oil temp fluctuation

[Roger_Lee]

Dynon,

I have a D120 wit a Rotax 912ULS. All my temps and pressures are fairly solid and steady in their readings, but the oil temp. The wires are snug and this seems to be a problem for many people. Is there a way or solution to make the oil temps more steady. They fluctuate as much as 20F. This seems to be more in high rpm range than when at the lower rpms down around idel range.

 

[airguy]

They fluctuate over what time period? A couple seconds or a couple minutes? Instantly back and forth or smooth changes?

 

[Roger_Lee]

They fluctuate constantly. Multiple times a second.

 

[airguy]

It would be very unusual indeed for ACTUAL oil temperatures to be fluctuating that much - I would start by checking for a good ground path between the connection and your battery, and good wiring for the powered side. If you have an "iffy" ground strap somewhere on the airframe you will develop a "floating ground" that will drive sensors nuts.

 

[Roger_Lee]

Hi Airguy,

All grounds are very tight. This is a problem on most Flight design CT's and many other aircraft. I would like to find a solution if one can be had for the many. I'm an LSRM-A mechanic and work on many LSA with Dynon's and many have the same issue on the oil temp. I know it isn't a real problem, but can cause the alarms to go off when you are a little high with it bouncing 15F-20F. There must be something to put in line to stabilize the reading.

 

[dynonsupport]

As others have said, that does sound like a grounding or actual sensor issue.

 

[Roger_Lee]

I'll add a separate ground wire right to the battery for a test and get back to you all.
Or I can go from the single pole VDO oil pressure sender to a duel poll with a separate ground wire.
I'll let you know.

 

[dynonsupport]

Note that adding a small wire from the sensor to the battery won't make a difference. The solution to grounding issues is a VERY big, solid wire that's run from the engine block to the battery.

 

[Roger_Lee]

Hi Dynon,
Why do you think that it is a bad groungmaking only the oil temp fluctuating? It is the exact same sensor as the CHT and wire set up and it along with all the other temps and pressures are rock solid. It isn't the sender as this is a new engine and it did the same thing on the old Rotax 912 engine.

 

[Roger_Lee]

I ran a #4 cable from the engine directly to the negative battery post for a test. That stopped the 20F fluctuations and now it's down to 5F-6F fluctuations. Now I'm going to ground directly from that point to the main ground bus which the D120 connects directly and see if it helps any more. This setup will basically be direct and go around all the aircraft Mfg's grounds that interconnect every where, but must have resistance.

 

[Roger_Lee]

Hi Dynon,

I now have a #4 wire from the battery negative to the engine and is attached just above and to the rear of the #3 valve cover. I also ran another ground from here to inside the instrument panel to the main ground block. So the ground is a direct link to the battery plus what was originally installed from the FD factory. Everything is steady as a rock except for the oil temp which still fluctuates 5F-6F. The CHT which uses the same probe is rock solid. Is most people's Rotax oil temp fluctuating or is it steady? The only other thing that fluctuates is the ammeter from -3 to +4 which I believe is normal.

 

[dynonsupport]

So that sensor (which is the same for CHT and oil temp, as you've said) is more sensitive voltage-wise at higher temperatures, and since your oil temps are probably higher than your CHT's (guessing here - correct?), that would explain the increased sensitivity of the oil temperature. The fact that the #4 wire helped is an indication that there's fundamentally a ground issue here, but you may not have either gone quite big enough or to the best locations to equalize all of the potential.

 

[Roger_Lee]

Hi Dynon,

There is about 15F higher in oil temp than CHT. The CHT will read 205F and the oil temp 220-225F. I have seen the CHT's up that high and they are still steady. I don't see how I could possibly get any better ground and that it only affects the oil temp which isn't just an issue for me, but many others. Wouldn't reducing the sampling rate on the D120/D10 would steady these out?

 

[dynonsupport]

Roger,

Slowing down sampling doesn't work. The actual temperature is stable, but the electrical system is putting noise on that signal at the rate of tens or hundreds of time a second. No matter how infrequently we sample, each sample is basically 0 seconds long. So if we slow down to sampling every 10 seconds, the oil temp will just jump how ever many degrees you have now, but will jump every 10 seconds instead of 4 times a second. The reading will still be equally as wrong.

We could add a filter that averages all the readings, but the issue there is that the temperature will always read high. Ground issues always make your temperature read high. So if your electrical system has noise, sometimes the reading is right, and sometimes it's high. The average of this is high. Averages only work when the noise is equally on both sides of "right."

So this is a complex issue. We want the temp to be right, but to do that, we have to rely on a good ground. We can hide the fact that the ground isn't good with filtering, but then you won't know you have a ground issue and all your temps are 20F high like yours were to start. Then you start chasing cooling problems because you think the Dynon is right.

We'll keep thinking about this, but ultimately it is a ground issue that the airframe manufacturer should be involved with.

