High Suction and dead AI

[dave50b]

Anybody know what would cause high suction from a vacuum pump? 

I have a 78J and was flying with my instructor on an instrument flight when we noticed the VAC light indicator on the annunciator panel.  A minute later my attitude indicator went dead.  The light indication was a steady on so I believe that indicates high suction. 

I would assume a low suction would indicate a possible vacuum pump failure (or leak), but what would high suction be caused by?  Perhaps a blockage somewhere?  The filters were replaced at last annual 6 months ago. 

Thanks for any help. 

[jaylw314]

Possibly a clogged filter on the engine-side vacuum relief valve (not sure if that's the exact name for it).  The vac pump puts out 5 psi at around 1000 rpm, but there's a regulator in the engine bay that lets air into the vac system to keep the suction at 5 psi even if the pump is turning faster.

[kortopates]

Depends on how high, but if out of adjustment range should be a regulator issue.

 

But I wouldn't trust my memory that solid light meant high - but that's what I recall as well. But high vacuum won't cause immediate failure like no vacuum. It will cause premature failure or shorten life expectancy.

 

Sent from my iPhone using Tapatalk

 

[takair]

9 hours ago, dave50b said:

Anybody know what would cause high suction from a vacuum pump? 

I have a 78J and was flying with my instructor on an instrument flight when we noticed the VAC light indicator on the annunciator panel.  A minute later my attitude indicator went dead.  The light indication was a steady on so I believe that indicates high suction. 

I would assume a low suction would indicate a possible vacuum pump failure (or leak), but what would high suction be caused by?  Perhaps a blockage somewhere?  The filters were replaced at last annual 6 months ago. 

Thanks for any help. 

Older Mooney’s had high and low vacuum lights, but I thought newer ones did not...only low.  No suction gauge?  I suspect the light you are seeing is the low vacuum, which is actually higher pressure (ambient when failed) and is likely the same light you get when shut down on the ground (?). My guess is you lost your pump.

[carusoam]

First verify what the solid red light actually means... the POH should be pretty handy at helping you there...

 

Wow... 1 minute from flying around normally, to a sudden partial panel exercise!

 

Many people add a pressure gauge to their panels... the high/Lo lights are not enough info for IFR flight... as you have experienced... is it high or is it lo... add some stress, and the memory really goes on hiatus...

Many people are just going vac free since there are several ways of doing that too...

What is your back-up plan for the failed attitude indicator?  A TC? Or a decent electric AI?

 

Following the logic above...

1) a blocked filter or other blockage keeps air from entering the system, and lowering the vacuum properly...

2) a mal-adjusted regulator allows for an improper vac setting...

Has anything changed lately?

Some things change slowly... have the filters been changed out of the system?

Your mechanic can properly change filters and set the vacuum regulator... and install a vac gauge too...

While you are there... check the age of your vac pump... how many hours on it? (Log book data)

 

Now for the really odd part...

High vac should allow for lots of air to be pulled through the AI... not usually a reason for instant failure....  this failure sounds like it might have not had enough air running through it... more of a sign of blockage...

Find your mechanic, have him connect a gauge (temporarily) to find out the status of your vac system... and what has been blocked or broken...?

PP thoughts only, not a mechanic...

Best regards,

-a-

[dave50b]

I confirmed with the POH and a steady light indicates a high value.  My hangar neighbor (A&P) thought it might be the regulator as well, so we will adjust that and see what happens.  Not sure why that would cause the AI to completely fail, though, so it may be a bad vacuum pump. 

[Guest]

6 hours ago, dave50b said:

I confirmed with the POH and a steady light indicates a high value.  My hangar neighbor (A&P) thought it might be the regulator as well, so we will adjust that and see what happens.  Not sure why that would cause the AI to completely fail, though, so it may be a bad vacuum pump. 

With the engine off and no vacuum what does the vacuum light indicate?  If the wires on the vacuum switch are reversed the indications will also be reversed.

Clarence

[Andy95W]

What else does your vacuum pump run?  Usually it also spins the DG.  Does that still work?

[dave50b]

The AI is the only thing that is vacuum driven.  The DG/HSI is electric. 

[dave50b]

Problem was the vacuum pump.  It was weird that we were getting high vacuum indicators but we took off the pump and the shaft was sheared.  When the battery was on and no engine, it indicated low vacuum so I don't think the switches were reversed. 

[jaylw314]

59 minutes ago, dave50b said:

Problem was the vacuum pump.  It was weird that we were getting high vacuum indicators but we took off the pump and the shaft was sheared.  When the battery was on and no engine, it indicated low vacuum so I don't think the switches were reversed. 

Is it possible a chunk of carbon got sucked into the vacuum system and got into the high vacuum sensor?  I think there is supposed to be a filter between the pump and the vacuum line precisely to prevent that, though...

[carusoam]

Hard to get a high vac sensor to go on when the vacuum pump is dead...

Great follow-up, so far...

keep looking to find out why that piece of errant info came about...

Also consider getting a vac gauge mounted on the panel... low cost, tells you what is going on.  Or not going on...

Flying solo in IMC... go with all the instruments you can get...

Best regards,

-a-