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Old 06-26-2008, 05:46 AM   #9
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Join Date: Jan 2007
Location: UK
Posts: 112
My Ride: 2003 M3
Originally Posted by zerodb View Post
We were close!

I managed to find one thread on another board that came up on the 5th page of results from a google search - the TEL ON lead needs to see +12V to enable the telephone audio inputs on the head unit.
I couldn't find anything when i was looking a couple of years ago, when I did my OEM retrofit BT harness.

I of course don't have a 12V lead that is switched at the same time as the Parrot mute lead, but it turns out, the BMW head unit doesn't really mind if you leave the TEL ON switched high all the time!
But you could do without the hiss, a switched relay and will sort that out.

Just solder another lead for the "tel-on" from the +12v ign and tie it to the relay and when the mute kicks in switches +12v ign to the "tel-on" pin (10).

That way no need to keep the "tel-on" high or have the relay circuit hot all the time (only when the ignition is on)

If I turn the stereo volume down really low, I can tell there's some hiss injected from the Parrot unit, so I'll probably put together a quick relay to switch 12V to that input when the Mute lead pulls to ground, but for the moment I've got a system that will work.
Result, that sounds ideal, you beat me to it

I figured I should drop in and mention this because there appears to be no more than one or two individuals in the history of the internet who have actually explained how this works. Hope this does someone some good who is up against the same issue and for one reason or another does not want to use a commercially sold harness.


TEL MUTE = Pull to ground
TEL ON = Pull to +12V

If you do BOTH of these things, you will mute all audio sources and enable the telephone input. If you just MUTE, you kill audio sources but get no telephone. If you just activate TEL ON, I suppose you could theoretically just mix your telephone audio into your music, but I can't imagine that being particularly useful for most people.
It makes sense the tel-on; the nav system must use this also as the nav instructions can "mix" with the cd/radio etc.

Maybe if you REALLY wanted to let a friend listen to the song that's playing on the radio at that moment. There's an idea. Put a hidden switch on the Mute lead. Just in case.
You could, just so you have the option, but as you say not that useful.

Looks like OEM BT can now be used with aftermarket Head Units now then, good find.

Let me know how you get on.
53, M3, Silver Grey. Reto-fits: Modded Mk4 DVD, OEM Bluetooth, VR Control, Rainbow/Alpine Speakers, Sony SOT Amp, HK Sub, CCFL, Intravee II, NightMode.
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