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Gauges Show LOW OIL Only When Lights are ON

PostPosted: Tue May 01, 2018 9:32 pm
by jsaint
Good Day, all. When I turn the lights ON (regardless of RPM, regardless of which lights), the EFIS oil pressure drops from its normal 45-ish down to 25-ish, with rising oil temperature at the same time, as if I was low on oil. I'm not low on oil. When I turn the lights OFF again, the oil gauges restore quickly to their previous healthier numbers. I've been told to look for a grounding problem, but have found none. All grounding connections are solid. Both GRT Displays and EFIS were sent back to be checked out - they're fine. Ideas??? Thanks.

Re: Gauges Show LOW OIL Only When Lights are ON

PostPosted: Tue May 01, 2018 10:43 pm
by Admin
Hello Jsaint,

Good to have you on board ...

What light are you powering up when the problem presents itself?
Is it landing lights or position lights/strobe ?
Do you have standard or LED lights?

It sounds like it could be a bad earth or something is altering the resistance of the OP and OT sensors which is causing Gauge to read erratically. You could try checking if there is a voltage drop (12V) when turning on lights at gauges (OT and OP) as that may lead to probs. I would also try and power instruments directly from battery (run a positive and negative directly to battery) instead of power bus to see if you get the same symptoms. As power required by instrument is very low, you could also try powering it from a small 12V battery ...

There is a good chance it is a ground loop. A tester will not necessarily show you that. Instrument and sensor ground, should go directly to engine block.

Do you have a circuit diagram of you installation?

Re: Gauges Show LOW OIL Only When Lights are ON

PostPosted: Wed May 02, 2018 6:26 pm
by jsaint
Any of my lights causes the oil gauges to change; the more lights, the more change. I'm passing along your suggestions to my mechanic. Good ideas. I had someone tell me to just fly it to let the wiring "jiggle", and it all might go away (after I told him the plane was 11 years old with only 400 hours on it).

Re: Gauges Show LOW OIL Only When Lights are ON

PostPosted: Wed May 02, 2018 8:48 pm
by Admin
You need an electrician, not a mechanic. It is very hard for traditional mechanics to get their heads around wiring.

Steve (one of the admins) has completely rewired tons of planes. He knows his stuff and would probably be the best person to involve as it sounds like a classical electrics issue. Ping him and see what he comes up with. He is just about as good as you'll ever get when it comes to radios, transponders, sensors and EFIS/EMS systems on aircraft.

I would strongly recommend that you establish an instrument ground BUS (ground to engine and connect all instrument ground to it) and an audio ground BUS (for radio). See below ...

Inst Ground BUS
( 0 0 0 0 0 0 0 0 ) ------awg 12-------->to engine ---------AWG12----------> to battery negative
| | | | | | | |
| | | | | | | |
Ground to sensors
and instruments


Audio Ground BUS
( 0 0 0 0 0 0 0 0 ) ------- awg12 ---------> to battery negative
| | | | | | | |
| | | | | | | |
Ground to radio, headphones, intercom

ensure you use thick wire (awg12) from bus to battery and to engine. Thin wire will not do!