Vacuum Low Light

[moontownMooney]

We are only four months into our ownership of a 1968 M20F.  Ever since we had purchased the plane, the vacuum "LOW" light would be illuminated when we have the master switch on prior to starting the engine.  This had made sense to us as the engine isn't running so there would be no vacuum.  The other day, on our first flight after "patching" a substantial leak in the step retract boot (perhaps entirely unrelated), the light did not illuminate when we turned the master switch on.  The light will illuminate brightly when pushed to test.  Upon close inspection it is apparent that the light is occasionally barely lit, but just barely, when the engine is not running.  Thoughts?  Is this a concern?  There seem to be no other symptoms.

[carusoam]

Vac step is drawing from the vacuum pump the instruments are on.

Engine on, vac is on...

Look for the vac sensor.

There are pictures of the vac sensor behind the IP posted around here.

could be a sensor, a connector, a wire, or the light bulb in the annunciator.... 

Can you chase with a volt meter?

Do you have a vac gauge to go with that?

PP thoughts only, not a trained mechanic...

Best regards,

-a-

[Guitarmaster]

This is the switch you are looking for.  https://goo.gl/images/Ycv5XU  You should find it on the back of the AI.  
With your multimeter set to Volts, attach the ground to the airframe or engine.  Use the other probe to test the switch.  White is 12V+, Yellow is Low Vac and should be showing 12V+, Black is High Vac and obviously be showing zero volts.

 

Edited October 2, 2017 by Guitarmaster

[Guitarmaster]

Oh, and, welcome to Mooneyspace!

 

[Guest]

That vacuum switch has 2 adjustment screws.  One between the terminal, the other inside the mounting fitting.

Clarence

[carusoam]

There is another thread currently showing pictures of the switch...

Best regards,

-a-