Oil Pressure Gauge

I am Having problems with my oil gauge. I have a 1993 Mastercraft Prostar. I bought the wrong oil sending unit last year. ( it was an automotive one ) It would peg the gauge all the way to the right when you start the engine. My new sending unit ( the right one ) does not peg all the way, but when I turn the key on, it goes to about 70-75 lbs. when I crank the engine, it stays close to that. I have taken the wire loose from the sending unit and grounded it on the motor. When I do this the gauge pegs, like it should.
Any one know why the gauge goes to 70-75 when the key is turned on.

You may have fryed the gauge with the wrong sending unit. If you can get an accurate pressure reading and compare it to the gauge, you could make sure. Or just replace the gauge with the proper SU.

When the key is in the start (NOT run) position, the automotive gauge/sending unit on the auto engine in your boat is switched into what's called "prove out" where all gauges and indicators turn on or go to max. It's supposed to show you that all the gauges are functioning, pre computer days It is working properly. So is the gauge. The sending unit may not be correct for the oil pressure that your engine puts out, there are basically two replacement sending units. 0-80 psi, 0-120 psi. Both are rated at 250psi, so it's just an accuracy thing. It's only real function is to tell you when you don't have oil pressure. Most boats have a HV/HP oil pump, so they take the 0-120 psi sender. It is available from Overtons for 20$ (thanks Shawn!)