Grounding my master switch M20K

[Bigpunch8]

I’m having some electrical glitches and the local avionic shop said is sounds like a bad ground.

Does the master switch ground to the panel with the mounting screws? I dropped the switch and see this wiring going to no where? Isn’t the panel aluminum? Doesn’t seem like a good place to ground anything.

Any help would be much appreciated!

Grant

1979 M20K

[chriscalandro]

What's wrong with aluminum? as long as it's mounted to the frame in some way it should work.

 

What exactly is the problem?

Edited September 24, 2020 by chriscalandro

[Bigpunch8]

1 minute ago, chriscalandro said:

What's wrong with aluminum? as long as it's mounted to the frame in some way it should work.

 

What exactly is the problem?

My Tach needle, and fuel gauge needles are bouncing around

[chriscalandro]

37 minutes ago, Bigpunch8 said:

My Tach needle, and fuel gauge needles are bouncing around

And the shop told you to look at the master switch?

[Bigpunch8]

19 minutes ago, chriscalandro said:

And the shop told you to look at the master switch?

No, I’m just following all the wires to see if something is loose or broken.

[chriscalandro]

6 minutes ago, Bigpunch8 said:

No, I’m just following all the wires to see if something is loose or broken.

I think you ought to have someone else do that...  

what Tach is it?  Do the gauges bounce up from correct or down from correct?

Is it both fuel gauges at the same time? Do any other gauges do this?

 

the senders don’t ground there.  They probably ground through their mounts and have just a single wire going to them.  
 

look in the service manual for the drawing.  

And find a different shop. 

Edited September 24, 2020 by chriscalandro

[Bigpunch8]

48 minutes ago, chriscalandro said:

I think you ought to have someone else do that...  

what Tach is it?  Do the gauges bounce up from correct or down from correct?

Is it both fuel gauges at the same time? Do any other gauges do this?

 

the senders don’t ground there.  They probably ground through their mounts and have just a single wire going to them.  
 

look in the service manual for the drawing.  

And find a different shop. 

Fixed the fuel gauge, it was only the right side.

The needle is bouncing both ways..

[carusoam]

Hey Grant...

Brad P had posted pics recently of how some grounds are connected to some CBs....

The CBs themselves are directly mounted to the instrument panel, their electrical connections are connected to a bus bar...

Some electrical panels have lost their ground connections over the years... find yours to see what it looks like....

You can look up the CB you have by going to the manufacturer’s website...
 

Solving problems randomly can get expensive...

Great pic... somebody was discussing this FF gauge in their K yesterday... it is a fuelP gauge calibrated in gph...

Some of these ancient instruments were really good in their day... If they are no longer working as new, you might look closer into what is being used as a sensor, and how that info is delivered to the gauge, and then check to see how many hours are on the gauge...

It may be OH time or replacement time for some of these things....

Cleaning up grounds is a great idea.  It may not help fix many mechanical problems...
 

Use caution when working as a mechanic on certified planes... know what is OK, what needs a log entry, and when you are in over your head...   

PP thoughts only, not a mechanic...

Best regards,

-a-

[chriscalandro]

that looks like a mechanical gauge.  Not an electrical issue.

[Baker Avionics]

Tach needle is most likely the cable needs serviced.  You can pull it out. cleaned the cable and the sheathing, then put some moly grease and move on.  

Edited September 24, 2020 by Baker Avionics

[Bigpunch8]

43 minutes ago, Baker Avionics said:

Tach needle is most likely the cable needs serviced.  You can pull it out. cleaned the cable and the sheathing, then put some moly grease and move on.  

My gauge is electric so there in no cable? Anything else I can look at?

Edited September 24, 2020 by Bigpunch8

[Baker Avionics]

3 minutes ago, Bigpunch8 said:

My gauge is electric so there in no cable? Anything else I can look at?

You need then to look at the ignition switch or the pick off where it gets the signal from.  I, personally would call Dan Reisland at Lasar.  He is tell you exactly what to do.  

[Bigpunch8]

2 minutes ago, Baker Avionics said:

You need then to look at the ignition switch or the pick off where it gets the signal from.  I, personally would call Dan Reisland at Lasar.  He is tell you exactly what to do.  

Here’s a video, not sure if I will upload..

[Baker Avionics]

4 minutes ago, Bigpunch8 said:

Here’s a video, not sure if I will upload..

IMG_5561.MOV

I truly hate to ask, but are you sure it is electric?  Let me do some thinking.

[carusoam]

 

Electric RPM gauges made it into Mooneys in the 90s...

People were afraid of modern displays... mostly because they weren’t proven yet...

The 94 Ovation has digital instruments that use mechanical needles for display... when it does something funny, like move, it moves in steps...

