Low vac light

[TheTurtle]

when should the low vac light be on?

If master is on and engine not running the low vac light should be lit right?  Pushing the light to test it does light up so the bulb isnt burnt out.  Vac switch bad?  They seem to be unobtanium and i dont really want to get an addon low vac system when can just replace a switch.

[Hank]

Does the light come on at idle, when the vacuum gage is below the green?

[nels]

Maybe the wire is lose on the switch. I think yours would be same as my 78 J. There are three wires, one is power in, one is low vacuum and one is high vacuum. You might try jumping a wire from the power terminal to the other two one at a time and see if your low and high vacuum light goes on. Also check and see that you have voltage going to the switch to begin with.

[TheTurtle]

5 minutes ago, Hank said:

Does the light come on at idle, when the vacuum gage is below the green?

negative.  never comes on.  It used to come on when the master came on (I think).  I have a gauge so I didnt pay much attention to the idiot light until I started trying to get all my lights in the cabin working.

[TheTurtle]

3 minutes ago, nels said:

Maybe the wire is lose on the switch. I think yours would be same as my 78 J. There are three wires, one is power in, one is low vacuum and one is high vacuum. You might try jumping a wire from the power terminal to the other two one at a time and see if your low and high vacuum light goes on. Also check and see that you have voltage going to the switch to begin with.

ill give that a shot, thanks

[Piloto]

3 hours ago, nels said:

Maybe the wire is lose on the switch. I think yours would be same as my 78 J. There are three wires, one is power in, one is low vacuum and one is high vacuum. You might try jumping a wire from the power terminal to the other two one at a time and see if your low and high vacuum light goes on. Also check and see that you have voltage going to the switch to begin with.

+1 I have the same switch on my M20J:

http://www.sigmatek.com/pages/prod_description/PD_ACC_vacswt.html

There is an adjustment screw on the back to set the pressure threshold. On mine (30yrs 0ld) the transducer module was not seating tight against the gasket. Put a little bit of silicone sealant and readjusted the set screw. You can check it easily by suctioning by mouth and checking for continuity.

José 

[Marcopolo]

Turtle,

 

  The answer to your question is yes, sort of.  the High/Low Vac light should be flashing when you turn on the master switch signaling Low vacuum as opposed to being on solid signaling High vacuum.

 

Ron

[TheTurtle]

1 hour ago, Marcopolo said:

Turtle,

 

  The answer to your question is yes, sort of.  the High/Low Vac light should be flashing when you turn on the master switch signaling Low vacuum as opposed to being on solid signaling High vacuum.

 

Ron

i have separate high and low vac lights.

[Marcopolo]

Sorry about that Turtle, but now you also know what I have in my K and how it works.  Information I'm sure you you were coveting!!!!

 

Ron

 

 

[TonyK]

Turtle did you ever get this figured out? I'm curious as to how it is supposed to work.

[TheTurtle]

 

Just now, TonyK said:

Turtle did you ever get this figured out? I'm curious as to how it is supposed to work.

yea theres a blue vacuum switch mine is attached to the back of the VSI.  If you have hi vac the high vac comes on.  low vac the low vac comes on.  it should be showing low vac anytime the master is on and plane is not running.  and probably when you are at low rpm as well.

That switch is made of unobtanium and costs $400 I think. http://www.sigmatek.com/pages/prod_description/PD_ACC_vacswt.html

You can test that its not the light bulbs or light sockets pretty easy by just jumping the connections on the switch and should get low vac light jumping one side and high vac light jumping the other.

heres another thread about it.

hope that helps.

 

 

 

[TonyK]

Well that is good news.  I had feared that it was expensive or hard to get hold of.  :-)

[Yetti]

hmmmmm something to check.... don't recall seeing this light come on

[TheTurtle]

well actually...  Mine was attached to the DG not the VSI.  And its not blue...  I guess I just remember all the blue fittings.  I pulled my AI and DG out today to replace them and run new hose and since I have the switch out im going to see if its fixable.  I thought I saw a post previously on repairing it.

 

[TonyK]

Take lots of pictures.

[Yetti]

Looks like someone put a bunch of goop on those screws.   Should just be a diaphragm with metal plates and contacts at either end of it.  calibrating it with a hand pump should work. 

[Guitarmaster]

Chief aircraft in Kansas City sells them for ~$400.  Ask me how I know.... 

[TheTurtle]

Here's what it looks like inside.  Wish I knew what it was supposed to look like.  Can't tell if the thin coat of hardend glue is from a previous repair attempt.  Not sure how the rubber boot is supposed to be attached.

[carusoam]

A vacuum sensing switch... (generally)

a piston, a cylinder, a spring, adjustment made by compressing the spring... Some electronic contact that opens and closes with the motion of the cylinder... source of vacuum in at one end, wires out the other...

Cylinder...the aluminum can

piston...the cylinder structure with rubber seals.  (Which probably ran out of lubrication over the years)

spring... doesn't seem to be visible in these photos.

adjustment...a screw in the center of the piston might be it?

electronic contact...some tiny wires go to it and exit the airtight can...

Age has a way of making these things not work.  Lubrication dried up. Rubber bits go bad. Wires break.  Springs lose their constant. Inquisitive people touch them...

Great photos.  What does the top and bottom of the can look like on the interior side?

Best regards,

-a-

 

Edited March 20, 2017 by carusoam

[TheTurtle]

Here's some more pics.  There a 'screw in the top and bottom but no way to open the unit to see the pressure contacts or wires.  Well there's always a way but it may really not be fixable after...

[TheTurtle]

Pulled the 2 rubber things off.  Looks like glue in the void between the outer shell and the white plastic inside as well as a set screw probably holding that brass part in the middle 

[carusoam]

The rubber things appear to be seals.  They also appear to no longer be round, uniform and centered in their locations.

The last photo looks like the seal came off the round home location and may have a tear..?

See if it is possible to clean and lube and reassemble...test to see if the mechanical part works, then the electrical part.

The chance of this being an AW fix... an important technicality....

 

This probably would be better on the homebuilt section.

Best regards,

-a-

[TonyK]

Hmm,  the insides of that thing make it look like a one and done kind of assembly.  

[Marauder]

The 75 Fs have this sensor as well. Mine failed a couple of times. The first time was the connection in the Canon connector that attached it to the glare shield. The second time, the silver solder weld failed at the neck where it connects to the DG.

I replaced mine with the Precise flight version.


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