Two questions here:

1. My oil pressure guage has been working intermittently. Where is the oil pressure sending unit and how to I go about testing it to see if it is my sending unit or the gauge? The previous owner had the sending unit replaced at the dealership in the past 6 months, but I am sure that won't cover my problem.

2. When it is working it marks between 1/4 and 1/2 on the gauge. What is the normal place that the oil pressure usually measures at idle and while driving?

Thanks in Advance for your reply.

Kevin

Just to the left of the oil filter. Mine barely shows above nothing at idle when the engine is warm. When running around 2500 RPM it covers the little handle on the oil can filler. Toyotas seem to be notorious for having innacurate oil pressure gauges. My mini always showed about the same as above and had it checked at a shop and actual pressure was fine. The manual says to remove the wire from the sender and ground it thru a 3.4W bulband the gauge should read "High". Resistance across the gauge should be 25 ohms. No resistance specs listed on the sending unit.

Intermittent readings are usually due to a grounding problem. The sending unit is fed by one wire that has a 12 v supply. The unit itself is just a rheostat that changes the amount of voltage that goes through it based on the position of the rheostat control, in this case, the pressure moves what is esentially a diaphram. Intermittent readings indicate one or both of 2 problems. Either the sending unit diaphram is not moving smoothly versus the pressure on it and/or the wire to the unit is not doing its job. A bad connection at the sending unit is one possibility. A worn wire insulation, say near an exhaust header or equivalent rubbing poinat can also cause the problem.
Chase the wire to make sure it is not grounding out somewhere. If the wire is clean and good, toss the sending unit and try another one. The previous owner's replacement of the sending unit leads one to think the unit is not the problem, the wiring is.