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Discussion Starter #1
Hey all, I've scoured the forums and info libraries and haven't found a solution yet, so I'm hoping putting out an APB will help.

Short version: I'm looking for a wiring harness diagram/pinout for a Premium 6 radio. It's dead and I'm trying to determine if it's a wiring problem before I go replacing it.

Long version:
I have an '04 Passat 4Motion with a Premium 6 single CD, Monsoon cassette radio. The thing hasn't worked since I bought it. Totally dead. No display, no sound from the speakers ever, nothing. (Got the car on the cheap and out of necessity, so a working radio wasn't high on the list of concerns when I bought it.) I've pulled and tested the fuses (both #37 in the panel as well as the 10A on the back of the head unit) and both checked out fine. After pulling the radio and testing the voltages in the harness, I have a hunch that somebody's aftermarket moves resulted in some bad circuitry or shoddy connections. Reasoning: The voltages in the harness are constant, regardless of whether the key is in the ACC position or removed entirely. Also, I have 2 wires in the 12v range - I can't find one that is tied to the ignition.

I don't have any original manuals, wiring diagrams or pinouts, etc, which has made this process a bit harder. The harness in the back of the radio doesn't quite match up with anything I've found, which is another reason I'm guessing a previous owner made some changes. I do have the factory sticker with model number, etc, which is how I learned it's a Premium 6.

As a perhaps related issue, despite the fact the radio has never worked, I often hear what sounds like a CD trying to eject, or a disc changer changing between discs, if that makes sense. I assumed it was related to the dead radio, however I noticed that I still hear it even when the radio is disconnected. Sounds like it's coming from just behind or beneath the plastic radio support. I'm not familiar enough with the car (yet) to know what's in that area that could be responsible, or maybe it's a short?

Any and all insight would be much appreciated!
 

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There is no wire that's "tied to the ignition" for that radio.
As of ~September 2001 production (when they went to the double-DIN head unit), the "Ignition / switched" line was eliminated.
The big red wire is B+ (constant.) The radio is switched on/off via CAN message (body CAN - the smallest of the twisted-pair wiring at the radio connector. Colors, those I can't recall right now. Orange & orange/brown, maybe?)

You should be able to turn the radio on, even with the key out - just press the power/volume knob.
Nothing? Verify +12v and ground are OK (big red wire, and big brown wire.) I would highly recommend checking it with the radio actually connected (if you measure 12v across those two wires, with the radio disconnected), as some wire faults will only 'fail' with an actual load on the circuit.
If power & ground are OK, check all four output circuits (the other four twisted-pairs) for short circuits to themselves, each other, and to ground (with the amplifier unplugged.)
If all of the wiring checks out, then the HU is dead.
 

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Discussion Starter #3
Thank you! I double-checked everything with the radio disconnected, and I'm not getting anything at the ground wire. Should I assume the issue is a ground short?
 

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Negative (no pun intended) - a "ground short" isn't, by definition, possible on a ground wire.
It's open.

Just to confirm: If you measure voltage across the B+ wire and the ground wire, you're reading 0 volts; if you measure across the B+ wire and an alternative ground (e.g. the lighter socket), then you get ~12v?
 

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Discussion Starter #5
Okay, if you couldn't already tell, I'm a total novice in this area, so many thanks for bearing with me!

So - I rechecked everything, with both the ground wire as my ref point as well as another spot on the frame. I do indeed have voltage across the B+ and ground wire - about 12v. I then went through and checked the voltages across all the other wires (again with both the ground wire and then the frame). This was all done with the radio disconnected and ignition off.

If I understand your earlier post, now I should check the rest of the wires (there are 5 other wires in the harness), right? How do I test for a short circuit between themselves/each other/ground? Would that be measuring voltages or checking for continuity?

Also wondering about how to test it with the radio connected - once the harness is plugged into the back of the stereo, how do I probe the wires? I'm a little iffy about sticking the probes in the back of the harness (i.e. where the wires are inserted into the harness), so I just want to make sure I'm understanding correctly before I proceed.
 

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You have another four pair to check (eight wires total) - the speaker outputs.
(the 5th pair (which is significantly smaller wire than the speaker outputs) is the CAN lines.)
Simply unplug the amp (it's in the trunk - wagon behind left side panel; sedan is on the bottom of the rear shelf), and check each pair. With both the radio and amp unplugged, each pair should measure 'open' - both to itself (wire to wire), and to ground.

I can check my Bentley later (it's at home, in my wagon) for wire colors / terminal numbers.

Don't fret about back-probing the radio connector - it's easy, the terminals are easily seen (and, therefore, probed.) You aren't poking the wires - just get the test probes on the back end (wire end) of the terminals.
 

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Discussion Starter #7
With the ignition off and amp and radio unplugged, I did a continuity check on the radio harness wires. Everything measured as open (OL) (to ground) except for the ground (big brown). Does that make sense?
 

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That makes sense, and is good. (y)
You also checked each output pair (sorry, I haven't had a chance to get The Book out of the car - been busy here), to ensure that none are shorted to themselves (e.g. output, LF + & - shorted to each other)?

Sounds like the head unit (radio) died - it's old, and it happens. This isn't an unheard of failure mode.
I look at it this way - it's a good excuse to upgrade to a modern head unit. They're quite nice these days..... (I put a Kenwood DDX574bh in my wagon a few years ago, and have been VERY happy with it. Rough equivalent today would be the DDX396.)
 

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The earlier radios, at least, had a fuse in the back of the radio. Standard auto-type fuse, accessible without removing any covers. It would be worth checking (measure resistance of fuse between its two pins, should be almost zero).

Depending on your audio needs/desires, you should also consider just replacing the head unit with another factory unit. The stock units are really quite good if you just want radio or CD; and I've got a CD changer you could have for postage. Just be sure you get the radio code for any unit you buy!

Granted, if you want to annoy the neighbors or connect with Bluetooth you'll need different hardware. (There are some decent iPod interfaces that connect instead of the CD changer.
 
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