Muting OEM Head Unit with cell phone

[Donny Carlson]

I am having a Nokia Complete Car Kit installed, and one feature is Car Radio Muting XCRM, or as the installation instructions say:

The car kit offers a feature that can mute the car radio automatically during a converstion. This feature is convenient and provides safer handsfree operation. When in standby, the yellow wire (XCRM) is not grounded and the car radio works normally, but during a call, the line is grounded and the car radio is muted.
Note that an auxiliary relay or muting unit must be used when the car radio doesn't have a mute feature available.

Okay, you guys out there with the service manuals (TTA?), can I hook this up to the OEM head unit so that the control module on the Nokia car kit can have the head unit mute the stereo when a call comes in? And how is this done. If you can scan or some how send me the page in the manual, I'd sure like to give it to the installers.

THANKS!

[N40GL]

I am having a Nokia Complete Car Kit installed, and one feature is Car Radio Muting XCRM, --snip!--

Hey Donny!

I conquered this problem (sort of) when I had my alarm installed and mated to the cell phone car kit data line as a backup transmission source. What I discovered is that the line that trips the mute circuit in the radio needs a 100% ground to effectively mute the circuit (you can fake this out by shorting the wire to chassis ground as an example), and that anything less than 100% (i.e. about 75% that the Motorola/NEXTEL car kit provides) isn't enough to shunt the circuit. What I did was rig a relay as a power interrupt so when the phone circuit goes live, the power to the radio is interrupted and it goes dark (the power circuit for the clock and memory is unaffected - just the display does out). This works effectively to mute the radio when the phone is active.

What I did discover, and is likely even if the ground circuit in the radio was working as I had expected it to, is that the cell phone comes 'up' for a brief second whenever I transit cells (i.e. go from one to another). This shuts the radio down for about 4 seconds and it then comes back on. This would also mute the radio for the same period, because the car kit is too stupid to know that the phone is only handing off, not actually making a call. It just measures power output as an indicator. So, either way, if you want to use the mute circuit, you have to live with that anomaly.

Apparently, Ford was to have designed a daughterboard for the phone to accept a full cell phone kit (like in the Lincolns, another place this radio is used) but didn't, due to cost constraints. We got the part of the radio that was most important, and not the audio daughterboard.

Mark