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Need diagnostic help with OEM head unit and CHA-S634 cd changer

1 March 2005
San Diego, CA
Hi guys,

I could use some help in figuring out what is wrong with my OEM audio. I am trying to install an Alpine CHA-S634 cd changer (purchased used), but I'm having weird sound issues.

Here's my setup:
CHA-S634 CD changer, not yet mounted.
Ai Net to M-Bus adapter
Acura to standard M-Bus adapter

Symptoms: The left channel sounds weird. There is an echoey, ringing distortion in the midrange. It is more obvious on vocals--they sometimes almost disappear or sound like they were removed, leaving only a reverb effect. If you move the balance all the way left, the sound is distorted/muffled. If you move it to the right it sounds normal. This problem is only evident when using the new CD changer. The OEM CD changer does NOT seem to have the issue. Reconect the OEM changer, and the left channel sounds normal. If you play the radio (either a station or with a bluetooth FM transmitter), the left channel is also normal.

Possible culprits:
S634 cd changer
left Bose amp
head unit
one of the adapters

My opinion is that if the radio and OEM changer sound normal, then it probably isn't the left channel Bose amp. Then the question becomes why does the OEM cd changer sound normal and the S634 changer sound weird? I thought the new changer might have a problem, so I sent it in for repair at Alpine. They said they couldn't reproduce the issue, but they replaced it with a refurb unit. The refurb unit came with cosmetic shipping damage, but I plugged it in and tested it and it showed the *same* issue in the left channel. I returned the refurb for a replacement. When the second refurb unit came in, it also showed the same weird left channel, so I have to conclude that the CD changers were ok.

The next most probable item I think would be the OEM head unit. This is a 1999 model, so it's pretty old now. I am imagining there could be a problem with in the input that handles the M-Bus signal. However, if the head unit is the problem, I would expect the OEM cd changer to also sound bad, and it doesn't. I don't know if there is a difference in the signal between M-Bus and Ai Net via a converter.

The cable adapters are probably the most reliable parts so I don't suspect them.

Any thoughts would be welcome. I'm considering getting a Grom Bluetooth adapter but I'm worried the same problem will happen, if the problem is in the head unit.
Last edited:
Acura swapped a couple of wires around to require a PnP install for an OEM changer.

You can easily make, modify OR repair your own.


I would look at the adapter, I suspect you have a connection issue with "Audio Left", which should be Pin 5. Search up on "m bus pinout", "DIN 8 pin circular"

Honestly, though the days of CD are gone. It is time an MP3 device or an interface with your phone. You can cosmetically install the changer and hide your new box behind it. The mbus can handle 594 songs and a new device has no mechanical parts....and probably costs less than your mbus adapter.

Hmm, looking at the adapter pins 2 and 4, when I use a multimeter to check the pins in my adapter, the pins don't appear to be swapped. So pin 2 at 6 o'clock has continuity with the 6 o'clock pin on the other side. Same with pin 4, it goes to the matching pin, so maybe my adapter is wrong. This might be the answer!