Mercy multimeter gauge and oil pressure gauge not working

[Jim Barrow]

2003 240 Sundeck w/ merc 5.0. When I turn on key I get the beep and gauges move. ( engine not running) Oil pressure gauge moves up slightly and Merc multimeter-gauge reads 4.4 psi, oil temp says 32degrees (80’s and sweating) and gauges don’t go up after starting - may actually go down to 3 psi. I have confirmed oil pressure with mechanical gauge of almost 50psi and goes up to about 60 if rev motor. I replaced the oil pressure sending unit, checked both neg ground blocks ( ohm test and circuit tester) and tight end connections. Confirmed ground ok at gauge. Disconnected, checked, reconnected all electrical connects on motor and gauges. Electrical diagram in owners manual doesn’t account for multimeter or match the wires on back of gauges.

There is only a single wire on sending unit, does it go to multimeter and share that with oil pressure gauge? I’m out of ideas?