I hope this is the right place to post this.
I am using a Nordskog digital boost pressure guage (0-16 PSI). I can't get this thing to work. The sender is installed in the upper plenum. I have power and ground at the guage, and 225 ohms resistance from the sender with the car off. When the car is running I get a .25 volt back feed from the sender. I have isolated all the wiring and know that there is no cross feed. I think I made be picking up electromagnetic interference from the ingintion. Anyone with the same guage encounter similar problems? Any suggestions?

 

Here is an update:
I was convinced that I had a back feed, but could not find any. Well, the battery negative was grounded through the supercharger, I only had .2 Ohms resistance at the ground so I didn't think moving the ground would help, but I was planning on relocating the battery to the trunk anyway.
So I went ahead and moved it, thus moving the battery ground to the chasis. Low and behold the back feed is gone! The guage works properly now.
The moral: Good grounds=happy circuits.