camaro430sut

iTrader: (2)

Im tring to wire in an autometer oil pressure gauge. I cant use the autometer supplied sender because I have no oil port to put it into on my block. The blocj is a 75 350. I have the factory oil pressure switch at the rear of the block up under the distributor. The oil port by the oil filter is plugged and cant be used. On the factory pressure switch there are 3 wires, orange, tan, and tan/white. I know orange goes to the fuel pump. I believe tan is to the guage. However when I splice the autometer guage onto that wire both guages just peg all the way to the highest reading. What am I doing wrong? Is there a better way to wire this?
Matt