Interfacing with NC Steering Wheel Controls

[ProjectMia]

Car: 2006 Mazda MX-5 (NC)
Existing Stereo: Standard Mazda Stereo with CD changer (non-bose)
New Stereo: HIZPO 7" 2 Din Android 9.0 Car Stereo GPS WiFi OBD2 Head unit Radio DAB+ USB Camera (ebay item # 192387041042)

Problem: I want to use the Steering Wheel Controls with my new stereo. When connecting the new Stereo's Steering Wheel Control wires to the car's steering wheel control wires, the stereo cannot detect the steering wheel button presses when in enroll mode.

Reason: The car's Steering Wheel switches work off varying resistance, depending on what button is pressed. The switches are connected via a control box that is behind the switches in the steering wheel (no need to remove these!). There are 2 wires running from the control box to the stereo loom plug. They are Blue/Black and Green/White. You can confirm you have the correct wires by measuring the resistance between them. With no buttons pressed, you should see 5k ohms. Each button will change the resistance to less than 5k. When the factory stereo is connected, you will see approx 5v when no buttons are pressed between the Blue/Black wire and ground. As buttons are pressed, the voltage will change on your multimeter. The new stereo appears to supply approx 3.24 volts on both SWC-1 and SWC-2 wires. Connecting these both to the car's steering wheel wires, as I did, will produce no effect, because both sides of the switches and resistors are the same voltage. Connecting only one will produce no effect, because the return wire from the Steering Wheel Controls goes nowhere, so there is no circuit. Not knowing how this system worked coming into this project meant many hours and google searches which largely produced nothing. I finally found a downloadable service manual on ebay for about $11AUD that allowed me to see how this works and the associated wiring. I would highly recommend to anyone. Cheap and the moment you pay, a download link is sent to your email (https://www.ebay.com.au/itm/OFFICIAL-WORKSHOP-Service-Repair-MANUAL-for-MAZDA-MX-5-MIATA-2005-2014/152849250219?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649).

Solution:
1) Connect the SWC-1 wire from the new stereo to the car's Blue/Black wire in the stereo loom plug. This will supply approx 3.24 volts to the steering wheel switches.

2) Connect the return wire from the switches (Green/White) to ground (0 volts). This provides a voltage differential across the switches and resistors. As each button is pressed, the voltage changes and new stereo can detect this.

3) Do not connect the SWC-2 wire from the stereo, it is not required for this car. Tape the end up so it doesn't short on anything.

4) Power on the new stereo and go to the settings>Car Infotainment>SWC page. Tap a button on the stereo screen and then press and hold the corresponding button on the steering wheel. If it is detected, the button on the stereo will remain dark blue. Rinse and repeat for the remaining buttons. Note that the MX5 has limited steering wheel buttons so you will on enroll about 6.

5) Your buttons should now be able to control the new stereo!