Custom Wiring for Use of Miata '91 OEM Head Unit with Sony CDX1000RF
Mark Foster
March 2004; minor update: Oct 2009

Here is a summary of the wiring cable I designed and built for my '91 Miata OEM HU and Sony CDX1000RF installation.  I elected to use a relay to switch in/out the line level output from the CD Player, rather than use the RF signal insertion.  I wanted the better sonic quality, and, more importantly (to me), closer behavior to the original configuration.  I also didn't want to have to worry about congestion on the handful of FM channels the Sony CD player supports. This design is derived from a similar one previously detailed on miata.net. The main difference is the use of a relay, instead of a manual switch, and use of the HU's power to operate the CD player, instead of splicing into the OEM wiring harness. My solution is completely reversible (no damage/change to original wiring), and works with the original unmodified HU.

As originally designed, the OEM HU accommodates an external OEM CD player by sending the audio signal out (pins 1, 3).  If the CD player is operating, it gates it's own signal back to the HU (pins 2, 4); if the CD player is off, then the audio from the HU is gated back to the HU audio in.  If there is no CD player installed, there is a jumper between pins 1 and 2 and between pins 3 and 4.  The relay allows a similar functionality to the original design: when the Sony CD player is operating, an "amp control" signal from the CD player is used to energize the relay's coil.  This disconnects the audio from the HU and connects the audio from the RCA line out of the Sony CD player.

One enhancement to this design was encouraged by Kevin Hennelly.  He wanted to also control his antenna motor, so the antenna is down when the CD is playing.  This can be accomplished with a 3PDT or 4PDT, instead of the DPDT I used.  Since I have a '91 (and no power antenna), I didn't have need for this.  But, as you can see, it's a pretty trivial change to the overall design.

The schematic was developed from information gleaned on miata.net and the '91 MX-5 shop manual.  Although all the documentation says pins 10-13 are "unused", they are most certainly connected inside the HU, since grounding pin 11 addresses the humidity-induced turn-on problem.  I left them unconnected in my implementation.
Update: although the climate I live in is fairly dry, I still periodically experience the humidity-induced problem; I would recommend anyone building this cable to ground pin 11.

schematic


The first picture shows the key parts for my cable.  The cable bundle on the left is the original wiring that comes with the Sony CD player (I bought mine new from Etronics.com).  To save a little effort, and to get a molded strain relief, I purchased an RCA patch cable, then cut off one end.  The shield of each conductor is connected to ground, per the notes in the schematic.  I used a Radio Shack 276-1101 diode to protect the Sony from any reverse voltage that can occur when the magnetic field on the relay's coil collapses.  I settled on the Radio Shack Double Pole Double Throw 275-249 relay; this one is kind of overkill for the low current of the audio, but it's what I could easily get. I cannibalized the 13-pin connector and pigtail from my failed OEM (Mazda/Panasonic) CD Player. 
components-sm

Note: I've seen sources for new 13-pin connectors with pigtails in a couple of places.  As of this writing:
http://www.mfjenterprises.com/products/mfj-5213  (for $16)  and http://www.farallon.us/webstore/54.html,  which is $15 on http://www.farallon.us  (search for item 8070).   If you're doing this, I would strongly recommend getting the connector with pigtails.  Those pins are pretty small and quite close together; you'll save a lot of aggravation and time for the modest additional cost.

wiring-relay

The picture above is a close-up of the connections to the relay.  I was able to tuck the diode right against the body of the relay. Note the orientation of the diode; it may appear to be "backwards" in the diagram. The diode is installed this way since you don't want it to normally conduct; only if there is reverse current from the relay, when it is shut off, do you want to bleed off. There is a bit of an explanation partway down this page: http://www.kpsec.freeuk.com/components/relay.htm. The anode (black side) should be connected to ground, and cathode (silver band) connected to the +12.   Also keep in mind when you're determining which color wire goes to which pin that the 13-pin plug is a mirror image of the socket (pin 1 starts on the left side as you look at the plug pins, on the right side as you look at the socket).  My schematic is from the perspective of the socket. 

 If you're doing as I did, and soldering the wiring directly to a relay that is designed for solder-through installation on a circuit board, be sure to affix something to the end of the relay pins.  They are fairly sharp, and with a modest amount of pressure, can poke through typical electrical tape.  I used some dense foam insulation, and trimmed it to size and thickness.

wiring-completed

This is the completed wiring bundle.  The 13-pin plug connects to the HU, and supplies power to the Sony CD player.  The RCA connectors are connected to the line-out of the CD player.  The four-pin plug connects to the CD player.  The ground spade connects to the HU.  

If I were making another one of these, I would shorten the length of some of the wires a little.  With the components removed from the dash, it looks like there is lots of space to tuck cabling pieces behind the devices.  While this may be true, getting the wires stuffed in "just so" is a lot harder than I would have thought.  It takes some patience and several tries to get everything reinserted with just this minimal bundle.  Folks with more goodies are in for a more challenging time.

Questions or comments? fosterm (at) sonic.net