MEX BT3900U Problem

[Vinman76]

Hi there

I have recently installed this headunit in my car but I am having trouble turning it on. I know there is power going into the headunit as there is a "whirring" noise when I connect the cables up. I can also put a CD in and eject the CD.

I have tried pressing the "Off/Source" button and various other buttons but cannot get it to turn on.

At first I thoguht this must be a problem with the actual headunit so I exchanged it but now this is my replacement and I am having the exact same problem.

What could be the issue?

Thankyou

[Drumzman · Accepted solution]

Hi Vinman76,

There are two power cables that you need to connect for the unit to work successfully - a red and a yellow cable.  It sounds to me like you only have one of them connected, or if you do have both connected that they are the wrong way round.

The yellow wire needs a permanent 12 volt supply connected to it (in other words there is a supply to the unit at all times, even when the car is switched off).  This is what maintains any radio presets you have stored, remembers the point you reached in any CD you are playing, keeps the clock running etc etc.

The red wire can also be connected to a permanent 12 volt supply, but is designed to be connected to a switched 12 volt supply, or one that is only live when the ignition is in the accessory position or the car is running.

If your car has got the standard multi-connectors that accept the two matching ones from your stereo, then you would expect everything to be OK, but for some reason, some car manufacturers have the permanent and switched 12 volt supplies the reverse way round.  For this reason, Sony provide a join in the red and yellow supply cables that allow you to unplug and reverse the connections ... I'd suggest swapping these two wires over as shown in the attached diagram, and see how you get on (you can't hurt the unit by trying this) but I suspect this might resolve the problem.

Hope this helps.

[Vinman76]

Thanks for the reply drumzman but still not workong

I tried switching the red and yellow cables around but this made the unit have no power whatsoever (the eject button wouldn't work)

The car is a BMW e90 if that is any help!

[Vinman76]

Edit

Drumzman I love you!

I had to put both the red and yellow cable into the joint one.

Means I have to remember to turn the headunit off all the time but better than nothing!

[Drumzman]

Hi Vinman76,

Really glad to have helped! :smileygrin:  You have the only other possibility where a car has no switched feed at all(!) and you have to use the power button on the stereo itself as you say (hence the reason you also have the double socket that you found).  Really pleased you got a result!

All the best.