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?
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.
Ok Carolyn thanks.
There is also the clock which is easy to get to.
And ciggie lighter