Oil pressure gauge maxed out in run position and drops down to 100 PSI when running??

[El Campo]

My friend recently bought a 73 Pontiac Ventura with a 455 in it. The problem is that the oil pressure gauge was not working at all. Has Autometer Pro comp electric gauge with a stock electric sender (looks the same as most small block chevys have). He replaced the sender on the motor. Now when the key is in the run position the gauge is maxed out but when he starts the motor it drops down to about 100 PSI. Says the amount that the needle drops looks about equal to 25 PSI. It is a yellow wire that is going to the sender. Is this maybe the wrong wire or is something flipped? (Incidentally when this wire is grounded to the block the gauge maxes out). What is going on? Thanks for your time