Relay or Diode?

What I've done:

Basically, I am installing a switch on the voltmeter in my car so that when I flip the switch it sends power to the gauge.  This is so that I can check the volt's on the car, without having to put the key in the ignition and turn the car to on.  Currently my main wire running directly to the gauge.  This wire is only hot when the car is in the 'on' position.  I tapped into this wire with a constant wire that I brought off of the switch I was using, and placed a diode on the main wire (wire that is only hot when car is on).  This should stop the rest of the vehicle from coming to life when I flip the switch.

My Problem:

When I flip the switch with the car off, the gauge reads where it should and used to read all of the time.  However, when the car is running, it reads closer to 12 volts, when it used to read 13.8 volts.  Should I do away with the diode altogether and use a relay to activate the gauge instead.  Relay wired as follows:

• pin 85-Ignition (hot only when car is 'on')
• pin 86-Ground
• pin 87-12V+
• pin 87a-wire leading into normally open switch, then into gauge
• pin 30-wire leading into gauge

What I have assumed my problem is, is that I am getting a draw from the diode, and I was told that a relay would only show a voltage drop of less than 200 milliamps. 

Is all of this worth my trouble???

-Terry

This is just my Opinion, but I would end up getting a digital palen volt meter display and hook it up to your battery directly.  That way you can see the voltage all the time.  No need for switches and stuff.  Also, it will draw very little current.  I think RadioShack or some surplus stores in your area would have that in stock.  Being digital will make it a lot more accurate.  Plus, I think it looks cooler. 

Hope this helps

you mean the part where it says "pin 87a-wire leading into normally open switch, then into gauge", should be

Man i missed that completely, sorry