Originally Posted by
TimRogers 
I'd like to bring up an FM noise problem I have in my '93 NX with a '90's or early 00's Kenwood head unit (KDC-S2009).
- The CD works fine with no noise; if I pause it with the key at "accessory" or "on", there is no noise.
- With middle strength to weak FM stations, they come in clearly with the key at "accessory" but when the key is turned to "on", noise drowns out the FM program, even without the engine running. The noise continues with the key in "on" after the fuel pump has shut off and if I disconnect the ignition coil.
I've had this issue ever since I got rid of the stock radio in the late 90's with another Kenwood that had this same issue.
The ground wire from the radio is connected to a bracket that holds the airbag computer.
The rear ground port on the radio and the mounting bracket (chassis) are each grounded to separate bolts that hold the shifter assembly to the chassis of the car.
All three locations show good grounds (multi-meter run between the hot lead to the power port and ground shows 12.xx volts).
I recently had the antenna off when I was doing bodywork and made sure the mounting points for it were corrosion free.
So, who has ideas?
It is most likely RF (radio frequency) interference generated by the ECU. I had this exact same problem with my '92 SE-R. Coincidentally I also had a Kenwood head unit, but the vintage Alpine deck that was in the car when I bought it also suffered from interference.
In my case I took a small, portable FM radio tuned to a weak FM station and walked around the car to isolate the culprit. With the key "ON", car not running, the noise was very high against the firewall, down low behind the engine and you guessed it, inside the car down around the ECU. There's no power to the ECU until you turn the key to "ON" and boom, that's when it happens. You probably get it to some degree on the AM band as well.
Do you know if the ECU has ever been removed from the car? In my case the previous owner had an upgraded Stillen/JWT ECU and may not have reinstalled it properly or the internal shielding was compromised when it was modified. Can't say for sure. I was going to swap in another ECU but sold the car.
See here and good luck...
http://www.sr20-forum.com/car-entertainment-general-electronics/43684-bad-radio-reception-only-key.html#post575682
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