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Andy, I'm not a good auto mechanic and I didn't stay at a Holiday Inn last night. But here is my take: you have two separate problems. That may not be true but I would start with that assumption.

1. You will not get to see if a code is being set as a result of the rough idle until you can get a scanner to read codes. I'd ignore the engine problem until I fixed the scanner problem. To do that, I'd try multiple scanners. O'Reilly Auto Parts, for example, has scanners that they will, for free, hook up to your vehicle. Since you say that it is driveable, that is an option. There are a number of different types of scanners out there and it may be that one will work on your Jetta and another will not. Trouble shooting that difference about why one works and another doesn't might get ugly. If you get lucky, a different scanner might tell you why the connector isn't working. Some of them are self powered and smarter than others.

2. I'm always suspicious when someone else "helps" me. By that, I mean that the person who supposedly applied the radio "fix" to the car may or may not have done it correctly. The other evening after reading your thread, I did a couple of searches and found the wiring change that is supposed to be necessary for that problem. I'd think seriously about looking to see if I could confirm that fix. It may well lead you to the scanner problem AND the rough idle problem. I'm really nervous by your not getting a 12 volt reading with a verified 12volt source (since it works with pin #4) and a supposed ground. If i #5 really ground, you should be able to run a jumper wire from the negative battery post to it and read a direct short with your ohm meter. If you cannot, something is wrong and I'd look into that.

3. If you do get the scanner working and there are no codes set, I'd suggest that you have a basic problem. I don't pretend to know the Jetta but a lot of cars have individual coils for the plugs and you could have one or more bad ones. I do understand that codes are supposed to set when coils fail but I'd start with the plugs and then the coils and work my way back from there.

I had a really nasty intermittent speedometer failure on our RV. It went to a certified shop several times at great expense (thankfully not all mine) and still wasn't repaired. It took me a while to figure out how the CAN bus was supposed to work but when I did, I figured out the the "pros" were not even in the right area and I fixed my own problem the next night. It was a loose connector. There was a code set but it suggested the absence of road speed pulse from the transmission. It was simply a pulse transfer problem. The point of my story is that sometimes you don't see the obvious because you are being driven to make it more complicated than it really is.

Andy,

+1 to the suggestion to get the veehicle scanned for codes somewhere.

Not being familiar with Vdubs, I am not certain what your connector readings are telling you. OBDII is an industry standard but the standard does not limit different manufacturers from doing their own thing with the system. Not all codes are universal. If it was a Ford I could probably help you more.

One thing that has helped me immensely is the internet, but the trouble with the info out there is you have to dig deep using different search queries to find what you need. I fixed a problem on my 2003 suburban recently but it took me 3-4 hours to find the right information. Think like "I am having a problem with that thing-a-ma-bob that is hanging on the whatchacallit."

Anygunanywhere

AndyC wrote:No stone left unturned; it's simple information-gathering, and if nobody here has an answer I'm in no worse a position as before.

Andy,

I agree, I'd be doing the same thing. I was once poor and know all too well what it's like, not that I'm rich now. Maybe this is more helpful...

#1 thru #3 are general rules I use when working on electrical. Most electrical problems don't happen by themselves, they were helped along.
When working on electrical systems it's best to have both a test light and a DVOM, a high impedance DVOM can sometimes lead you astray. Never use a test light where the amount of current the bulb draws could damage anything, i.e. don't probe the computer with a test light. Test lights are cheap and one shouldn't cost more than about $10.

1) - Check all fuses - even if they seem unrelated. Missing fuses are also suspect. A test light is best for this.

2) - Look over the entire vehicle and anywhere you find that looks like someone has been working on it is suspect.

3) - If the car has been in an accident, my experience is that body shops are notorious for hacking things up when it comes to wiring.

4) - I think you should focus on why the OBDII connector is not working as it could potentially lead you to the solution to all of
the other problems. If not at least a scan tool will work.

5) - If the ground pin on the OBDII connector has power there are 2 options.
a)Power is shorted to that wire, however, for that to happen without blowing a fuse the other end would need to be disconnected.
This one is not likely.
b)Other circuits tie into that ground at a common point and someone left a ground wire loose. The power is coming from the other
circuits that also need the ground that has been left loose. This is the most likely and could cause all kinds of grief by
feeding back into other circuits.

6) - You should be able to safely ground pin 4 without any damage since that should be it's normal state. After grounding it see if
that makes any difference to the operation of anything. If no smoke escapes it's seems likely you've raised your odds of a
scan tool working, take it back to the nearest parts that offer a free scan.


Regards,
Larry

I'm probably physically close enough to you that I could put another set of eyeballs on your problem if you want. As I said, I never ever claimed to be an auto mechanic but I've been working on my own for over 40 years. Because of my computer background, electronics are usually not a problem for me.

If you get into electrical stuff that has you scratching your head, shoot me a PM and I'm sure that we can work something out. My only disclaimer - I'm a fair weather guy - rain, snow, and cold stop me in my tracks.