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Replacing PCM 3.1 with standalone Bluetooth receiver

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Hi all,

I'm in the midst of planning a bit of an unconventional PCM replacement and wanted to see if I might be able to get some feedback on a few questions that have popped up as I've gotten along in the process.

Long story short, the PCM 3.1 in my 981 GT4 has been replaced twice now due to the prior units essentially going into an endless reboot cycle. And before you ask, it's not the XM problem—the PCM doesn't even power on long enough to be functional, essentially like the PCM in this 991. 

I'm now on the 3rd unit and am pretty much done with Porsche OEM head units; the latest replacement in there now was installed last week and works fine, but I consider it a ticking time bomb given past experience and am planning on simply uninstalling it preemptively and replacing it with a Bluetooth receiver that connects directly to the speaker system (the Out of Sight Mark 4 Bluetooth receiver, to be more precise). My reasoning for this is that nearly all of my music & nav audio is driven off of my phone at this point, so a head unit is really just a conduit to connect my phone to the audio system—I derive virtually no function from the head unit itself. (I also acknowledge that jslo73's HDD to SSD swap from RL/P9 would possibly be a solution, but I'm already past the point of even wanting to use the PCM full stop)

Here's a list of parts / equipment that I've purchased / am in the process of purchasing for this little project:


Broad strokes, I think the Out of Sight unit should be pretty plug and play, but I had a few questions that I was hoping folks here might be able to help with.


  1. What circuit(s) is the 12V radio power connected to? – I'm going to test with a multimeter once I have everything apart, but in terms of the power output coming from the Quad Lock connector, is it strictly on the Ignition/Run circuit? I ask because I notice that with the harness adapter wiring diagram it calls for you to plug in an accessory signal line into an empty fuse; however—given that the OEM head unit doesn't have this signal wire—I'm wondering if the 12V line coming from the Quad Lock is hooked up to both the Accessory and the Ignition/Run circuits.
  2. What wire(s) is the integrated hands-free microphone hooked up to? – Given the Out of Sight unit has a mic input, I'd love to make use of the preexisting hands-free mic in the steering column, but haven't been able to find any info regarding how exactly it's wired up to the PCM in the first place. Anyone have any info on this? I'm operating under the assumption that I'd be able to splice the wires to a 3.5mm TRRS microphone jack which I could then plug into the Out of Sight, so I'm crossing my fingers the integrated mic isn't wired up in some kind of exotic fashion.
  3. Glovebox USB / AUX input—how are these powered? – When my last PCM died I was puzzled when the USB port in my glovebox no longer worked, but then the tech at the dealership clued me in on the fact that the glovebox USB / AUX connections are powered from the PCM itself, so it'd make sense that when the latter no longer works, neither will the former. AUX is essentially going to be useless in my new application, but it'd be nice to still get power to that USB port for charging. Anyone know how that's wired up so I can get power to it?
  4. Amplifier wake-up signal wire: where does it connect to?? – The Suncoast wiring harness instructions are kind of vague on this, and there's like half a dozen different things I've read about how to wire up the amplifier wake-up signal wire to the point where I'm utterly confused. Some say to connect it to the antenna power line on pin 4 or 5 of the Quad Lock? Which pin exactly are people speaking of?
  5. FAKRA connectors: what are they connecting to? – I've read a handful of threads that talk about the FAKRAs, but mostly gloss over what each and every one of these is / is connected to. Any insight (or, even better, a diagram!) on these—if just for my own self-edification—would be much appreciated 👍🏼
  6. Should I hook this up to the amp instead?– I realize hooking all this up directly at the amp would probably be more straightforward wiring-wise, but I'm trying to make this as easily reversible as possible in the (however unlikely) event I ever want to swap the PCM back in. All the same, if anyone has any reasons why wiring things up to the amp would be more advantageous, I'm all ears.
  7. Am I missing anything? – As evidenced by the above, there's a ton I don't know about how all of this works (or really car audio, for that matter—I've never worked on automotive sound systems, so this has been a learning experience)—everything I've gleaned has been from the Suncoast radio harness wiring diagram, and a patchwork of searching the forums for folks that have done aftermarket radio swaps. That said, are there any blind spots I ought to be aware of? I just want to make sure I'm doing all my homework before removing everything.

Thanks for any info or feedback any of you can provide!

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  • 8 months later...

So this project finally got tackled this week!

