How to test if oil pump is bad?

Temporarily hook up mechanical pressure gauge in place of oil. pressure sending unit. What makes you think oil pump is bad? Pretty hard to wear out oil pump in these boats with the low hours most of them get.

Bad or corroded wire connections can cause that.   Lots of threads here regarding this issue.   Neighbors 1990 sn has that same issue. Really annoying to drive his. Just waiting for some time to show him how to run a new positive and negative wire fromengine to dash. His voltmeter will drop to 10-12 volts if you turning anything on.

He has already been given that info but apparently he wanted a second opinion.

old thread

Since both the engine light and oil pressure gauge indicate a problem it was a waste of time changing the oil sending unit.

Thanks for letting us know! So why do you want to check the oil pump if the gauge is reading correctly?

Since the engine light is driven by the switches you can disconnect them, run the engine and see if the problem goes away. The switches short to ground when a fault is detected, either low oil pressure or high water temp. You can also simulate a fault by touching the wire to ground (engine block etc..) and the light/alarm should turn on with the key on.

The dash gets it's ground from the engine through the main harness.   You can also try disconnecting the main engine harness plug, cleaning it and reconnecting it. If you have some electrical contact cleaner then spray some in the connector before plugging it back together or use some isopropyl alcohol.. There is also a main connector behind the dash which you could also clean. The permanent fix for dash power and ground problems is to run additional power and ground wires from the engine to the dash.