Oil Pressure gauge for you gear heads

Just looking for some reassurance and info.

1995 454
I have two oil pressure gauges. One went to zero (mostly, but read good once in a while) and the below deck one was reading erratic but always showed 40 psi.

I replaced the oil pressure sending unit and the lower gauge always works. 40 to 60 psi. The top gauge always seems to work at start up but drops to 0 at some point.

I’m pretty sure it’s either the gauge or the wire.
So how do I check the gauge?

Am I missing something?

Thanks!

Can you swap places with the gages?

Thanks Dutch, I just thought of that!

I just need to move the wires from the starboard upper gauge to the port gauge and that should give me the answer I’m looking for.

Are your engines Mercruiser? I know some of those had two oil pressure senders offered, a standard sender for a single helm and dual for dual helm. Did you get a dual helm when you replaced it? Easy way to check the gauge function is to ground the post on the gauge that the sender wire connects to. The old service manuals always had a resistance chart in them as well that told you exactly how many ohms the sender should read at specific RPM’s whether you have a single or dual helm sender. You might be able to dig that up online or talk to a mechanic with access to that old literature.

Thanks Mr Hiker. They are crusaders but since the run on resistance it would make sense there are different sending units…another thing to check into.

If I remember correctly the Crusaders ran the same senders… for that matter essentially the exact same electrical system as the MerCruisers.