If your Oil Pressure, Oil Temperature, CHT, EGT or RPM / Tach is reading incorrectly (low, high, intermittent), the very first thing to check is the Reference Ground to the Engine Block.
About 95% of all sender reading troubles are related to inadequate engine block Ground or total lack thereof.
Many Engine Sensors use the Engine Block as the Ground return path. And, the Engine Block Ground can have a different Ground potential when compared to the Aircraft Ground Bus or Battery Negative. Therefore, if you do not Ground Correctly to the Engine Block this can have a significant effect on the sender measurements and can cause big indication errors. Most senders measure resistances or small voltages and these will be inaccurate or intermittent if there is not a Reference Ground to the Engine Block.
The EFIS itself or the Single Instrument still needs to have a Supply Ground at the Avionics Ground Bus. However, The RDAC Ground (in cases where this is used) or the wires labelled Reference Ground (in Singles) need to be Grounded directly to the Engine Block.
Directly means that the terminal needs to be wired DIRECTLY to the Engine Block Ground, with the shortest possible wire, WITHOUT first Grounding to Airframe or Battery Negative. Just a straight line to the Engine Block. Use heavy gauge wire (18 AWG or better) if possible.
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