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2012 Jetta TDi turns itself on!!

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05 Oct 2019 16:29 #34040 by BetterCallBrody
2012 Jetta TDi turns itself on!! was created by BetterCallBrody
Hello all, by the time I got around to posting this I had figured it out, so I wasn't going to post it. Then I thought "well that's rather selfish", So here ya go:
2012 Jetta TDi, customer complaint was "sometimes it wont turn off". They had just bought it from a car dealer who is a customer of mine, and he agreed to take it to me to get to the bottom of it and fix it. Of course he did a little Google-fu first and said it sounded to him like an ignition switch and he wanted me to toss one in there for him. I don't bother to argue with these guys anymore and they're always understanding when their diagnosis turns out to be wrong, so I agreed. Of course the car didn't exhibit the symptom while we had it so we put in the switch and shipped it. An hour later it was back LOL.

So now the real diag begins. Finally after almost whole day of turning the car on and off and running errands in it, it did it. It didn't stay running, but what it did was the instrument cluster came back on all by itself, flashed and beeped a couple times, then went back off.

I'm going to switch computers, this keyboard is awful

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05 Oct 2019 16:56 #34041 by BetterCallBrody
Replied by BetterCallBrody on topic 2012 Jetta TDi turns itself on!!
I established a pattern and was able to replicate the symptom from there forward, it would do it fairly consistently if you left the key on for 5 min and then shut it off. Right away I thought, maybe BCM is shorting internally, or something on the CAN is "waking up" the cluster. I scanned for codes in all modules and had several lost comm codes in various modules, including the cluster. Then I broke my own cardinal rule, I didn't go straight to the wiring diagram! Anyway, I decided to go down the CAN rabbit hole (which I'm not very familiar with) and saw that the CAN was flatlined for the most part. I thought BINGO I'm on the right track, then got nowhere for an hour or so and went home. That night I found SD's video about how VW doesn't run all their comm's through the DLC. Then I joined this forum LOL.

Next day I started over and went straight to the wiring diagram and saw that the cluster gets its power from an accessory relay and there is no "wake up" signal per se. I did some back probing and saw that during the event, anywhere from 2.5-4v would be present on that circuit intermittently even though the key was off. I checked for sticking relays, unplugged stuff and no change. I was leaning towards a short in the fuse box or the harness when I had shoved myself under the dash for the tenth time and noticed an after market looking wire way up in the dash. I gave it a tug and out popped a GPS tracker.....

I'm sure some of you know that GPS installer guys sometimes don't have a wiring diagram or even a test light at their disposal, and for those of you not familiar with German cars, in Germany black wire is usually NOT a ground.....

Yup GPS ground was wired to the power circuit of the instrument cluster, and back-feeding a small voltage (while trying to reach ground) and turning the cluster on!!

Sorry for the TLDR but I hope someone gets a kick out of this one!
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05 Oct 2019 20:08 #34045 by Noah
Replied by Noah on topic 2012 Jetta TDi turns itself on!!
Nice find! The whole cannon full-O-parts couldn't fix that one.
Thanks for sharing.

"Learn, apply, repeat."

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