed to direct current. This is done inside the alternator, using integral diodes. The process is called “rectification,” and older vehicles that used generators rather than alternators were equipped with rectifiers to fulfill this need.
In brief, typical alternators have two bundles of diodes; a pulse of one phase of the alternating current passes through one bundle, then a pulse of the other phase passes through the other bundle. The diodes are synchronized so that two non-sequential -- alternating -- voltage pulses go in, but only one stream of direct voltage comes out.
Ripple
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The direct current downstream of the twin diode bundles is not an entirely smooth, even current. It has a tendency to “ripple” -- to piggyback a trace of the alternating current on the direct current. That ripple is lost in a properly operating system, and causes no problems.
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Alternator Whine
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If the diode bundles deteriorate, the ripple increases. An exaggerated ripple is picked up by nearby radios, which interpret it as an antenna signal. They translate the ripple into an audible whine. Less frequently, increased impedance -- an exaggerated resistance to electrical flow somewhere in the circuit, most typically at the battery posts -- can have the same effect.
Resolutions
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Capacitors can be installed in the system that bypass the ripple voltage, sending it back to the alternator, and resistors can be installed that simply block its passage. Both these methods address the symptom, not the cause. Solving the problem is better than disguising it. An auto electrician should be able to determine which of the two potential problems is at fault. If the diode bundles on the alternator are causing the radio noise, replacement or rebuilding of the component will address the problem. If the alternator is fully operational, a comprehensive check of the system, including grounds, should reveal a faulty or corroded connection.
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References
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