 

[DaveJnr]

Roger,
As you have discovered, this is a problem which plagues the Rotax 912.
I have written about this problem on another thread on this forum.
I have good steady readings on all sensors except the oil temperature.
I added a dedicated ground from the body of the sensor back to the ground on the Dynon. This solution has made the oil temperature reading more steady but not solved the problem.
At the moment my solution is to just live with it.
Regards
Dave

 

[Fgerr]

Dynon,

A few questions. First, I have a well grounded engine (Rotax 912 FD CTsw) and my oil temp varies by +/- about 3-4 degrees F, rapidly (i.e., changes second by second) at temperatures above 200 degrees F. The CHTs, at nearly identical temperatures, are rock steady. As Roger notes, since these are the same sensor, reading nearly identical values, how can it be that grounding or electrical noise affects them differently? This is not logical.

Second, when the engine is running, I get the variation noted above. As soon as I turn the engine off, the variation is gone, completely. This confirms to me that grounding is not the issue, but that either noise or voltage instability in the electrical system when the engine is running is the issue. Still, the difference in stability of oil temperature vs CHT suggests some difference in how the DYNON processes the information from each sensor, since, as noted above, they are identical sensors operating in the same electrical environment.

Thanks.

Fred Gerr
Iowa City

 

[dynonsupport]

Dynon,

A few questions.  First, I have a well grounded engine (Rotax 912 FD CTsw) and my oil temp varies by +/- about 3-4 degrees F, rapidly (i.e., changes second by second) at temperatures above 200 degrees F.  The CHTs, at nearly identical temperatures, are rock steady.  As Roger notes, since these are the same sensor, reading nearly identical values, how can it be that grounding or electrical noise affects them differently?  This is not logical.

Second, when the engine is running, I get the variation noted above.  As soon as I turn the engine off, the variation is gone, completely.  This confirms to me that grounding is not the issue, but that either noise or voltage instability in the electrical system when the engine is running is the issue.  Still, the difference in stability of oil temperature vs CHT suggests some difference in how the DYNON processes the information from each sensor, since, as noted above, they are identical sensors operating in the same electrical environment.

Thanks.

Fred Gerr
Iowa City

Fred:
Please read the entire train of questions, and responses from Dynon, above (or below, depending on how you're viewing the thread). We believe that this issue is ultimately an issue of insufficiently good (enough) grounding between the sensor, the engine block and the EMS.

If you prefer (if this is an LSA, this is not really a possibility) to use a different oil temp sensor, we have a list of two-wire oil temperature sensors that are compatible with Dynon's EMS, but not supplied by Dynon. Note that to make use of these sensors, you must have firmware v5.4.2 or later on your EMS. (Note - these sensors are not supported in SkyView.) The advantage of a two-wire sensor that you're no longer dependent on engine block ground path back to the EMS.

 

[Fgerr]

Dynon:
Thank you for your response. I had read the entire train of questions, and responses from Dynon, and clearly, the issue is not solely one of grounding, as you insist. Others have had the problem persist after solid grounding (see DaveJnr, above, July 1, 2012). Also, as I noted in my post, when the engine is not running, the problem goes away (with temps of 225 degrees, rock steady). The resistance of the ground didn't change between engine on and engine off. Your explanation is not consistent with the observed phenomenon. Blaming the aircraft manufacturer for poor grounding is not adequate.
Thanks.
Fred

 

[jnmeade]

From the installation manual:
"The oil temperature sensor is very susceptible to voltage differences between the engine case and the negative terminal of the battery."
"If your panel is not metal, or is otherwise isolated from engine ground, connect a 14 AWG or larger wire to the instrument case."
"Because of the current drawn by the EMS-D120, even very small resistances between battery ground and instrument ground can cause voltage differences which adversely affect engine sensor readings. An easy way to test the quality of the instrument’s ground is to measure voltage between the ground pin at the EMS-D120 and the ground lead at your aircraft’s battery. With the EMS-D120 powered on, connect one lead of your voltmeter to a free ground lead coming from the EMS-D120. Connect the other lead of your voltmeter to the ground terminal of your battery. The voltage between these two points should measure very close to 0 mV (within 5 mV). If it does not, you must improve the ground connection between the ground of your battery and that of your avionics bus."
Looks like I'll at least start in this direction. Apparently, 0.5v difference between the Dynon reading and the cigarette lighter doesn't really cut it.

 

[dynonsupport]

Sorry for the short-sighted response above. On further research, we've noted the same thing that you have: that the identical sensors ostensibly go through the same ground path through the engine. There's only a slight difference in the way that they're wired, and they're equivalent. An interesting experiment that you could perform, if you're able is to simply take the wires at the CHT and Oil Temp sensors themselves and swap them (if this is an LSA, you really can't do this without the manufacturers' support, unfortunately, because you can't make changes to the aircraft that aren't in the maintainance manual (and then only with an A/P or LSA repairman)). But the idea here is to see whether the bouncing moves with the sensor or the input?

And, note that your experiment doesn't disprove a ground issue of some kind, as the alternator current being passed through the resistance of the ground path is what generally causes noise in sensors, not just the resistance of the ground path. The fact that the noise goes away when the alternator stops is actually a strong indication that it is actually a ground issue, but we're still a bit baffled how it can affect one sensor more than another.