FM Property Value?

You can increase the value but as soon as you exit the diagnostic menu the value defaults back to normal. I'd like to take credit for this, but I read about it on a different GPS forum.

That's the value used to code/decode the FM transmitter signal - also called an IM or Intermediate Frequency. Your FM receiver won't pick up the decode the signal from the Nuvi if you were allowed to change this.

For those interested in the tech stuff:

The audio spectrum (The sounds we can hear) range from about 20 Hz to 20,000 Hz (or 20 KHz)

Stereo FM radio stations roll off the audio frequency range at 15 KHz and inject a 19 KHz tone (pilot) into the first harmonic sub carrier of the signal.

Your receiver listens for this 19 KHz pilot. When it hears it, it knows that the station is transmitting a stereo signal. Since Garmin makes the FM modulators of cheap components, they probably do not roll the audio signals off at 15 KHz, allowing sound frequencies all the way up to 20 KHz to pass through. When the receiver hears both the "injected" 19 KHz pilot AND the 19 KHz audio (out of phase, of course) or if you over modulate the transmitter and cause the audio to "splatter" or "distort" and crash into the sub carrier where the pilot is located, the receiver can no longer detect the stereo pilot and becomes confused. The result is over modulated and clipped sound,making for very poor audio quality.

The fix is to severely limit the amount of audio that will pass through the transmitter to -6DB

The real fixes are:

1) Use Aux inputs on your radio when possible.
2) Hardwired FM Modulator (FMM) injection of signal to radio.
3) Cassette adapter ( audio device for input of others)

BobDee got this right but only partly. The 19khz signal is a subcarrier that is only used for stereo FM broadcasts. One of the stereo channels is modulated onto the subcarrier and the complete mono signal is modulated onto the main carrier. The Rx demodulates this 19kHz subcarrier along with the mono signal from main carrier. The audio signal from the subcarrier is inverted and mixed with the mono signal. This cancels out the stereo channel modulated on the subcarrier leaving you with left and right stereo signals.

If you change the subcarrier frequency the stereo demux in your receiver might not be able to decode the stereo signal leaving you with only mono audio.

There is another setting that you can change while in the debug screen. FM power can be raised to about 115 dB which really improves reception. Unfortunately it reverts back to the original setting when you exit the debug page.

Jack j