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What order does the PCM2 MOST fiber optic system go in?

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Hi, wonder if anyone can help. I've got a fault somewhere in the PCM2 headunit, telephone module, autochanger, wiring, sat nav or amplifier.


I've read that the telephone module is often the culprit and gives you the 'system error - sat nav,phone, amp not available' type message.


What I am planning to do is loop out the phone module with an optical bridge and then get it coded out by my local specialist. As part of the fault finding though, I'd like to know what order they are connected in so I check the connectivity? All I know is that if I pull the plug out the amp I can see light on one of the cables (the input i assume). When I pull the plug out of the telephone module I see no lights so was wondering what is before the telephone module? Is it the headunit itself?


any help much appreciated.



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I think you are best to start the other way round.  If you get it plugged in to a PST2 or PIWIS tester, you can display a list of the equipment connected, and the order.  It also can display a list of required (coded-in) equipment.  If everything showing as fitted is also shown as connected, there will be fault codes stored which will tell you what's wrong.


I have experienced a couple of PCM2.0's which have lost their coding for no apparent reason, and restoring the coding has fixed the problem.


I have also heard of a couple of occasions where the fuse has blown inside the CD Changer.  There is a small glass fuse on the circuit board inside it.  You would know if it wasn't working if you couldn't eject the CD cartridge.


Depends which side of town you are, but if you are in the west, I'm not a million miles away and would be happy to plug in my PIWIS for you.

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Hi Richard, many thanks for the PIWIS...I'm over in Essex so not round the corner but if I'm down your way and can't get it resolved i might take you up on your offer!


I'm off to a local specialist at lunch who can tell me the error codes. He didn't seem to think it would show the order of connection but as he stated he code enable/disable certain bits he must be able to (not sure which diagnostic tool he will use)


If the glass fuse is broken in the autochanger wouldn't this result in permanent error? Sometimes the system works, sometimes it doesn't and sometimes it comes on for a few seconds and THEN sound dissapears and several seconds later it comes up with the error.


I also notice that when it's working and you pull the MOST connector from the phone module under the seat, I can see a red light on the cable. When it's not working there is no light so I was wondering if some module/wiring prior to the phone module. Hence me asking which order they run in


ta for the help, much appreciated


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Yes, I think you can rule out the CD Changer.  I didn't realise it was an intermittent problem, but the PCM should still retain any fault codes, so a diagnostic check will give you a clue.


Get them to go into the Coding sections of the PCM module, and look for "List of Required Components" and "List of Actual Components" (or words to that effect).  This will show you what is coded (Required) and what is connected (Actual).  This should tell you what isn't working, but of course it needs to be not working at the time of the test.  That might be the problem with having an intermittent fault, but at least the fault codes will give you a pointer.

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Hi Richard, of course, when I go to take the car down there...the system decides to work! He was using PIWIS and the check he was going to do was the 'MOST connection check'. Obviously with it all working, this came up fine.

  There were two stored error codes. One was a fault with the external phone handset and one was relating to battery/starting. (Sorry, I should have photo'd the screen or written the numbers down). The car doesn't have an external handset so he cleared that and then 'removed' the handset. The battery/starting one he just cleared so there are no fault codes present.

  He's reasonably flexible though so the next time it plays up I can nip down there as only 10 minutes from work.


Ta for the help



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Hi Richard (again). Right, had a tinker this evening and might be making progress. I pulled the connectors out of the telephone module, autochanger and amp and then shone lights down the in/outs on the MOST connectors to see where they went to. I haven't taken the headunit out but I could establish the amp leads to the autochanger and then this leads to the telephone unit. I assume the telephone unit then loops back to the headunit? Either way I can establish a path between the amp/autochanger/telephone.

  With the system in faulty mode I only have one red light i.e signal and that is the input to the amp. If I then look at the input to the autochanger i.e from the amp, there is nothing.


I then used my optical bridge that I had made and connected across the connector for the amplifier. This then removes the amplifier from the circuit so, in theory, any signal from the source should make it to the autochanger. Lo and behold, this is what happens, a red light at the input to autochanger. If I then plug in the autochanger, I then also get a red light at the input to the telephone module.


Turning the headunit off and on, instead of getting amplifier, autochanger and tel not available, I just get 'amplifier not available'. This suggests by removing the amp and bridging the in/out I complete the circuit and it is working (obviously with no sound because no amp)


I've looked at all the connections to the amp and nothing seems amiss. What I didn't do though was test the cable in the position it goes into the amp...maybe it can be bent into a position and not work? (I've made sure all the radii are as large as possible).


