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CD changer audio sound quality problems after changing head unit

Poor quality sound on the cd changer input can occur after the headunit has changed from a second generation headunit to a third generation headunit or vise versa. The fix is simple and requires pins 2 and 3 to be swapped on the head unit connector.

Second generation headunits

volkswagen_rcd_510_main_N91-10016Multi-pin connector 4, 12-pin, for CD changer control and CD audio input signals

1 –  Right headphone output signal, positive
2 –  CD changer, left and right channels, negative
3 –  Headphone output signal, audio earth
4 –  CD changer, voltage supply, positive
5 –  Left headphone output signal, positive
6 –  CD changer, bus DATA OUT
7 –  Not assigned
8 –  CD changer, left channel, positive
9 –  CD changer, right channel, positive
10 –  CD changer, control signal
11 –  CD changer, bus DATA IN
12 –  CD changer, bus CLOCK
1 –  AUX signal input, left (yellow)
2 –  CD changer, left and right channels, negative  (black)
3 –  AUX signal earth (green)
4 –  CD changer, voltage supply, positive
5 –  AUX signal input, right (grey)
6 –  CD changer, bus DATA OUT
7 –  Not assigned
8 –  CD changer, left channel, positive (yellow)
9 –  CD changer, right channel, positive  (grey)
10 –  CD changer, control signal
11 –  CD changer, bus DATA IN
12 –  CD changer, bus CLOCK

Third generation headunits

Multi-pin connector 4, 12-pin, for CD changer control and CD audio input signals

1 –  AUX signal input, left (yellow)
2 –  AUX signal earth (green)
3 –  CD changer, audio signal earth
4 –  CD changer, voltage supply, positive, terminal 30, contact continuous load greater than 1 A, temporary peak load 5 A
5 –  Not assigned
6 –  CD changer, DATA OUT
7 –  AUX signal input, right (grey)
8 –  CD changer, left audio channel, CD/L
9 –  CD changer, right audio channel, CD/R
10 –  CD changer, control line, switched positive
11 –  CD changer, DATA IN
12 –  CD changer, CLOCK (internal check protocol for data flow monitoring)

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