Bouncing Oil Temp

[Andy]

I've got a bouncing oil temp gauge that I"m still not covinced isn't a grounding issue, but now I'm not so sure.  The gauge is rock steady until about 40 minutes into the flight and then it starts wandering around.  I had the "Roger Lee recommended" grounding straps installed last year in reponse to an oil pressure gauge problem like this, and it did cure that.  I'm hearing some static over the avionics, mainly on the ground...small popping sounds..like a couple of wires hitting.  (Had an avionics guy look for that and he thought he had it but didn't get it.)  Before I call my A&P, I'm looking for a "steer" about things to suggest to try to keep the expenses down.  Obviously, I'll mention the possible grounding issues but if this sounds more like a sensor breakdown, please let me know.  Thanks for your help.

[FredG]

Do you have a mechanical gauge or a Dynon EMS?

 

A year or two ago, Dynon wrote a software update that smoothed displayed oil temperature.  It worked for my 2006 CTsw that had a rapidly varying displayed temperatures (more variation with greater temperature).  The range of rapid variation was as much as 10-15 degrees F.  Improved grounding helped, but did not completely fix the problem.  Changing the sensor did nothing for my problem.  

 

Of course, there may be other reasons for your variability.  

[Andy]

It's a mechanical gauge.  UMA type, I believe. The variation runs 40-50 degrees, i.e., almost the entire "green" range.

[Andy]

Thanks, Roger; I really appreciate the response.  The temp change generally has been occuring at a constant throttle setting.  I'll certainly execute what you've suggested.

[Tom Baker]

Andy, you might also want to check the capacitor, and its connections.

[Andy]

Thanks.  I'm turning it over to a local A&P who's worked on it before.  My Rotax guy reminded me that he had repaired the connector to the sender previously for the same problem.  That's where we're staring to look in the morning.

[Andy]

The erratic oil temp was the middle of three signatures associated with a breaking wire, specifically the one running from the oil temp sending unit to the gauge.  The three symptoms we saw at various stages were: (1) static in the audio system that got worse with aircraft vibration, primarily on the ground during taxi and right before takeoff with zero flaps (around 50 knots)--earliest stage in the breaking; (2) erratic oil temp as I described above that is exactly the same as what you see with bad grounds--middle stage of the break; and, finally, the gauge became totally unresponsive except it would drive to the null position when power was supplied--final stage of the break..essentially all strands broken except for one.  Replaced the wire and now all is well.

[2FlyAgain]

Andy,

 

Thanks for the info.  I have been having a similar problem with my oil temperature sensor.  I hope that it is as simple as replacing the wire.  I just returned a couple of weeks ago from Rotax service training at Lockwood.  Dean Vogel said another potential cause is old Loctite in the oil pump interfering with the ground connection there.  Obviously, I am hoping that is not true in my case.  

[Andy]

Getting there wasn't simple for us; in fact, my A&P didn't have a clue. The first two signatures made it look like a grounding problem.  His guys checked the grounds ( as I did) and we couldn't find any problems. (I have an extra grounding strap installed. Good thing he didn't know about the old Loctite issue or he might have taken my oil pump apart looking for it).  They supposedly cross-strapped the CHT sender and decided it was the oil temp sender (still scratching my head about how they got there) so I ordered a new one.  When they installed it, the gauge went to zero (My guess is they probably broke the wire completely during the installation).  That really threw them for a loop.  When my A&P called me to tell me what had happened and called it "a grounding issue", I put on my engineering hat, cranked up the airplane, and saw the gauge drive to null when power was applied (no data but gauge sucessfully powered).  I have a digital copy of the wiring diagram, went through it, and concluded that it was probably the wire from the sending unit (which I independently verified was working to cover the case of a new but bad sender being installed).  So I showed my former A&P the diagram, explained to him how a breaking wire would induce all the signatures we had seen, and told him I wanted him to check the wire continuity as the next step.  He did, and I was right. 

 

If it's a ground at the pump, wouldn't that give you the same signature as a bad sender?  Seems like if you simply connect the oil temp wire to the CHT sender, that will tell you whether the problem is at the sender (sensor or ground at the sensor) or downstream.  I'll let the mechanics here chime in on this; I appreciate your input.  I guess that's just another possible grounding issue for the CT, which it  has aplenty.

[FlyingMonkey]

For anybody else with oil pressure issues:  I replaced the sender twice on mine, and the problems I was having did not resolve until I used the Lockwood kit to relocate the sender to the firewall.  I also have the UMA gauge, and the symptom was oil pressure dropping down into the yellow and approaching the redline.  Right before I replaced it the last time it was reading 45psi at engine start, and then once it started warming up it dropped down to 10psi.  

 

When the sender was relocated we also ran dual grounds, one to the regulator and the other to the ground point the battery attaches to.  The pressure now shows 70psi+ on start and quickly settles to a very steady 40-50psi.

 

The VDO senders are obviously "vibrationally challenged" and benefit from being off the engine.  I recommend the dual grounds as well to try to take the weak CT grounds out of the equation.

[sandpiper]

When oil pressure fluctuates that much with throttle it's a grounding issue. Small changes of 2-6 psi or just a low oil pressure around <40 psi is usually the sender. Major fluctuations are grounding.

I chased this ghost once for 2 hrs. then I learned quick.

Two hours you call quick??? You must be slipping.

[WmInce]

. . . "I try and post here so someone else doesn't have one of those fun days." . . .

 

And Roger . . . that is very much appreciated by us!

Thank you.

[2FlyAgain]

Getting back to the oil temperature sensor issue...

 

The oil temperature sensor and the cylinder head temperature sensors are similar (identical part numbers...?...).  I had an experienced Flight Design person and Dean Vogel from Rotax both suggest swapping those sensor leads as a trouble shooting approach for the oil temperature sensor issue.  

 

I had a mechanic do that for me yesterday.  During a 20 minute test flight, both temperatures were stable.  During the 70 minute flight back home, the oil temperature sensor was still stable.  The mechanic said that some times there is a small amount of corrosion on the connection to the sender.  Simply removing and reattaching the connector will "clean up" the corrosion.

 

I did notice that the oil temperature sensor was 5-10 F degrees warmer than the CHT value.  As I recall, the oil temperature is usually about 20 F degrees lower than the CHT.  Am I remembering the difference correctly?  Or is 20 F degrees either way still within normal operating ranges...?

 

Thanks!

[2FlyAgain]

Thanks Roger!

 

For both this specific response and all of your many other contributions to this site...