I have a 71 442 with factory guages. The oil pressure reads backwards. With the key on and the engine off the guage reads high. Once the car is started the guage drops to 1/4 and when you rev it up it drops a little more. I have a manual guage hooked up also and it read 65 pounds at 2000 rpm.
Is it possible I have the wrong sending unit?
Thanks for any help.
Dave

 

If nothing else was altered, then it sounds like you do have the wrong sender, or a bad one. My sender did this one time. I tapped it once with a wrench and has worked ever since...

 

Sounds like you need the sending unit to decrease its resistance as oil pressure increases for your gauge to read correctly. There are a few other scenarios that could cause the gauge to behave like that, but most require a wire getting switched somewhere other than on the sending unit.

A Bullet proof way to test the behavior of your sending unit is to get a multi-meter that can measure ohm's. If you don't already have one, get one. A basic multi-meter is dirt simple to use, cheap, and they are a God-Send when checking for shorts in a cars electricals. Anyways, Disconnect the wire that is connected to the sending unit, then touch one probe (doesn't matter which) to where the wire was attached and the other probe to something grounded ( "-" battery post, car frame, engine block, any bare metal really) The ohms should read something around 100 Ohm's (maybe more, maybe a little less but not much) If the meter reads zero or only a few Ohms then your sending unit is definitely the problem. You can repeat the test with the car idle to double check that the unit is actually changing its resistance in the wrong direction.

If the sending unit does actually decrease its resistance (which would be the correct behavoir for your gauge) then something is boned up further "up stream".

 

make sure that the connector is tight on top of the sending unit.