Power to gauges

[Gaz mr-s]

I recently wired a couple of oil temp & pressure gauges. They are not yet connected to the senders, only the +ve & earth. I looked in the back of the instrument binnacle & the easiest 12v wire to get to was to the plug behind the speedo. I teed a wire off to the gauges. When the engine was started the rev counter won't work. Disconnecting the gauges,- back to normal working.

I assumed the gauges must effectively been shorting the power via them to earth, but measuring them, they are each over 100 Ohms. Connecting them directly across a battery, they only draw milliamps. 

Can anyone explain what's happening, & is there a different 12V supply within the instruments that someone has used?

[Carolyn]

There is a big switched (red and white as I recall) wire coming off the back of the ignition switch.  It powers all the stuff that comes on with the ignition.  I put a piggy-back on that. 

You're messing with the voltage to your instruments and they are voltage sensitive,,,, hence the problems.

[Gaz mr-s]

Ok Carolyn thanks.

[m1tch]

There is also the clock which is easy to get to.

[shnazzle]

And ciggie lighter