Direct Bypass of HiFi Amp

[vmimarine]

I have a 2000 C4 (w/ PCM1 and HiFi package (non-Bose and speakers in door)) to which I am starting to do some audio upgrades.  While there are many methods with which to attack the problem, I am swapping the head unit first for personal preference.  I have read that the current amp will not function with an aftermarket radio, but want to make sure that is indeed the case.  In that event, I have contemplated using the Metra harness at the head unit and then just wiring a direct bypass at the amp (in theory, thus simplifying the process of installing an aftermarket amp in the future).  Has anyone ever done this, and if so, how?

[Richard Hamilton]

There is no reason why you can't use the existing (M490) amplifier with an aftermarket head unit.  There are several cheap adapter cables around which connect the line outputs from the new head unit to the car's wiring loom.

The problem with bypassing the amplifier is that the speakers are an unusual impedance.  IIRC the door speakers are 2 ohm, compared with the more normal 4 ohm.  I bypassed the amp on my old 996 Turbo, but I also replaced all the speakers.  I had to fit crossover units to the dash & door speakers to use them as the front channel, and I used a pair of large ellipticals in a box behind the back seats as the rear channel.  Not ideal, but it sounded OK.

[vmimarine]

Thanks Richard.  I appreciate the response.  For the sake of clarity, I have a 2000 C4 Cab Tip with M490 and M680 installed.  From an interior view, there is just the PCM with the tape deck and the Nav disk unit below.  A/C controls are already located on the bottom due to the PCM and Nav consumption of real estate.  I have not started disassembling anything. 

If I can retain and use the factory amp for the time being, that is fantastic, maybe not for sound quality, but certainly for time, money, and personal preference reasons.  My current plan is to leave the PCM1 in place, and put a single-DIN unit in place of the NAV controller (upper slot on lower console).  I purchased the unit (Pioneer DEH-X8800BHS) from Crutchfield and it has been shipped with a Metra 70-1787 harness as well as an antenna adaptor.

After perusing the uploaded wiring diagram (borrowed from another thread…appears to be the correct one) and then comparing it to the wiring diagram for the Metra harness (unsure whether it could be posted here as the diagram is a Crutchfield product), in my layman’s point of view it would appear that with some slight modification, the harness should “connect” the radio to the amp in such a way as to function.  However, I have read in several places that the amplifier is not compatible with aftermarket head units.  The statements have been that generic, so I have no other details as to why at this time.

 So in light of that, below are my assumptions as to the connections.  If you wouldn’t mind giving them a onceover, it would be greatly appreciated.

1. Anti-theft connection will have to be disconnected and permanently insulated once the PCM1 is actually removed.

 2. C1 Connector (Yellow)*

     PIN 1:  Rear Left (+)  ---> connects to --->  Violet RCA on Red Metra Harness

    PIN 2:  Rear Right (+)  ---> connects to --->  Green RCA on Red Metra Harness

    PIN 3:  Ground (-)  ---> connects to --->  Black wire on Red Metra Harness

    PIN 4:  Front Left (+)  ---> connects to --->  Gray RCA on Red Metra Harness

    PIN 5:  Front Right (+)  ---> connects to ---> White RCA on Red Metra Harness

    PIN 6:  12V+ Switched  ---> connects to --->  Blue and white wire on Red Metra Harness

 3. A Connector (Black)*

     PIN 1:  Unassigned (normally speedometer but not with M680 installed)

    PIN 2:  Unassigned

    PIN 3:  Unassigned (no phone prep on my car)

    PIN 4:  Terminal 30 (12V+)  ---> connects to --->  Red wire on Black Metra Harness

    PIN 5:  Antenna Booster  ---> connects to --->  Blue wire on Black Metra Harness

    PIN 6:  Terminal 58d**  ---> connects to --->  Orange wire on Black Metra Harness

    PIN 7:  Ignition Lock, Terminal 86s  ---> connects to --->  Yellow wire on Black Metra Harness

    PIN 8:  Terminal 31 ((-) Ground)  ---> connects to --->  Black Wire on Black Metra Harness

 *Crutchfield wiring instruction for Metra harness indicates yellow and red wire functions are reversed on my model year.  Colors listed above to match correct pinout using that assumption.

**Not entirely sure what this is…labeled as illumination on Metra Wiring Diagram

 Thanks again for the help.  Hoping this is all that is necessary for the moment.  I drive my car A LOT, and the added features of the new head unit would be most welcome.

996_Group_9_Circuit_Diagrams_Part_1 Radio.pdf

[Richard Hamilton]

Are you sure you have M680?  On 2000 996's it was the Digital Sound Processing module, which would normally be located in the bottom position of the centre console.  (On later models M680 was Bose).

I have a feeling you will have a grey/pink wire connected to black plug, pin 1.  This is the speed signal, and is fitted to most 986 and 996's.  Terminal 58d is the switch backlighting.  I am not familiar with the Metra adapter cable so can't really comment on the colours, but it looks similar to the one we have here in the UK is made by Autoleads which I have shown in the attachment.

At the end of the day, I think you are going to have to pull it out, to see what you have actually got.  My experience (with the Autoleads adapter) is that the black power plug on the car loom needs some re-working to match up to the standard DIN spec used on aftermarket units.  The RCA adapter cable is plug & play, apart from needing the switched feed.

 

[vmimarine]

M680 is definitely on the sticker in the boot, and I have access to DSP options (that do affect the audio playback) via the PCM1.  However, there is definitely not the control panel in the passenger compartment you are talking about.

