Supercop0184 Posted February 29, 2020 Report Posted February 29, 2020 Vacuum pump problems - AI doesn’t work well when power is pulled in the pattern - It’s erratic - before I threw money at a pump - I decided to hook a gauge up to my pump to see what kind of vacuum I’m getting straight at the pump. Bought a basic gauge at the recommendation of one of my AV gurus - hooked it up and noticed it doesn’t even have the values I’m looking for. Should be between 4.5 and 6.5 vacuum. My suction gauge shows 6 when full power - 5.5 in cruise - but at idle the airplane suction gauge shows nothing! When hooked up directly to the external vacuum gauge - it shows 20 at idle and up to 24 in HG at 1500 RPM. is my external vacuum gauge missing a decimal point? No way it should be registering 20-24 in HG when the suction gauge in the airplane shows max 6 in HG. Suggestions please. Thanks! Quote
EricJ Posted February 29, 2020 Report Posted February 29, 2020 Are you reading the inner scale or the outer scale? The outer scale is cm Hg, the inner scale is inches Hg. Quote
RLCarter Posted February 29, 2020 Report Posted February 29, 2020 Also which side of the vacuum regulator are you on? Quote
carusoam Posted February 29, 2020 Report Posted February 29, 2020 Sc, Sounds like the planes gauge is working... and your instrument is following what it says... if so... Skip to line 18), below... You have selected an interesting and sort of complex hand held gauge to read... what is the blue mark for? And why didn’t you take a picture of the gauge while it was connected... so we could look over your shoulder? You may be sending yourself further away from a solution this way... Expect the amount of vac you are seeing... depends on a few things... 1) vac pump vanes are spinning in correlation to the engine rpm... (independent of MP) 2) This allows the vac pump to generate a stronger vac than needed... 3) The vac regulator is settable... and should be adjusted to bring the raw vacuum down to the green arc range... 4) vac pumps wear slowly over time...sooner or later they may not be able to provide the excess vac that is needed... 5) Pumps over 500 hours get replaced because maintenance on them cost more than a new pump... 6) Check the logs to see how many hours are on the pump... 7) Check filters and their age... 8) Inches of Hg is what you are looking for... according to the gauges for sale... https://www.aircraftspruce.com/catalog/inpages/suctiongauges2.php?gclid=Cj0KCQiAtOjyBRC0ARIsAIpJyGP2e7k0VZtpKHgrrKfAxJke-cgwD6-Xdaq04prQ8Ne7wxRn88iNE7AaAvUVEALw_wcB 9) You have more than required... by your description, share your actual data... 10) Can’t help without actual data... 11) prove what is working first... then go down the line... 12) if getting too much vac... this is bad for gyros... 13) know if you are reading vac in the wrong place... before or after the regulator... 14) get mechanic to help... because they know the responsibility/skill for this type of important systems... 15) 5” of vac looks like it should be somewhere near 7-8:00 on the dial...? 16) This conversation looks like it may have blended vacuum instruments with engine vacuum that is also called MP... 17) vac pumps typically break, long before they wear out... one blade breaks and messes up the rest... 18) If you see low vac when the rpm level drops... expect a leak may be occurring in the vac system lines... 19) The POH for your plane will have a drawing of what the vac system looks like... 20) Or... the gyro is showing it’s age... worn drive system... worn bearings... meads to be cleaned... (?) 21) How is the DG doing? Is it drifting at a higher rate than usual? So... Let’s start again, from the top. PP thoughts only, not a mechanic... Best regards, -a- Quote
takair Posted February 29, 2020 Report Posted February 29, 2020 I think you are bypassing your vacuum regulator which is inside the cabin. It is a regulated leak. (You are reading the full output of the pump and starving it of air). What you describe seems like you have a leak somewhere beyond the regulator. So, likely a hose is cracked, broken....or an instrument is leaking. If you have a PC or vacuum step you have additional opportunities for leaks. I would start with the AI lines...if the lines are old enough they will crack open. 2 Quote
cliffy Posted March 1, 2020 Report Posted March 1, 2020 Going direct to the pump inlet is only telling you that the pump is sucking. You need to connect the gauge to a line on the back of your horizon to get a real picture of your panel gauge and this one. The vacuum regulator between your vac pump and your horizon is a regulated "leak" to bring the suction to no more than what it is set for. Your vac pump will ALWAYS suck more than the instruments require hence the regulator between them. Here's what you need to know about the system you are working on- (with your A&P looking over your shoulder) https://www.eaa.org/eaa/aircraft-building/building-your-aircraft/while-youre-building/building-articles/instruments-and-avionics/a-gyro-instrument-system 2 Quote
carusoam Posted March 2, 2020 Report Posted March 2, 2020 One interesting thing to look for... Some of our vac driven instruments are daisy chained together... The last one in line, may not be getting the vac it needs... because something in front of it has a leak... The POH most likely shows the order of instruments in line to the vac pump... PP thoughts only, not a mechanic... Best regards, -a- Quote
Prior owner Posted March 5, 2020 Report Posted March 5, 2020 If you have (or ever had) a vacuum warning light, and the switch for that is still screwed into an instrument, you might want to check that as well- mine was broken in half and someone had apparently glued it back together decades ago.... it was leaking something fierce. Quote
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