stuck my voltmeter where it wasn't welcome
 

Im trying to find a source of 12v in my rear trunk to power a piggy back computer and some diagnostic stuff. I need something that only provides power when the ignition is on. Sounds like a simple enough task right? So I have my voltmeter handy I started poking around in the large plastic connectors (X connectors I think thay are called in the Porsche manual) and looking in my manual for a possible wire on the DME cable bus. So far all I have manged to find is a wire that provides power for about 5 mins then shuts off (secondary air supply possibly?). Then, last night, I managed to trigger something. Dont know what but I turned something on and it started humming in the engine compartment. I shut off the ignition and it stopped. This morning my car wouldn't start, like the battery was drained. So it makes me want to stop poking my voltrmeter in places its clearly not welcome. Has anyone got a idea on where I might find such a wire back there that I can splice into?

I have 3 questions for you:
1. Are you out of your mind?:D
2. What year is your car?
3. What exactly do you want to power?
I'll find something for you.

1. Possibly - but this seemed pretty sensible compared to some stuff Ive done. Im sure things would have turned out differently if I hadn't drank that Guinness.

2. 1999

3. An SMT6 and a wide band O2 controller made by 14point7.

Black/brown. Pin 56 at ECU.
Backed up by fuse B8 (at main fuse box. Driver kik panel).

Do you have a supercharged engine?

BTW.
The fuse is 15 amp,and bucking up the alarm,DME and tiptronic mudule (if your car is tip).
Not sure what is the power consumption of your setup, but if it was me, I would use the wire to trigger a 30 amp relay. You can feed the relay from the 40 amp SAP maxi fuse located on the bottom of the realay panel.

You need to be very careful where you bridge electrical contacts in these cars, as you found out. Many of the control systems, particularly the ECU, do not like stray voltage inputs or spikes. Rather than use a meter, a test light would have been a better choice.

A test light would be much worse! Much, much worse! Electrons allowed to flow through a conductor freely is not a good idea.

What you need to do is use a diode so that current only flows through the meter and into ground...but I'm not going to elaborate because if you didn't know that already, you shouldn't be poking around with a mm.

What type of MM were you using? A quality meter should have very high input impedence in Voltage mode, and not couduct any measurable current.

Which is exactly what a test light would be doing............. The problem with using a meter is most people bridge across two terminals or wires, not to ground. To be totally safe, a low range clamp meter would be even better, but most do not own one.

I would suggest buying a complete wiring diagram. This way you don't have to go poking around to find a hot wire - instead you can sit at your computer and know what you're looking for BEFORE you start on the wiring. This is how I found a hot wire in the front fender well to power my track transponder. Easy as cake.

+1
most of today's test lamps, are protected by an electronic circuit.
unlike the old school lamp that was made out of ... lamp, the new ones has LED's and/or buzzers seating on a protected circuit.
i use this one for basic stuff:
SST 42-Volt Test Light-SST42V at The Home Depot
and this for more advance:
Power Probe 3 Demo Video - YouTube

both are not cheap, but if you want to be safe, you need one of them or similar.
needless to say, i'm not affiliated with one or the other ( although i wish i was :D)

that's funny.
as soon as i posted my last post, i received an email from one of my techs.
this is the starter wire for the 2013 panamera.
going to the data base.

http://986forum.com/forums/uploads01...1354910245.jpg

wire is cut. no CEL no problems.

Thanks for all the help guys. I try to be careful and I was using a meter because I know they don't draw current from the wire you are testing. For that reason I also didnt think a lamp would have been better but an LED version would draw very little current. I had the ground side of the meter held at a ground point so I really dont know what happened. Thats where the Guinness comes in.

To answer some of the questions........

I have a cheap little digital meter from radio shack - nothing fancy

The motor is not currently supercharged but thats the direction Im heading. Im trying to get the controls and extra diagnostic stuff in place first. I realise Im going to have to look at how much current each component needs. Sounds like a wire from the DME bus is the easy solution (Meir suggested black/brown at pin 56) but pulling too much current from the is scary.

the wire i suggested is basically the 15 feed to the ECU (AKA true ignition ).
the information was taken from the circuit diagram in the Bentley book (highly recommended if you have any intentions working on your car yourself)
this wire also feed power to other element in the car except the ECU (as mentioned in previous post).
adding a relay will drop the load on that wire, and will allow you to use a separate fuse for your setup.
select the correct fuse based on the current drew.

X59 Plug pin location 4 White plug next to ECU (left of ECU) Red wire blue stripe fairly heavy wire come from ignition relay and turns off about 30 seconds after ignition is off. Does not come from ECU so can't hurt ECU. It turns on a half dozen other relays. I'm using it to turn on power to my Accusump solenoid ( .6 Amp) for over a year with no ill effect. I installed a 1 Amp fuse link after the connection. It come from fuse block C fuse 1, 25AMP fuse supplied from 80 Amp fuse at distribution block near battery.

Thanks jsceash. I found that red/blue wire and the wideband O2 controller is running great!

How exactly do you tie into the power source wire? Do you use a posi-tap or?
Thanks, Stephen

I sure hope your poking around did not harm something.

I was trying to use my voltmeter on the airflow sensor connect to check for a bad ground and I ended up frying the ECM.

I had to replace the ECM in the trunk/Security computer under drivers seat and the ignition module as they are all a matched set.

I fortunately found a set for my model and year for $350 from a dismantler.