1972 Chevy El Camino Oil pressure sender switch

OK now I have replaced the sender unit. I disconnected the wire from the sender and grounded it, the needle on the guage went in the opposite direction. This test confirmed that the sender was the problem, as before the guage went all the way to the right when teh ignition was turned on even without starting the vehicle.

I purchased a new guage and installed it, so I switched on the ignition without starting and the guage looks better now, just moves a little from zero to indicate there is power in the guage. When I start up the motor, however, the guage goes to the other end, maximum, all the way to the right.

Any ideas on what the problem is now, perhaps its still a faulty sender even though its a new one?