A little different oil temp behavior - temp drops, then returns

rjcthree

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Jan 29, 2009
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18
D180. Single wire oil temperature sensor. 117 hours on a RV-9A. Normal oil temps during flight of about 180F (deemed plausible in previous experimentation/oil cooler testing). During today’s 1.6hr flight, the oil temp would periodically drop to 120F-ish, then return to the 180F range. A high rate fluctuation, sometimes at less that a second in cycle 180-120-180, call it “twitchy”, that occurs for minutes, then goes away. At one point it stayed at 120F-ish for a couple minutes, then returned to 180F-ish. No switches were flipped, for most of the period of time no throttle changes made, no other indications of any other anomalies. Odd indications on this sensor usually send people down the ground loop path, but I could not find any discussion of dropping temperatures. I have no evidence but the data log. I did disconnect the lead to the sensor about five hours ago, but it’s snug and has a lock washer. It reads correctly at ambient, ive had no reason to doubt it.

Before I go tearing into it, I’m hoping there’s some wisdom that can help! Thanks, Rick
 

airguy

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Nov 10, 2008
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Gods Country - west Texas
100% guaranteed it's a sensor or wiring issue - you cannot physically change the temperature of the oil that quickly. Could be a failing sensor, or more likely a poor ground. If it's a single-wire temp sensor it will ground through the engine, so you'll want to pull the sensor out, clean the threads of both the sensor and the mounting location, and reinstall - then make certain you have a good non-corroded signal wire connection and ground path from the engine block to the airframe ground - preferably the same ground point where the Dynon EMS is grounded.

If you have the old-style sensors, which you almost certainly do with 117 hours on the airplane, then this would be a good time to consider the recent SB on changing out the Kavlico sensors.
 

rjcthree

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Jan 29, 2009
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Air guy, typically most people see a rise in temp with grounding issues, it’s all over the forum, here and VAF. This seems unique in that the oil temp goes down, and all other sensors - fuel pressure, oil pressure, stay rock solid. That alone points to sensor or single wire issue.

I do have the legacy VDO sensors, and plan to make a change at the next condition inspection for fuel and oil pressure. The oil Temp sensor doesn’t change, I don’t think.
 

rjcthree

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Jan 29, 2009
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It does, but it only suggests it’s not the D180 sensor ground. All the other sensors are grounded to a common sensor ground (fuel pressure) or a sensor housing ground (oil pressure) and the oil pressure housing ground ties back to the firewall. The oil temperature ground is the only one (Non-TC) that is Grounded through the crankcase.

I think.

i did some investigation tonight, cowl off. There was no difference in oil temp regardless of what was powered or not powered, except I did not run the engine. A got current draw over 20A all the way down to the Dynon running on I internal battery. I saw the oil temp fluctuate 1 degree, once. All grounds are solid, I find no loops/ground offsets battery to main ground block or to the D180 case. I run 6ga from the battery to the main cabin ground block, so that’s pretty solid.
 
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Dynon

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The oil temp sensor, when connected to the D10/D100 series, is fairly susceptible to grounding issues. Other sensors aren't, so the fact that it's the only one isn't surprising. Because its grounding path is through the engine, you want to make sure that engine ground is well-connected to the airframe / other grounding points (running a small wire between the sensor and the EMS won't fix anything here, Fwiw. You need to have a solid, robust ground from the engine all the way to the sensor).
 
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