Testing Oil Pressure Gauge and Sending Unit

[Oldbluesman]

Having just gone through all of this I’ll pass on what I have learned. You’ll need an OHM meter, 100 Ohm resistor (Radio Shack 5 for $1.50) and some test leads with alligator clips on each end.
Test the gauge. Remove the sensor wire from the rear of the gauge. Clip onto the gauge sensor terminal with a test lead. Have another test lead clipped to a ground ready. Turn ignition key on. The gauge should peg out to max. Touch the sensor terminal test lead to ground, the gauge should go to zero. Turn key off. Connect test lead from gauge to one side of 100 ohm resistor, connect the other end of resistor to ground. Turn key on. The gauge should have a reading. Note the gauge reading. Turn key off.
Test the sending unit. Disconnect the wire from the sending unit. Clip one end of a test lead from the sending unit terminal to one lead of the Ohm meter. Connect the other lead of the Ohm meter to ground. Start the engine. The Ohm meter should read around 100 Ohms, give or take. No reading and the sensor is bad. Turn off engine.
Test the wire from sending unit to dash gauge. Assuming the gauge is good, reconnect the gauge sensor wire back onto the gauge. Disconnect the wire from the sending unit. Connect the wire just removed to one side of the resistor. Connect the other end of the resistor ground. Turn key on. The pressure on the gauge should read the same as it did above when the gauge was being tested.
What confused me was that I found an oil pressure switch on the engine block above the starter. The wire from this goes into the Engine Control Model and has nothing to do with the dash oil pressure gauge. The sensor for the dash gauge has to be located. On mine it was right above the starter, hard to see without sticking my head up in there. Monaco tech told me to look near oil filter, mine was not there.
To summarize, with everything connected a gauge that reads zero with the engine running means the sensor wire is grounded (or of course no oil pressure or port is blocked) . If the gauge is pegged to max it means the connection is open, not “shorted” or grounded. If everything was working ok and the wire from the sending unit was removed (with engine running) the gauge would peg out to max.
Also learned the different types of sending units. One type is just a switch, like for an idiot light with one terminal. Oil pressure breaks connection to ground, or would make an electric gauge peg out. A resistor type (one terminal) that varies resistance with pressure for an electric gauge. And a combo type (2 terminals) with a switch and a resistor. I replaced a combo type, one terminal going to the Low Pressure Warning Light on the dash, and the other going to the dash oil pressure gauge.


2002 Monaco Dynasty, ISL 400