[speedygonzales]

Find out the operating range of the sending unit then duplicate the condition it would be in artificially, if the pressure is where you think it is.

For example, let's say for this example, the sending unit ranges in resistance from 10Ω to 100Ω and at 0PSI it's 10Ω and at 100PSI the resistance is 100Ω. Then extrapolate the resistance at the pressure you're looking for.

If we pick 30PSI and want to know the resistance (I can't stress enough for you to know the actual method and range of the sending unit) we calculate 27Ω. So you get ahold of some resistors at radio shack to add up to 27Ω and connect it in the cable in place of the sending unit. Then observe the gauge reading. If all else is good, but the gauge reads incorrectly, you know the problem is the gauge and not the sending unit.

Some folks are lazy and don't want to go through all that work so they get the sending unit and if that fixes it, they stop there. If it doesn't fix the problem, they get the gauge. But here's where they are really not doing themselves any good.

Let's suppose the numbers I used are correct above. That is 27Ω at 30PSI of oil pressure. You didn't test the gauge, you bought a sending unit and it didn't fix the problem, so you just went and bought a gauge. Now you put the new gauge in and your reading at idle is say 18PSI. Do you have any confidence in that reading?

Now let's suppose that due to manufacturing variances, tolerance or what ever, the gauge at 27Ω actually shows 20PSI and not 30PSI. You would have known that up front, you would be more knowledgeable about the operation of the system (gauges, oil pressure and sending unit) and you would not have wasted money buying unnecessary parts.