1987 GMC oil pressure gauge

[cazdog]

I recently put a new 350 in my 1987 GMC. Things are going well except . . .
The oil pressure gauge is not working. So, I replaced the lower and upper sending units. I put a new electrical connection to the lower sending unit. . . Nothing. 
When I disconnect the lower sending unit the gauge will peg out to the right side of the unit. Do I need to replace the gauge or ?


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[bd]

What happens if you ground the 'lower' sending unit wire?  Does the gauge drop to zero?

[cazdog]

When I ground out the lower sending unit the oil pressure gauge hovers in the middle ish portion of the gauge.


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[bd]

Assuming you established a good ground to bare metal, if the lower sending unit looks like the image below, there are probably connection problems either in the gauge wiring, between the harness plug and the printed circuit (PCB) on the back of the cluster, or between the PCB and the gauge.  A faulty gauge, however unlikely, is also possible.