Low vac at high rpm???

[N9405V]

Riddle me this, what could cause a low vacuum light at takeoff and cruise power, but not at low rpm or during the run up? I don’t have a gauge, just an idiot light on my 1970 M20E. The wing leveler/autopilot and artificial horizon work fine.

 

 

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[Mooneymite]

That High/Low vac switch can be twitchy....most seem to be installed to float somewhere on the vacuum line.  Perhaps it is a vibration issue.

See if you can borrow a vacuum gauge to install to check the actual vacuum pressure before you spend a lot of time/effort looking for a vacuum problem.

[larryb]

I had the vacuum switch go bad on me. Fortunately my avionics guy had one lying around from his own Mooney, removed when the vacuum system was removed. I now also have a spare one, which was removed from my K this past summer. I’ll sell it if you need it.

[Jim Peace]

Just had my vacuum system and standby vac removed......two G5's being installed......

So much more access behind the engine now.....

[takair]

9 hours ago, N9405V said:

 

Riddle me this, what could cause a low vacuum light at takeoff and cruise power, but not at low rpm or during the run up? I don’t have a gauge, just an idiot light on my 1970 M20E. The wing leveler/autopilot and artificial horizon work fine.

 

 

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Do you have a single light vs two lights?  Is it possible that it is acting as the low and High vacuum light and it is responding to high vacuum?

[N9405V]

I have the two light setup.


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[Yetti]

[carusoam]

Previous owner installed a vac gauge on my O...  

The idiot lights just don’t give enough information.  You don’t need to be an engineer to appreciate the added data...

Looks like a mechanic may need to install a temporary gauge or a real gauge to get you to the next step...

Do you have a JPI already?

PP Thinking out loud, not a mechanic...

Best regards,

-a-

[takair]

On 2/14/2018 at 8:32 AM, N9405V said:

I have the two light setup.


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Does the low light come on when the engine is not running?

[N9405V]

Does the low light come on when the engine is not running?

Yes, and at low idle. Above 900 rpms to 1800 rpms light goes out. Then, when I apply take off power, it comes on.


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[takair]

Really strange.  Would be best to insert a gauge to see what’s going on.  Could be a bad switch or electrical.  Maybe it responds to vibration vs pressure.