I recently purchased a 73 convertible that someone had added a tach in place of the dummy light. I decided to buy a set of factory gauges for the center console and hard wire them in. We installed the gauges and used the voltage regulator behind the speedo and everything is fine when the key is on but as soon as we start the car the temp gauge pegs and the oil temp shows just below the high mark on the gauge. We orginally thought that we might have a bad sending unit for the water temp and changed it out. It did the same thing. I am not sure what wold be casuing the gauges to peg. We even double checked to be sure that we had the proper gauge sending unit and not the dummy light sending units. I have ordered a new regulator for behind the speedo and hope that might work. Any information would be greatly appreciated. We also found that the fuel gauge wire has a short in it somewhere and we are not getting power to the sending unit.

Steve  

"used the voltage regulator behind the speedo and everything is fine when the key is on "

Wire your(Aftermarket Gauges) electrical power(+) to the battery with an inline  fuse of 5-10  amps
Should solve your problem

Verne reel55