High vacuum light

[Brett Kallish]

Hi folks I was wondering if somebody can help me with something that happened. The last time I went flying I used the Wing leveler for the first time, after about 30 minutes the high vacuum light came on and I flew back to the airport and landed. Today when I turned on the master the preflight list says the low vacuum light should go on but it did not. I pushed the light in and it lit up. So I decided to run the engine up to see what was happening. As I started to taxi the high vacuum light came on then went off as I continued to taxi. I ran the engine up and all was fine. Then I gave full power and a few seconds later the high vacuum light came on again so I went back to the hanger and shut down. I do not know if this has anything to do with it but the battery was low and I had a hard time to get it started. As I said I do not use the wing leveler and keep a rubber band around the button. Any help will be greatly appreciated.

Brett

[Cruiser]

variation in vacuum can be from several sources, including the vacuum pump itself. How old is it?

[carusoam]

Vac pump generates the vacuum, another device is in charge of regulating the pressure.  Often a gauge will tell you what the pressure is.

Your mechanic can check your regulator to see If it is set properly and if it is only an adjustment.

Do you have a vac gauge?

Your mechanic will have one that he can use to see if you are getting a false alarm.

Vacuum pumps themselves don't usually generate too much vacuum on their own.  They usuall wear until they break and don't generate any vacuum at all after that.

These are only PP ideas.  I am not a mechanic....

Best regards,

-a-

[TTaylor]

This is the reason I replaced the "idiot" lights with a vacuum gauge.   A simple gauge is much easier to troubleshoot than just high and low readings. 

[Yetti]

I am offended by your use of the word idiot my lights self identify with lightweight indicators of problems

[Mooneymite]

I can't  tell what year/model is involved, but on my C, the idiot lights are controlled by a very simple (but darnned expensive) adjustable switch.  While it's  possible the switch has gone out of adjustment and a simple turn of the adjustment screw would fix it, without reading the vacuum pressure, you're  probably not helping things.

As previosly noted, once RPMs come up, vacuum is reglated by the regulator at the firewall.  This is where your problem may be.

Edited May 21, 2016 by Mooneymite

[Hank]

9 hours ago, TTaylor said:

This is the reason I replaced the "idiot" lights with a vacuum gauge.   A simple gauge is much easier to troubleshoot than just high and low readings. 

My venerable C has both. The original idiot lights in the stack below the gear switch, the vac gage added in above and slightly to the right. At runup of 1700, it reads well, I've not paid attention to where it cuts off. The only time it read below the green, it was stuck on "0" and my AI was doing weird things. Oh, the Low Vac light was on, too, but I didn't see it until much later.

[cnoe]

A dirty garter filter on the vac regulator can cause high vacuum. Replacement is on the Annual Inspection list IIRC.

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[geoffb]

The vacuum switches are garbage.  Best to add a gauge.

[PTK]

Question: can the low vac light stay on at higher rpms than it used to as the pump ages? Is this possble?

[63C]

On May 21, 2016 at 10:21 AM, cnoe said:

A dirty garter filter on the vac regulator can cause high vacuum. Replacement is on the Annual Inspection list IIRC.

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What I was thinking as well.

[carusoam]

I think this is how the basic vac system may work...

 

Vac pump <———instruments<———restriction/regulator<———filter<——— outside air...

                                        |

                                       V

                                  vac gauge /switch&light

 

the regulator is a controllable restriction to flow.  The restriction causes a vacuum to be generated down stream.

Any additional restriction to air flow upstream of the instruments will cause higher vacuum to be measured.

Check when the filter was actually replaced.  It may have been visually inspected?

Check to see if any dirt has been able to sneak into the system?

Have your mechanic adjust the regulator?

Keep in mind the amount of air being moved by the air pump is rpm dependent.  The regulator is constantly working to maintain the vac pressure.

PP thoughts, I am still not a mechanic... Or digital artist!

Best regards,

-a-

PS for Peter, a worn pump could technically cause a low vac situation as it loses the abillity to push the same CCs of air with each revolution.

[Mooneymite]

I'm no engineer, but is seems that dirty filters will decrease the volume of air, but the regulator will maintain pressure within the proper parameters?  While the various vacuum driven instruments are designed to work at a specified negative pressure, they must also have an adequate volume of air to work properly.

I"m guessing that volume and vacuum pressure are important and inter-related.

Like Carusoam....just the musings of a layman.  I guess I'm the reason Mooney put idiot lights on my panel.

Edited May 24, 2016 by Mooneymite

[Awful_Charlie]

23 hours ago, carusoam said:

I think this is how the basic vac system may work...

 

Vac pump <———instruments<———restriction/regulator<———filter<——— outside air...

                                        |

                                       V

                                  vac gauge /switch&light

Sorry, I think you're, errrmmm, wrong!

Expanded for twin pumps, for a single pump just remove the manifold and the second line

Main Vac pump       <--< Pump filter <--< Manifold  <--< Regulator <--< instruments <--< Instrument filter <--< outside air                
Standby Vac pump <--< Pump filter <-------<

The regulator works by bleeding in air between the pump(s) and the instruments, lowering the depression. It has the garter filter on it to avoid letting the pump suck junk it that will damage it

Whether you have the warning light or a gauge, in either case if you get an out of limit condition, you are going to have to verify with a gauge of known quantity. If you have both and one indicates a problem, then you can hazard a guess as to which one is lying

To the OP, it could be the warning switch or it could be a real problem, in which case the regulator or the garter filter on it would be the first point to inspect. The garter filter is pennies (although a total git to get to on my Bravo), and most regulators are adjustable - but before you go adjusting it you should verify there are no kinked lines and the filters are servicable

[captainglen]

High vacuum is never caused by the pump or the instruments themselves. There are still three possible causes 1. the regulator that stands between the instruments and the pump is out of adjustment. It is located dead center between the two removable panels in front of the windshield, screw in increases vacuum and screwing it out decreases. 2. A clogged instrument air filter can cause high vacuum like putting your hand over the hose on a shop vac. # the most likely for two reasons is the vacuum switch. It is nearly impossible to get excessive vacuum at idle and with the engine off you should have gotten the low vacuum light. The only way to know is to hookup a vacuum where the hose to the switch is and see if it vacuum is right. I myself gor rid of the switch and installed a panel gauge.

[Brett Kallish]

Thank you all for your suggestions. At this point I am trying to get the local mechanics at Queen City airport, near Allentown, to look at the problem but they do not seem to have much time to work on my Mooney. So when I get some information I will update here.

Brett