Bill Chenoweth
Joined: 10 Jul 2008
Posts: 9
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 Posted: Sat Aug 01, 2009 3:05 pm Post subject: UMA engine gauge problem |
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I have UMA tachometer, oil pressure, oil temperature, and cylinder heat temperature gauges installed as part of a recently completed Rotax 912S installation in a Kitfox that previously had a 582. The airplane (and its electrical system) are about 8 years old except for the changes made to accommodate the brand new engine.
I am using an electrical system design as outlined in the Aeroelectric Connection including the over voltage module and the ground block bolted thru the firewall to the 6 awg engine ground cable.
At about seven hours of trouble-free operation the CHT and oil temperature gauges registered 140 degrees at power-up with the ambient temperature at about 60. The oil pressure gauge barely crept up to 12 psi as I ran the engine for about 60 seconds (before shutting down). The plane is hangared and had sat idle thru about 10 days of damp, rainy weather. Prior to this point the gauges had behaved in a normal way - temperatures pegged at zero then rising as the engine warmed up and oil pressure climbing immediately at start-up to 70 psi and then declining as things warmed up. Oil temperature would reach 120 after about 5 to 6 minutes. The tach behaved normally.
Simple explanation, right? Evidently not. After testing the instruments in the plane and discussion with UMA I sent the gauges back, they were declared to be working correctly, I got them back, installed them (one at a time with tests in between), and they worked fine. For about two flight hours. Then after the plane sat for three or four days the false readings reappeared.
This time I made a test rig so I could completely bypass the plane's electrical system. I have a db9 with separate leads. I ran one to the battery positive, one to the sensor on the engine, and held the ground lead against the engine block within a couple of inches of the sensor. Same false reading. Battery voltage at this point was about 11.9 (as shown on the aircraft voltmeter just after the test). We checked the resistance of the CHT sensor where we did the test and it was 1145 ohms which is consistent with the ambient temperature of 70 or so.
The following day the "falseness" of the temperature readings was less. The day after it was still less so I flew the plane a bit. Oil pressure readings were lower than "normal"; temperatures were higher but all within a range that I felt ok running the engine in flight (briefly).
The fact that three instruments are involved makes me wonder just what I've done to cause this. The fact that an instrument, isolated from the plane's electrical system, is reading inaccurately makes me wonder about some kind of problem in all three gauges.
Can anyone on the list suggest a cause or perhaps an additional debugging technique.
Thanks very much.
Bill
Kitfox IV 1200
Albion, Maine
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