Fluctuating oil temperature indication

[till]

Hi there,

we own a Piper Cub with a Stratomaster Velocity E-1 instrument for engine monitoring. Over the last couple of months the oil temperature indication started to display worrying and weird indications. First the temperature just went higher than usual. So I thought the engine might get hot. But it then dropped down 5 degrees within 1 second. This sort of fluctuation and degree of variation then increased over time. Yesterday it went up from a normal 76 degrees up to 110 within 5 minutes and then suddenly dropped back to 77 degrees just to stay there for the rest of the flight.

Obviously it's an indication issue. Can anyone point out what to check first to tackle this problem?

Thanks,
Till

[Jodelflipper]

Hi Till,

Did you get this issue resolved? I am having exactly the same problem, after 3 + years of operating successfully I have started to get wildly fluctuating oil and CHT temperatures. I have re-done the earth circuit and checked the terminals all with no success.

this problem only manifests itself after 10-20 mins of flying once the temps are getting hotter. Very baffling and annoying !!

Mike

[till]

No. It's not fixed. I was hoping to get a response from MGL through this forum but no support unfortunately.

[gd5362]

I copied this reply from Ranier on another thread. It might help you out.....

If you are not getting consistent readings - consider what the EMS sees. What does it measure ? It measures a voltage between an input and it's own power supply ground. That voltage is either created by some sensor or it is a voltage drop over a resistive sensor connected to ground (usually the engine block). The voltage is as a result of the EMS injecting a small current through the resistor.
Thermocouples are differential sensors - often they are grounded at the thermocouple itself (to the engine block). This means one of the two wires goes negative while the other goes positive - and we are talking just micro-volts per degree.

The most common issue is incorrect grounding of the item that measures the voltage. Does not matter if it is the EMS or an RDAC. The same thing applies.

To make it work - it's very simple. The EMS (or RDAC) get's it's power supply ground from the engine block. One wire. You do NOT share that wire with any other electrical user. You do NOT connect that wire to the airframe, power supply ground distribution point (or bus bar), you do not connect it to the battery negative or anything else. One ground wire from the EMS or RDAC to the engine block. No more no less.

If your EMS or RDAC has other ground terminals (like the RDAC gives you a few more) do NOT connect these to another ground point. These terminals are intended to be connected to ISOLATED sensors. Typical example would be a resistive fuel level sender - often these have a nice big metal flange screwed to the side of a metal fuel tank which happens to be connected to ground. That flange is the negative connection of the variable resistor. Not good. Fine for a car but not for an aircraft - isolate that ground connection and run a ground wire back to the RDAC or EMS.

What happens if you do not do that ? If your RDAC or EMS is the ONLY electrical user in your system (not even a alternator/battery charger or any sort) then you do not have a problem. The moment there are other currents flowing in various ground connections your EMS or RDAC ground now no longer sees the voltage drop from your sensor but instead sees that voltage PLUS whatever voltages are dropped over your various other ground connections. Currents are flowing in all sorts of directions and your grounding system is nothing but a complex resistive network.
Do not for a moment underestimate the effect this can have on your readings - it can be really bad.

The only other thing that causes havoc is a (much) less than perfect DC supply - that is more common than you think. The only way you will find out is to connect a scope to your power supply. A voltmeter tells you absolutely nothing in this regard. Errant ignition system currents, regulator artifacts and electrical noise generated from faulty or badly designed equipment (some cheap USB chargers come to mind) can seriously affect the quality of your power. If bad enough - that will also affect your readings as these various signals that are on-top of your power can be very fast exceeding your devices ability to regulate and filter them out.

if it starts going wrong after several years of no issues - and you have checked your wiring for damage and corrosion - simply run your EMS/RDAC with your engine off. Heat one or two sensors and see what your readings do. If everything is stable now - start looking very closely at your regulator/rectifier - they do have the habit of failing. Even a battery that is getting tired can contribute as its internal resistance starts to increase - the battery forms a vital part of the regulator/rectifier circuit which tends to be just a fast on/off switch and relies on the battery to smooth things out.

[till]

Thanks for the helpful information. I'll try my best when I find the time!

[Paulie321]

Grounds. It's almost always poor grounds.