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First let me say a great forum and resource for my first Porsche from a new member. While snooping around under the bonnet of my 99 C4 coupe I ran across 2 mysterious black boxes that were wired into the battery and taped to a harness in the compartment just to the left of the battery (directly in front of the particle filter). Upon removing the tape from the boxes found this.

1) One of the boxes is a 12V battery back up that connects to the main (bigger box) via a plastic molex plug

2) The main box had a 7.5A fused power wire (not hooked up) a ground wire and an antenna wire which attached to it was a aprox 12in flexible plastic antenna.

My initial thoughts are that this is some type of Lo-Jack device so the car can be found if ever stolen but I thought I would ask here in case anyone can help. It is most certainly an aftermarket device in looking at how crudely it is installed.

My question is that when I hooked up the wire to the battery and drove the car I found a few things 1) the voltage gage which normally reads 13V was at about 14.4 (not to odd as now this small back up battery is acting as some what of a capacitor and holding a charge in line) but more concerning is that my oil pressure gage which normally is at about 2.5 at warm idle was now sitting around 4 and when the motor was under any type of load would peg out at 5 where as it would not before. Now I can’t imagine that a battery is affecting the oil pressure so I would have to guess that the additional voltage is somehow affecting the gage cluster. Anyone ever experience anything like this before. The only other thing that is not stock in the electricals is a hardwired K40 radar detector but I would imagine that this cant be a part of that system because there would be no need for a battery backup.

I did read another post where a guy was experiencing higher than normal oil pressures if the gages are sensitive to voltage fluctuations this may be a good place for him to troubleshoot as well. Thanks for any input and regards.


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