Story of the Oil Pressure Sending Unit

I have a 1949 Ford Tudor with a 1965 289-V8 engine. All the dashboard gages work except the oil pressure gage has been erratic. Sometimes it reads quite low, sometimes fairly high and sometimes about where you would expect, 40-50 lbs. The readings stay near one level for several days at a time, not jumping around on a regular basis as you might expect with a poor ground. I checked the oil pressure with an accurate external gage when the dashboard gage was showing quite low and indeed the pressure was in the 40-50 lbs range depending on RPM.

The conclusion is that a new sending unit is needed. However, the sending unit and gage combination is somewhat unusual. The gage appears to be original and moves very slowly as you would expect with the electrical/thermal action of a stock unit. However, when I purchased the car the gage was reading above 80lbs so I placed a 33 ohm resistor in series with the sending unit to get a mid-range reading most likely due to the 12 volt electrical system. In addition the installed sending unit doesn't look like the one in the service manual or the one that the parts suppliers say is correct for the 1949 Ford passenger car.

In looking for oil pressure sending units I found those that look like the one that is installed. However, they say they are for indicator (idiot)lights. First strange thing: the catalog information says 80 lbs pressure. Well that can't be the switch point where the light goes on, it should be more like 10 or maybe 20 lbs pressure. It only costs $14 (well $20 with shipping/handling) let's see what it really is. Shown here are pictures of the catalog information and the online catalog information.

Next strange thing. The new unit is open circuit when there is no pressure. That doesn't make sense for a low pressure warning unit that is supposed to ground a connection to turn on a light.

Next, evaluate the old unit in operation. It is presently operating in a reasonable manner. With no pressure (engine off) and no circuit connection it is open circuit. With the engine running the resistance varies around 50 ohms. Connected to the gage it shows mid range.

Now install the new oil pressure sending unit. Engine running resistance varies around 40 ohms. Connect the gage circuit. It works! However, it reads slightly above 80 lbs. Add 60 ohms of resistance to the circuit for calibration and it reads around 45 lbs. (It's a good thing I've had those resistors for 40 years or so.) This is definitely NOT an idiot light warning sensor in spite of the label on the box as shown below. Also shown are the gage reading and the unit installation. It's on an extension for ease of removal. (I had to take out the fuel pump to get the old one out because of the condition of the nut-area.)