Since I’d originally posted, another forum member had got in touch with me saying he was undertaking the same swap and was running into a couple problems, namely that the minimum volume level was too loud and that there was a ton of crackling/popping. I’ll address those further in the post, but first I wanted to answer the questions that past-me had:

  1. What circuit(s) is the 12V radio power connected to? – the PCM is on a constant / always-hot fuse. The PCM itself controls the amp wake-up signal, so I wired the amp wake-up separately to a free switched fuse slot in the left-side fuse panel using an add-a-circuit tap. If you leave amp wake-up hooked to a constant fuse (e.g. if you tap off the PCM power wire), the speakers all draw about 5V, which is probably enough to kill a battery (I didn't check amps, but I think it'd be enough where were you leave it over a long weekend, you'd probably come back to a dead battery).
  2. What wire(s) is the integrated hands-free microphone hooked up to? – Never figured out any mic stuff, but for anyone interested the mic wiring is in the blue plug on the Quad Lock connector (lower left): 

    Imgur: The magic of the Internet

    (The above pinout is courtesy of an excellent thread on retrofitting a PCM4 in a PCM3.1 car)

    You can see there’s two mics indicated (which is evident on the top of the steering column), as well as a shield (which presumably gets connected to ground? I’m an automotive electrical neophyte, so forgive my ignorance). My plan at some point is to take apart the mic wire that came with the Out of Sight and wire up the positive connections & gang up all the negatives + shield on the 2.5mm plug to see if that is able to work; here's an article I've found informative on the subject.
  3. Glovebox USB / AUX input—how are these powered? – Also didn’t work on any USB stuff this time around, but in the pinout above you can see some pins on the green plug on the Quad Lock labelled as “IPOD CHG” which I would presume would be for USB power. However in the service manual wiring diagrams (see attached PDF) there’s a wire that’s labelled “VUSB”, which I presume to mean “voltage USB”? That bit honestly has me confused, so just as well I didn’t decide to tackle the USB port this time around. Anyone with knowledge on the USB wiring, I'm all ears.
  4. Amplifier wake-up signal wire: where does it connect to?? – Pin 4 on the blue plug! (Labeled "AMP On" in the pinout) As I said above, probably best to wire it to a switched fuse in order to avoid power drain.
  5. Should I hook this up to the amp instead? – When I was running into issues with buzzing/popping I considered this, but as you’ll see below there’s a solve. Short answer: yes, you could wire to the amp, but you’ll be taking out a seat, then splicing into the wiring harness itself or wiring up a plug to go into the amp. On balance, it’s probably easier (and less destructive) to just do all the work in the head unit area.

I’d mentioned another forum member that had gotten in touch saying he was having issues with a loud minimum volume and crackling/popping when the amp was switched on.

The former issue is because outputs on the left side of the Out of Sight (i.e. the ones on the green block) are at speaker level (vs. line level); given that every Porsche sound system that’s Sound Package Plus and above has an amp (found under the passenger seat), the system is wanting line level output. This is further confirmed from this user who “[changed] the outputs [coming out of] the head unit from the speaker outputs to the preamp outputs”. If you have a CDR-31 I believe you'll want to use the speaker level outputs.

To solve, I used the pre-amp RCA outputs on the right of the OOS unit, which output at 4V vs. 12V. For this I got some RCA plugs, ganged up all the positives and negatives of each side, and soldered them to the respective RCA lead. This results in a quieter max volume, but it's sufficient for my needs. If in the future I want something louder, I think I could hook everything up to the speaker level outputs and put a potentiometer between the OOS and the wiring harness to act as a volume knob. It's something I've briefly looked into, but I wanted to just figure out the basics the first time out.

For the buzzing/popping on amp wake-up: all the stock amps under the passenger seat directly contact metal, which creates a ground loop condition. I hooked up a ground loop isolator between the Out of Sight and the speaker harness adapter and it killed the buzzing right away. There's still a little buzzing, but that kind of high frequency noise always been present in the sound system even when the PCM was installed. The ground loop isolator also has some additional ground wires that I didn't splice into the ground on the harness this time around, but maybe I'll give it a try next time I'm in there to see if it helps.

And that's kind of it! Here's a photo of the entire setup before it went in: 


Imgur: The magic of the Internet


It takes a little cajoling, but it all fits really snugly behind the dash. See here for a quick video of the unit in use. You can see that the amp is powered off when the ignition is turned to "off", and powers back on in "accessory".

Some odds and ends that I found useful: 


  • Tesa wiring harness tape really helps in tidying a project like this up so that it doesn't look like a rat's nest
  • I got a ferrule crimper for the power and ground leads and it similarly helps to keep things tidy and provides a very secure connection
  • Having a pack of T-tap connectors on hand is really helpful (you can see one tapped into the yellow power lead that I initially used for the amp wake-up signal; it's unused now, but could come in handy later)
  • Terminal extractor keys are super handy if you need to pop out any terminals in the Quad Lock plug (I didn't need the antenna wire, so I took it out instead of leaving it dangling)
  • Wire strippers!! Oh my god, why didn't I get a set of these sooner?! For years I've been using wire stripping pliers, but with stranded wire you always end up cutting a strand or two out. Proper wire strippers make things so much easier.

Anyway, that's it for now—if I ever decide to tackle the built-in microphone, I'll be sure to post an update. If you decide to take on this swap, please post your results!

981 Porsche Cayman - Nav Wiring Diags.pdf

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