Ever since I bought my car I've taken issue with the previous owner's audio install handiwork. Not only was it overkill for my needs, but it was a mess. I've also always hated the whole 90s era Pioneer Head unit installed.

I found the Continental tr7412ub-or radio for around $150 which has bluetooth, and a fairly OEM look. They also make a CD version as cd7426u-or which looks even more stock, but I liked the idea of the smaller form factor of the tr7412. The radio has no pre-outs, so you need either a line converter or an amp that takes line level input. For me, I don't need anything excessive for my AP1 front speakers so I just used the front as is. For the rear, I grabbed a small form factor Clarion XC2110 amp that I mounted to the sub box in the trunk.

Right away after the install, I'm much happier with how clean it is. The radio color isn't a perfect match to the S2000, but it's pretty close. Enough that I don't mind having the radio door open sometimes.

That being said, I wanted as much stock functionality as possible so I looked into getting the steering wheel controls working. I was going to buy a Modifry unit, but after talking to Bob, we weren't sure if it would work with the Continental. So I bought an ASWC-1 to try. Unfortunately, this did not work which put me down the road of learning how these steering wheel controls work and found it was quite simple with some reverse engineering. Each button on the dash has a specific resistance value it sends down a single wire to command the radio to do something. The radio has a set of resistances it looks for in order to do something (volume up/down, etc). Using a multimeter, here are the resistances from the S2000 dash control pod:

Pin 3, Gn/Rd, referenced to ground:
No Function - 10,000ohm
Volume Down - 47ohm
Volume Up - 307ohm
Channel - 900 ohm
Mode - 2,400ohm

Mute is special, Pin 14 Yel/Blu, when on and referenced to ground, it sends a 12V signal. I have not found any resistance that mutes the Continental so I'm leaving this alone. Though I am tempted to get a new label for the button and repurpose it.

I hooked up a potentiometer to the Continental Radio and swept through varying resistances to see what triggered what and this is what I got:
Volume Down - ~0-10ohm
Volume Up - ~110ohm
Source - ~165ohm
Seek Up - ~310ohm
Seek Down - ~460ohm
Next Station - ~690ohm

Now the trick is to convert the S2000 resistance network to the Continental Resistance Network. I was originally going to use a digital potentiometer with an Arduino Nano. A voltage divider circuit would measure the input resistance from the dash pod and then I would trigger an output resistance with the digital pot. Unfortunately, the resistances of the Continental are so low that any reasonable digital potentiometer won't quite work. Since I'm using known discrete resistance values, my current plan is to use an Opto-Coupler array with resistors.

I do also have a spare dash pod I will likely tear into and maybe just manually insert the resistors for each physical button. A bit more work to come back from, but it is a spare pod.

Maybe this will help someone in the future. I know it would have saved me some time!



Radio Pinout

Mute - 12V



Original Amps

Original Head Unit

Continental Head Unit

Continental Head Unit

Amp tucked under sub

Wiring tucked aside

 

Oh wow, I was just looking into this radio and doing the same research to make it work on my AP1. The lack of the mute feature is pretty annoying, but I wonder if we can send it to a relay that stops power coming out?

 

Yup! I was actually thinking about using a relay with the mute as a trigger to cut the ignition power to the head unit, but I don't actually use mute myself so I'll probably skip it.

May probe a bit more and see if I can find a remote mute on the Continental.

Also going to do some more probing to see if I can skip using resistors and just use a PWM signal directly to the Continental input pin. I'm away on business for a couple weeks but will report back on how that goes.

At the very least I want to determine the internal resistor of the Continental so I better understand the voltage divider circuit they're using...

 

from looking at the manual, terminal A3 in the headunit's connector is a telephone mute. These usually mute (or attenuate) when grounded,
so if you use a relay to convert the +12v on the car's mute wire to ground, you should be able to mute the headunit.

 

Good catch, don't know why I was expecting all the controls to be in the resistive band but this makes more sense. I'll test it when back. Should have read the manual sooner
​​

 

Well dang, I might have to buy one of these now if you can get mute working too. That's the only thing that really had me holding out for a different head unit.

Shame how single dins haven't really changed in the last 15 years.

 

Tested the mute, it works well grounding A3 in the head unit. I had to go to a clicky relay due to the 12V signal from the car not working well with the Arduino 5V logic and I didn't want to bother stepping the voltage down. All other functions work too, thought I need to restructure my code to get it more responsive.

Another note is the Batt and Ign wires are flipped from the standard ISO harness arrangement for some reason. Was easy to depin/repin that

 

Finished testing today. I ended up needing to use a mechanical relay to give a "no signal" to the head unit. The overall circuit is clunkier than I'd have liked, but most solid state switching devices I tried to use simply did not have a low enough impedance to not affect the signal to the radio. Next step will be to use a custom PCB.

 

Sir, you are an absolute legend! I have the Continental CD head unit in my Rover 75 and have been trying to get the steering wheel controls to work, no thanks to the manual which has no information at all about it. Thanks to your work, I am sure I will now be able to get it going. Thank you so much!

 

Nice work! I did a double take initially thinking this was an OEM radio. Blends nicely with the interior without shouting "look at me" as many (most?) aftermarket radios do with the door open and neon light displays.

-- Chuck