Also see this thread which indicates that the Motorola kits need a pull up resistor.

"The line floats low which means that if you have a phone sense that connects the line to zero volts you will need:

1. A 10K pull up resistor between the mute sense line and twelve volts."


"The Motorola car-kits are pull-down, so I guess you'll need a pull-up on the control line"


/Michael

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/Michael