One thing I have noticed is starting is a bit sluggish and the battery voltage is showing up at roughly 11.8V on the dashboard gauge. I read somewhere that someone 'fixed' his PCM2 issue just by fitting a new battery. Of course, could be other factors there or just coincedence but have you ever heard of that amp being prone to flaky on boot up if voltage less than 12v? (i tried starting/resetting PCM2 from engine running/not running and no difference)








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Good work Sherlock. Sounds like you have pretty much tracked it down to the amp.

There is a software update CD for the Bose amp to correct sporadic sound failure, which might be worth trying. You haven't said if you have Bose, and what year your car is, by the way. If you have Bose (option 680) drop me a PM with your email address and I'll send you the instructions for checking the amplifier software version, and if you need the CD you can borrow mine.

If all else fails, I have a friend who may be able to help with a used replacement amp.

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Hi Richard, I did read about the checking the software with the 'main' and 'trip' buttons depressed simultaneously. I got this BUT the amp wasn't working at the time so would it be able to read the version? Do I need to wait until it is working and then try?


It's a 2003 with the 680 Bose option as below. Any help with the CD (if required) or sourcing a used reasonably priced replacement amp much appreciated...definate drink in it for you!




PCM2 basic module including radio 


PCM2 telephone (GSM) 


PCM2 navigation 


-MY 01 digital sound package
MY 02- sound system Bose analog 



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MAIN and TRIP won't show you what you need.  This is an extract from the document I would have sent you by email, which shows how to check the amplifier software version:


Vehicle Type: 911 Carrera (996)/911 Carrera 4 (996)

Model Year: Model Year: ‘03 (3) and ‘04 (4)
Concern: BOSE amplifier – Sporadic sound failure in vehicles with PCM2 (I No. 665, 666, 668, 670) and BOSE sound package (I No. 680).

- Modified software in BOSE amplifier MY ‘04 (as of the date of introduction).

Information Sound failure occurs sporadically (no sound playback of PCM2 during radio or navigation operation) on vehicles from MY ‘03 and MY ‘04 with the equipment mentioned above, which were manufactured up to the introduction date mentioned below.
In the event of a complaint, the software of the BOSE amplifier must be updated using the CD "BOSE Update-CD 2003-2; 911 & Boxster", part number, 000 043 205 50.

After the update, the software matches the version in vehicles from MY ‘04 (as of the date of introduction) and thus BOSE amplifier, part number, 996 645 333 05.

Software version:

1. The current version of the BOSE amplifier software can be displayed in the PCM2
display as follows:
After switching the PCM2 on, press and hold the SET button (Figure 1, Item 19), then press the "FM-AM"- (Figure 1, Item 1) and the "AUDIO" button (Figure 1, Item 10) at the same time. The Service menu is shown in the display. Select the "SW versions" function by turning the right rotary knob (Figure 1, Item 17) and confirm by pressing the right rotary knob (Figure 1, Item 17). Depending on the software, the following version is shown in the display under "amplifier":

"00011927" (or lower):
The software matches the amplifier software version for vehicles from MY ‘03 or MY ‘04 up to the introduction date.
=> Perform an update as is described in Item 2.

The software matches the amplifier software version for vehicles from MY ‘04 as of the
introduction date.
=> An update is not necessary.

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Ah, understood...I will get this info ASAP, thanks!

  I left the car with the good lady this morning as she is taking a break from work to got to some spa place. I might just forward her the instructions and ask her to let me know whether it's got MY'03 software or later...


Then again...I'll wait until I get home :)



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Well, firstly a massive thanks to Richard for some offline help that lead me the, hopefully, correct diagnosis. I ordered a s/h amp from 'boxsterbits.co.uk' who were extremely impressive (speed of delivery, communication and packaging/condition of item all perfect) and fitted in 5 minutes last night. Ok, so my fault was intermittent but the hifi wasn't working before and now does so, fingers crossed, that's cured it.


Finding out the order the components were in was key here and then bridging out the part with a fiber optic loop to change the error message finally confirmed where the problem was lying. For future reference the error message appears to list the components that won't be active from the first failing part i.e mine would say 'amp, autochanger, telephone not available' i.e in my case it was the amp that was then knocking out the autochanger next and then telephone. If the autochanger module was faulty I'd hope the error message would say 'autochanger, telephone not available'. Either way, using a fiberoptic bridge/loop confirmed this. For the UK, I got mine from http://www.cav-essex.co.uk/ for £15 posted




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Fingers crossed Richard, it's worked 3 times now in a row so I hope it was the amp. Next step, take the amp apart and see if I can find anything obvious....if not, give to the boffins at work who will take great delight in making it better than it was before.


Enjoy the bottle of wine ;)



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