That tach, in the video, is behaving like it has a dirty or worn mechanical cable...
 

Aviation technology moves slowly compared to cars...

Corvette first experimented with digital displays around the time this K was built... And they built 20k at a time... and millions of similar technology enhanced cars in the rest of Chevy...

The only thing that is going to be electric about this tach will be its light bulb... if it has one...

 

I would love to be proven wrong...

Take a pic from behind the tach... if it has a center mounted cable resembling a bicycle speedometer....

It is essentially the same thing...

Hope I didn’t ruin any dreams... 

PP logic only, my 65C’s tach looked similar to that one... and needed a cable shortly after behaving this way...

Best regards,

-a-

 

[chriscalandro]

You should check more grounds on that mechanical gauge per your genius avionics shop. 
 

please tell us who this shop is. 

[carusoam]

Let’s see if there is something positive to write about master switches and grounding.... For Grant...

1) The Master switch doesn’t carry much load... it’s not like the master supply of electricity runs through the switch...

2) There is a master relay that does the heavy hauling of electrons....

3) The Master switch turns on/off the master relay... nothing more...

4) attaching things to the body of the switch like where it is bolted to the instrument panel will ground things to the panel...

5) The panel benefits by having some really nice grounding straps connected to the airframe and engine block...

6) The POH for the M20K will be very helpful to find out what is connected to the electrical supply and what is not...

7) By the time you get to the level of M20K... things can get really complex, fast... having the MM to review electrical schematics is key...

8) The electricity that runs through this switch... starts in the bus bar, through the top of the switch, out the bottom of the switch on its way to the relay... the relay is grounded to the airframe...

9) If the master switch isn’t grounded, or even bolted in place... it would operate normally...

10) Additional grounding of switches is a good safety mechanism, in the event the switch wears abnormally or breaks... and starts somehow leaking electricity to your fingers...

PP thoughts only, not a mechanic...

Best regards,

-a-

[carusoam]

11 hours ago, Bigpunch8 said:

I’m having some electrical glitches and the local avionic shop said is sounds like a bad ground.

 

[carusoam]

Can’t seem to get the quote to work right today...

 

Grant,

It appears that this plane may...

1) Be new to you? Or is this the same plane from back in 2019...

2) Be showing some signs of being old?

 

 

It sounds like you may have had a casual conversation with a mechanic, away from your plane...
 

he may not have understood your request...

Or his advice got lost in translation to being written here...

When this happens to new owners... they typically seek some help from people that are familiar with their model...

Often this occurs during a Pre-purchase Inspection... where a knowledgable mechanic makes a list of everything in the plane and makes notes regarding how well they are working or not working...

Post purchase training or Transition Training is where you find out how things are really going... where you have hours in your plane and a person sitting next to you that has seen these things wear out before...

 

Briefly summarized...

1) You saw some instrument needles bouncing...  Described as fuel gauge and tach...

2) You tried to have a conversation with a mechanic to learn about your machine...

3) You disconnected something unrelated to the problem... possibly mis-understood the guidance offered?

4) You are now surprised by what else came off the two screws you disconnected...

5) You post a video of your Fuel Flow gauge, but the needle isn’t on the screen...

6) What looks like a mechanical tach appears to behave like it has worn cable driving it...

7) Did you mean...?  Your fuel level gauge needles are bouncing around?

8) Have you checked the fuel level floats for their operation?

9) Fuel level sensors can be OH’d or replaced... there are many for sale around here....

Welcome to the world of old airplane ownership...  

 

PP thoughts only...

Best regards,

-a-

 

[N231BN]

That is in fact an electric tach. It is fed from the P-leads via 15k isolation resistors. It is probably an internal issue with the tach, these things don't last forever.

I am surprised your fuel flow gauge is still working, that is also electric and uses a floscan transducer.

[carusoam]

15 minutes ago, N231BN said:

That is in fact an electric tach. It is fed from the P-leads via 15k isolation resistors. It is probably an internal issue with the tach, these things don't last forever.

I am surprised your fuel flow gauge is still working, that is also electric and uses a floscan transducer.

@Bigpunch8 1BN left you some additional insight...
 

Best regards,

-a-

[David Lloyd]

Without looking up a wiring schematic of the 231K, I believe most aircraft master switches depend on a ground and the switch connects the master relay to ground as a safety related design.  The master relay itself get power directly from the battery and only needs a ground to energize.  In our Mooneys that have a combined master and alternator field switch, the field side of the switch does connect the bus to the voltage regulator.

Again, without knowing exactly how the electric tach works, the jumpy needle could be the result of the tach itself going bad, loose connection or broken wire between the tach and the resistors or one of the resistors failing. Phew!  I once had a sometimes jumpy tach on an electronic display that corrected itself when one mag was rebuilt.  Problem went away.