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Hardwired An Aux Input To '91 Pioneer Ls400 System


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Hi there, I thought I'd share this information with you because I haven't found anyone who managed to do this. With this I mean adding an AUX input to the Pioneer system without cutting into the CD wires or the radio. It does require to open up the dash unit. But this quite easy. For by-passing the cassette deck a bit more effort is needed, but still is very much do-able for the average DIY Joe.

If the units are the same throughout the years this will work for every '90 - '94 LS400 with the Pioneer Premium Sounds system.

My intention was to not cut into the CD wires, but to use the tape deck in some kind of way, because I still love the sound of the CD player and don't want to mess with the signal path, I'm an audiophile ;)

Because the cassette deck does not have external wires I had to open up the Pioneer unit. What I've found there are three unused pins sticking out the board with the labels: 'L', 'G' and 'R'. Now that looks interestingly familiar. I tried these for an input and believe it or not IT WORKS! :lol: The sound input through these pins sounds through every input (CD/Tape/Radio). So it should work with the 'empty CD on repeat' method. But as I said, I don't want to mess with the CD-player. So I don't want to put in an empty CD and put it on repeat. I wan't to use the cassette deck.

As with the CD-player, the unit only gives of sound when a cassette is in the deck. After thoroughly examining the cassette deck from the inside, I found switches which trigger the cassette deck to think a correct cassette is inserted. After connecting these it didn't work. Turns out the cassette deck also has two sensors to check if both 'coils' (don't know what they are named) are turning, because one is turning through a motor and the other is turning by the pulling of the tape. So if the tape breaks, one coil stops turning and the unit would spit out the cassette. This is by-passed by simply adding one wire to give of both signals from one sensor. This does mean the motor in the cassette deck should be running to trigger the sensors. I don't think this is a problem. After this I disconnected the 3 wires from the head which reads the tape. When I press 'TAPE' a sound is still audible. It sounds like an empty tape, but this is almost not noticeable. Actually, the sound quality is awesome and I think if a great source is used, so no headphone plug in my iPhone, it can be better than the CD-player. If I've got the time I might go and install an iPhone to SPDIF/I2S with a Buffalo 2 DAC and the Legato I/V stage from Twisted Pear Audio. I think that will come close to the ultimate source.

Now that is all! I ran my cable through the cassette deck to the outside, which is ugly but very easy. I might do this otherwise sometime. I can now press TAPE, plug in my iPhone and start listening!

I have shot some images, they do not show the process of getting to the board, but it does show what I am talking about. Getting to the board is actually very easy. Counted in my head I think it requires just 12 philips screws to be removed (after removal of the unit from the car which can be located here: Tutorial from to remove head unit). 8 to remove the front with the controls, and 4 to remove the cassette deck from the board.

The picture shows the board separated from the unit, which is not necessary. I did this to examine the back of the board.

The board with the wires connected, no it's not a small board, those wires are huge. Again, audiophile ;)


The wires connected to the pins. The pins are not visible here, they are beneath the tape. The marks are visible. I chose to solder the wires to the pins, but there are standard connectors to go on these pins. I have no idea how they are called:


The switch that is triggered when the flap is pushed up:


The switch that is triggered when a cassette is pulled back and pushed down.


Here I removed one switch, and you can see the wire connecting the sensor which gives of a signal...

5675791004_af08351514.jpg the one that does not, if no cassette is inserted:


The wires to disconnect the pickup-head:


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It really is interesting indeed, because no soldering is needed when used with the 'play empty CD on repeat' method. I think the pins might have been used for testing the board, or it's just a standard board and this was a CD input or something.

The pins look like these (in the Pioneer unit there are just 3 pins):


And I think a cable like this fits it, this one has 4 inputs but you get the idea. This cable is used to connect a CDR's audio signal to the motherboard in a computer:


I'll be opening up the unit in the future again to make it look clean and maybe attach a separate DAC to it via this: the Onkyo NS-D1

I might post a step by step procedure then.

Cheers, Maarten

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  • 6 years later...

Hi All,
I have successfully wired in an aux 3.5mm audio jack to a first gen LS400 Pioneer head unit via the AM/FM radio audio circuit.
When a music source is connected to the 3.5mm jack, the audio is played together with the radio on both AM or FM channels. To fix this I simply cut the audio channels on the printed circuit board for the AM radio (I don't listen to AM!) so that there is no longer any AM audio signal being fed to the processor.

Therefore, this method converts the "AM" button on the head unit to effectively become the "AUX IN" function.

Here is how I did it:

80-img_20171102_223358_74032685b17caed79715c919c5f70e0984ab0730.jpgOriginal Pioneer LS400 Stereo
80-img_20171102_223438_d61f9062b794e61bf81d81ea42f6579afe296f72.jpgRemove the upper AM/FM circuit board from the radio

80-img_20171102_223453_efc5320a6925eb8b877c14c2b4c656d3f8d8f193.jpgLocate TUR and TUL audio circuits

80-img_20171102_225248_63782951c7aa55f57440b66d52bfd90615cbf653.jpgSolder in your Left and Right audio circuits of the 3.5mm audio jack to TUL and TUR circuits.
Connect the ground wire to a ground point on the PCB (grey wire in pic).

80-img_20171102_231122_744653b1cfdb13652cc7fa0d34f189b73d209c26.jpgLocate AM audio circuits AMR and AML.
Cut the tracks on the PCB using a knife to create an open circuit.
AM radio audio will no longer play through the stereo system.

After carrying out these small modifications simply reassemble the head unit and refit back into the dash. Route your new auxilary cable to your preferred location, select AM and connect your music player.


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