Did you see any improvement, Greg? I'm seeing something similar and in our case taking the alternator offline makes things behave much better - temps drop overall, and increased loads (lights, pumps etc) no longer cause temp jumps. Have you tried that test?
If you're having the same issue we are having, I think there is a good chance your our sensors are fine and it's a matter of grounding, which these sensors are inherently sensitive to.
Even if the sensor is ok, I do agree that these engine grounded sensors with only one wire are definitely suboptimal since they introduce these possible grounding problems, and I bet this is what we are both experiencing. Unfortunately we currently have 3 of these sensors (oil, coolant, gearbox tmp) and it may not be feasible to swap them all out in the near term, so we are trying to fix what we have first. We also have a few other mounted temp sensors (non G3X) for cross checks on engine temps.
One clarification: if you truly only have one wire I doubt it is a thermocouple. Those are a bundle of two wires that can look like one, but two connections are needed to read the temp and I don't think you ever ground them to the engine. I'm guessing you probably meant thermisistor? This is what we have and I think would be more likely for an oil temp sensor the way you are describing it, and the behavior you are seeing would then make more sense.
Since the alternator has such a big influence in our case, I think our issue must have something to do with the path between alternator ground and the battery and to the G3x - so through the engine, some ground straps and garmin wiring. If I'm thinking about this correctly, resistance there would increase voltage of the engine metal relative to the G3X reference ground causing the G3X too "see" lower voltage drop on the engine grounded sensors, thus higher temps.
Will be poking around trying to solve this in the next couple weeks and will report back!
Andy (RV9A, Subaru EJ25)
Hi Andy,
Yes, I’ve been able to reproduce exactly what you’re describing.
In flight, I’m seeing a clear load-dependent change in oil temp indication. For example, in cruise the oil temp will sit at ~177°F. Turning on the landing lights bumps it up to ~184°F, and adding pitot heat pushes it up further toward ~190°F. The change is immediate and scales with load.
With the alternator off, I turned on both landing lights and pitot heat (about an 11A load) and the oil temp stayed completely stable—no change at all. When I brought the alternator back online, the oil temp jumped up significantly (approaching ~210°F+ briefly) and then settled back down.
That seems to strongly point to something associated with alternator current flow rather than the sensor itself.
For what it’s worth, this is the older Van's single-wire oil temp sender, so your point about it not being a thermocouple is correct. Given that, the behavior makes more sense, any voltage difference between the engine and the G3X reference ground is going to show up as a change in indicated temperature.
I’ve checked all the grounds and continuity looks good, and I also tried adding a direct ground from the GEA 24 to the engine with no change. At this point I’m leaning toward a small voltage drop somewhere in the return path under alternator load, or possibly some alternator induced noise affecting the measurement.
Your description of the engine metal voltage shifting relative to the G3X reference ground under load lines up very well with what I’m seeing.
I’m planning to take some millivolt measurements between engine case and GEA ground under different load conditions to see if that correlates with the temperature shift.
Definitely interested to hear what you find on your end as well, it sounds like we’re chasing the same thing from two different installations.
Greg
7A - New Orleans