[Richard Hamilton]

Well, you'll lose the DSP feature when you remove the PCM.  The wiring diagrams for the PCM are about as clear as mud, as I can't see the audio output wires from the PCM to the amp.  (Sheet 14A).  Unfortunately, I think you'll just have to pull it out, and see what you have got.

[vmimarine]

With any luck I can give it a shot this weekend and see how it goes. Regardless I am going to disable DSP on the PCM before I unplug everything. I'll let you know how it goes. Really appreciate the input. Thank you. 

[vmimarine]

So I called the audio installer (purchased an installation item from there that I will post as I go too) where I saw this note:

 

"PCM (factory double DIN) equipped vehicles with DSP of all years require a replacement amplifier when upgrading the radio.  The factory DSP amplifier is not compatible with aftermarket radios.  You can identify a DSP equipped vehicle by looking at the amplifier in the trunk.  It will have DSP written on it."

 

So after talking to him,referencing a pinout listed in another forum post here about a PMC1 to CDR220 swap and a post about upgrading to an M680 amp, and then pondering the issue some more, I think I have arrived at:

 

1.  The DSP "controller" in my setup is the PCM1. (intuitive I suppose based on the PCM1 DSP Options)

2.  The PCM communicates with the DSP amp through  PINS 6 & 14 on Plug II: "DSP Ground" (-) and "DSP Amplifier" (I assume data not 12V+) respectively. 

3.  Unless the DSP controller is in the loop, the amp potentially will not pass through a signal.

 

So I am wondering if the PCM requires power to support the data loop.  In other words, if I turn the DSP off before I disconnect power from the PCM, will I get a pass through signal on the amp?  Doubtful I think, but I suppose it's time for an experiment because...why not?  I'll let you know how it goes.

[Richard Hamilton]

What you say is probably correct, but the standard M490 amp has inputs for DSP, although they aren't connected. I reckon it is worth trying to disconnect the DSP and see if you still get sound. 

[zak996]

I have a '99 with hi-fi but no DSP. I completely removed the stock nokia amp and replaced it. That amp splits the door speakers so if you replace speakers also, you need to replace the amp if the speaker impedance is different. It sounds alot better with a new amp and it was easy-ish to do. My replacement 4 channel kenwood amp even fits in the same space. I did cut the harness for the amp to get access to the speakers but the wiring diagram for everything made sense. I also pulled the battery and on the drivers side theres a big firewall plug you just pop out to route new wires ( i made new dash speaker wires, rear sub and direct power).

 

I can't speak to the DSP,but replacing the amp would make it easier. 

Edited September 9, 2016 by zak996

[vmimarine]

Well, experiment finished and suspicion confirmed.  No sound signal is allowed through the amplifier with the DSP not in the loop regardless of whether it is turned on or off (powered or not).  So the M680 is indeed incompatible with an aftermarket amp when controlled through the PCM1.

 

Seeing no easy way to direct wire past the amp, it would seem that I am going shopping for an amp.  Is the impedance of all of the installed speakers 2 Ohm?

[zak996]

i checked the door speakers, they are 2 ohm. I cut up the fronts and backs I can't remember. Ill see if I have pics.

[Richard Hamilton]

18 hours ago, vmimarine said:

Well, experiment finished and suspicion confirmed.  No sound signal is allowed through the amplifier with the DSP not in the loop regardless of whether it is turned on or off (powered or not).  So the M680 is indeed incompatible with an aftermarket amp when controlled through the PCM1.

 

Seeing no easy way to direct wire past the amp, it would seem that I am going shopping for an amp.  Is the impedance of all of the installed speakers 2 Ohm?

 

That's a shame. You did have the Plus Switched (Black/Red) wire connected?

Edited September 14, 2016 by Richard Hamilton

[ahereng]

Hi,

I'm in the middle of the same process, replacing my PCM1 Hifi on my 2000 C4.

I have bought the Pioneer F980Dab, and this harness https://www.connects2.co.uk/Product/ProductItem/CT20PO02 that should connect the new headunit to the original Nokia HAES 6*40w amp. 

But I get no sound through the amplifier and to the speakers.  I have connected a separate speaker directly to the headunit just to confirm the headunit actually works, and I get sound on that speaker.

Does anyone know if there are any additional trick that has to be performed to "wake up" the amp? 

 

Alexander

[Richard Hamilton]

You need to connect a switched ignition supply to the blue/white wire on the Connects2 adapter plug.  This is the 'wake up' signal for the amplifier.  Most people splice into the aerial amplifier feed in the black power plug (The white wire, IIRC).

Edited January 27, 2019 by Richard Hamilton

[ahereng]

I have connected the blue/white wire to the antenna/remote output on the headunit, but no sound.  I also removed the "signals" plug on the amp, and verified that the power-signal actually got to the amp.  Since there was a lot of wires in that plug I don't know what is for, I was wondering if some of these should be connected to anything special in order for this to work.

[Richard Hamilton]

Have you checked fuse D9? (for the amplifier - 15A).

[ahereng]

Yes, double-checked today.  Fuse is ok.

I checked that the lineout on headunit actually produces sound, it does.  Also connected the blue/white on connects2 directly to power and connected my phone to the front-phono plugs, no sound from the amp sadly.

The amp worked when I removed the old pcm-unit..

 

[vmimarine]

This is the same issue I ran into.  The DSP controller is resident in the PCM in this setup.  There is a separate wake up signal AND a data signal that are needed for the DSP to function.  Unfortunately, there is no way to "convince" the amp that the DSP controller is in the loop and thus the amp won't produce sound due to the fault condition.  

 

I have not not found a way to avoid replacing the amp with an aftermarket solution.  My project is on hold until till then.