Recently installed an oil pressure gauge and a water temp gauge from glowshift. They worked good for about a week after I installed them but recently my battery went dead and I had to get a new one. I installed it yesterday and after I did I noticed my oil pressure gauge was reading 0 or close to zero at idle. After driving around a bit and messing with stuff I realized what was happening. When ever the headlights were on or I pressed on the brake and the head/taillights came on, the gauge would read less, like it was getting less power.
So first thing I did was check my grounds and they were all good. Then I tried splicing the power from a different fuse and that didn't change anything. I am powering both gauges from the same wire which is tapped into the engine fuse in the fuse box, and was considering it could be that but I am not super confident in that being the case. But what else could it be?

 

Which fuses did you tap into, and where did you run the ground? I have a glowshift temp gauge in my car with no issues. They reference constant 12V, and switched 12V, and ground (as you had mentioned).

 

I was also curious about the ground. If the coolant temp sensor is on the radiator hose and not grounded, it would build up crud on the sensor and show an incorrect readout. From first hand experience.

 

I have since posting this hooked up a relay with a 14.3v output when the car is running, so its getting sufficient power from the battery and only drops by about .3v with headlights and other things on. It is grounded on the drivers side firewall under a screw that holds on a tube or hose support piece.
i don't think the temp sensor is being affected after checking it a few times, unless the difference is so minimal that I can't see it with my own eyes. Also the ground for that sensor is in a nice secure spot where it shouldn't build up any debris in the future.

I have another more in depth thread in the Int/Ext/Audio Forum where I have gone into more detail about everything I have done and how it turned out but TLDR; The grounds are good, the power is good, so all thats left it could be is the gauge being bad, which from the amazon reviews doesn't seem super unlikely. But I will find out today when the new one gets here.