Erratic oil temp and alternator behavior

[Monty]

I recently was flying home and noticed oil temp starting fluctuate erratically. Oil pressure was consistent. During the end of the same flight I noticed battery voltage sitting at around 12.5V with no apparent current from the alternator being displayed on the VPX monitor. I thought this would resolve easily to both a new oil temp sensor and alternator replacement. I have both of those new parts ready to install.
Today, just to verify behavior I started the aircraft and noticed static oil temp indications and the same lack of amps flowing from the alternator. These indications appeared to validate initial trouble shooting for cause. However, after running the motor at higher RPM (~2200) for 2-5 minutes and returning to idle I observed an erratic indication in the oil temperature followed by what appeared to be positive amps flowing from the alternator and battery voltage climbing to ~13.5 ( as if the battery was recharging from a working alternator.)
Is it possible that a failing oil temp sensor could be causing a short or strange behavior with alternator outputs? Seems unlikely but checking to see if anyone has experienced anything similar or understand the observations better than me.

 

[jakej]

I’ll make a guess - you have a Plane Power alternator ? No ?

 

[Monty]

I currently have an internally regulated auto alternator.

 

[jakej]

Ok, but why did you not buy the B&C unit you mentioned a couple of years ago?
One advantage you have now is that B&C offer a new ( I’m figuring it is to the typical B& C high standard) internally regulated unit for ~ $us595.
IF you want to still mess around with a cheap auto unit then that’s your choice - personally I prefer not to & feel more at ease with a product that just works for a long time without me worrying about when it may fail. Tread your own path

 

[airguy]

Is it possible that a failing oil temp sensor could be causing a short or strange behavior with alternator outputs? Seems unlikely but checking to see if anyone has experienced anything similar or understand the observations better than me.

No - a failing oil temp sensor cannot cause an alternator problem - but an alternator problem can 100% cause symptoms that look like a failing temp sensor.

You likely have (not certain) a failing alternator regulator. You 100% certainly have a weak (electrically) ground strap connection between the engine and the airframe ground. There is no clear indication that you have a failing temp sensor.

 

[Monty]

Jakej and Airguy - thanks so much for the feedback. Jake - I had not installed the B&C because they did not offer an internally regulated unit....until recently. My newly ordered one arrives this week and I do plan to replace the inexpensive option, which I also was not a fan of. I also have a new temp sensor which I will most likely install during the condition inspection that I am working on now.
Airguy - I will investigate the ground strap situation that you mention. I am still learning and curious how you make your assessment with such certainty. Would you be willing, please, to expand on your explanation.

 

[airguy]

Jakej and Airguy - thanks so much for the feedback. Jake - I had not installed the B&C because they did not offer an internally regulated unit....until recently. My newly ordered one arrives this week and I do plan to replace the inexpensive option, which I also was not a fan of. I also have a new temp sensor which I will most likely install during the condition inspection that I am working on now.
Airguy - I will investigate the ground strap situation that you mention. I am still learning and curious how you make your assessment with such certainty. Would you be willing, please, to expand on your explanation.

The temp sensor is one of two designs - the first being a simple voltage feedback device and relies on ground to be constant to the airframe. Voltage flows to it, goes through a resistive device whose resistance is variable upon temperature, and then flows to ground. The indication on the panel depends on how much current is flowing through the device (depends on the total resistance, which depends on the temperature). The other type of temp sensor is a simple thermocouple which generates a given voltage and reports this back to the panel indicator as voltage above ground. Both devices depend on the ground voltage of the engine to be equal to the ground voltage of the rest of the airplane - which will be true if the ground strap between the airplane frame and engine is of good quality. If it's NOT - then you CAN run into other issues...

Such as when your alternator tries to charge (or not). Your alternator also shares that same ground strap. It creates positive current which flows to the battery via the B+ line from the alternator, but that current has to make a full circuit back to the alternator and requires a ground connection - which is supplied by the ground strap. If the ground strap is a bit flaky and offers some small resistance to the power flow from the alternator, the result is that the entire engine assembly is now a "floating ground" and is not truly at zero volts - it is floating at some small voltage ABOVE ground, depending on how much current the alternator is trying to put out and how much resistance is in the ground strap (Ohms Law). Your oil temp sensor now lives on this island of raised voltage, and the additional voltage is ADDED to your true sensor output. The device in the panel reports the total - it doesn't know when/where/how that total was created.

Now for the flaky on/off/on variable oil temp results - that can happen if the alternator is not always putting out smooth current. If it is always smooth, then the "floating ground" may be there anyway but it is constant and nonvariable and you never saw the error in the temp, never suspected it. But if the alternator is turning on/off/on (it should not be) then the floating voltage is coming on/off/on and results in a variable error in the oil temp reading, depending on how hard the alternator is pulling. That's what you are seeing, and it's caused by an alternator fault (should be constant and is not) and a flaky ground strap (should be near zero ohms, and is not).

Next time you go fly - wait till it starts doing that, then pull the breaker for the alternator field. If the oil temp sensor reading does not settle out immediately, I'll buy you a steak and a beer.

If that solves your problem, I'll be parked in homebuilt camping at Oshkosh, you can buy me the steak and a beer.

 

[jakej]

Airguy handled it well. Too bad about the steak for me as I’ll be in Milwaukee/ Chicago area next week & not OSH this year

 

[Monty]

Airguy - Awesome description and thank you. I will be checking on the grounding strap during the upcoming condition. Re: steak and beer - I'd be happy to buy you a steak and beer either way. If I end up as Oshkosh, I will walk around with a six pack until I find your campsite.