I am going to look at an issue a local guy is having with his 73 Road Runner, I have never seen the car before or met the owner but I was contacted by him through a mutual friend. He is having an issue with some of his dash gauges, he said the Fuel Gauge and Oil Gauge is pegged to the full or high side and I believe the Amp gauge is too, he said the tach isn’t working properly. The car is supposed to have a new dash harness and the factory style tach isn’t original to the car but it is a rebuilt unit from Year One and it has a new harness on it. He said he has checked all the grounds already and tried moving the ground wires to make sure he has a good ground and he said they seem fine, but he still has the issues. Any ideas of where to start with this one before I go over there?
Thanks.

dash voltage reg is broken. should be a silver rectangle plugged into the dash circuit board somewhere.
that will take care of oil and water, if amp is pegged too, then another issue will have to be dealt with.

Check the voltage coming out of the voltage limiter on the back of the dash & if is sending full 12V instead of the appropriate ~5 volts to the gas/temp gauge input terminals & if so replace it with an electronic version. The ammeter at full charge is a seperate full fielding issue. Check the voltage regulator ground & the VR itself may be bad. I'm assuming the batt ain't dead