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Re: No AM Radio while Engines Running?[700/85] posted by Don Foster on
Saturday, 31 October 1998, at 6:18 p.m.
Gordon --
It seems that Steve and Jean have good answers to "how to fix". The "why" part is that AM is extraordinarily sensitive to noise, and there are three major sources in your car:
- Ignition
- alternator
- arcing sources, such as relays -- and specifically the "radio suppression" relay (wonder why it was named that?).
The AM (amplitude modulation) circuit of your radio has something called AGC (automatic gain control). When it detects a strong signal, like a local station, it reduces the RF gain of the radio so it won't overload. When you have an external source of AM signal, regardless of its source (as mentioned above), the AGC can't distinguish between noise and an intelligent signal, so cuts back on everything. Hence, radio stations disappear. --