Ron4x4

For Defender, you can buy any standard oil pressure gauge, 52 mm, e.g. at AliExpress for some 10 USD or even less. Make sure you also buy a proper adapter so it fits the oil temp sender which you probably will not need. The adapter is a brass 3/8 BSP male to 1/8 NPT female; also available from several suppliers like AliExpress who deliver anywhere in the world in approx 3 weeks.

I found an even easier solution to tackle this problem, and I should have thought about that earlier. It appears that the gauge receives a too high current which should be lowered by a resistor. Use a variable resistor between the blue/green sender connector at the fuel tank and any 12V wire to initially calibrate the gauge. Then replace by a fixed one in the dash. So, on the back of the PRC3107/PRC7313 gauge, just bridge the 12V terminal and the blue/green wire terminal, in my case this was a 127 ohm resistor. Problem solved.

I see that I could not upload the picture explaining all this. Here is a description: use the following resistors in order to bridge the 0 - 2000 ohm: Full - 3/4 = 330 ohm; 3/4 - 1/2 = 330 ohm; 1/2 - 1/4 = 330 ohm; 1/4 - 1/8 = 330 ohm; 1/8 - empty = 680 ohm. You can skip the 3/4 indication by using 680 ohm between the top connector and the second position from above. The needle stays at FULL till it reaches the second dot position and then gets 680 ohm, jumping to position 1/2. And so on. The cutting should be done in the variable wiring section, just above these connector points; this way you create 5 (or 4) sections that should be bridged with resistors. Good luck.

On my Defender Ninety V8 from 1986 I noticed that the original PRC 3107 fuel gauge read "full" at all occasions. I have an STC1139 sender in my tank, originally used for diesel. This seems to be an often occurring issue. In order to solve the issue I found out that the STC1139 sender has a resistance range of 15 - 300 ohm ( full to empty) whilst the fuel gauge operates with a resistance range of 15 - 2000 ohm. Hence the little movement of the fuel indicator. How to solve this? Simple. The STC 3107 is made out of 5 sections, this is full - 3/4 - 1/2 - 1/8 and empty. See the red dots. First, cut through the resistor wiring using a Dremel grinder disk, at the position of the green lines. This is just above the red dot connector points. Then connect the red dots using 4 resistors, as indicated. Glue the resistors to the housing using 2-component epoxy glue which is resistant to fuel to avoid them coming loose. You will now see that the gauge reads in 4 sections: full, 1/2, 1/8 (indicator lamps jumps on) and empty, rather than on a gliding scale from full to empty. In practice however this reading is sufficient. Hope this is helpful! STC1139-done.pdfSTC1139-done.pdf