I recently had to repair the oil pressure gauge on my 55.1 3100 with a stock 235. I have a GMC dash with an older Autometer gauge suite. The oil pressure gauge had been erratic and a couple of weeks ago it died. In order to determine whether it was the gauge or sender, I disconnected the sender and connected up a variable resistance in its place. The gauge worked fine. Being a curious engineer type, I decided to take some data to find the relationship between the resistance and the gauge reading.
Graphs The blue line in the left graph shows my results. I wondered if that was how it is supposed to work, so I rummaged around the web to find the specs. I found a reference that said it was a 73-10 ohm system. I assumed it should be linear and put that on the graph as the dashed black line.
When the new sender (Autometer 2241) showed up I tested it with shop air to make sure it worked before installing it. The results were so unexpected that I did it twice. Those are the yellow and red lines on the left graph.
I then decided to hook up the sender and gauge and run the test with shop air. The results are in the right side graph. In general, the systems reads low, being really off at higher pressures.
The gauge now reads a steady 40 psi with the engine at cool idle, so I'm calling it fixed. I haven't had a chance to get it good and warm yet. My tests indicate that 40 psi on the gauge is 50 psi at the sender. I'm not sure I believe that, but it does show that there's enough pressure to keep the bearings happy.
I apologize if I'm over-sharing, but I thought you guys might find this interesting and/or informative.
regards,
Leon
