Ground paths

[bradp]

so looking at the back of a panel for an early J

master goes through the bus bar on the panel to the CB bus bar.  Makes sense.  
 

what makes no sense at all is that the ground paths for the some but not all of the switches (essentially the left wing ground paths) come back to the klixon switch frames (sandwiched between the frame and the instrument panel).  There’s also Another ground path from the engine instrument frame to the switch frames.  I always got a little dance of my engine gauges when the strobes were on.  Similarly got a little ignition noise in my headsets.  The ignition switch ground is grounded to the tubular frame where the headset jacks plug in.  
 

I guess the theme is that Mooney grounded a lot of items to the face of the instrument panel, but not to common grounds.   Therefore there is potential for ground loops and RF noise from a bunch of sources. 

[MikeOH]

The buss bar is 'hot'; i.e., it is a source of voltage/power.  The multiple wires on the other side of the CB are NOT grounds but feeds to various loads.  Those loads are commonly grounded to the airframe to provide the return path.

Edited August 2, 2020 by MikeOH

[carusoam]

Brad,

When you get a chance...

I didn’t understand some of what you described...

I see Mike explained what I was thinking...

Possibly the incorrect words got used?
 

Best regards,

-a-

[PT20J]

A few comments.

1. Mooneys generally have very well engineered power distribution and grounding. Look at the wiring diagram in the Service Manual for details.

2. Electrical buses are BUS not BUSS (even Mooney documentation gets this wrong sometimes -- take it from a EE: Buss means "kiss" -- look it up )

3. If you want to have some fun, bet someone your favorite beverage that you can turn the radios on by pulling a CB. Mooney realized that an avionics master was a single point failure and wired the relay as normally closed. The Radio Master powers the relay open when it is in the OFF position and removes power from the relay when in the ON position. So turn on the Master but leave the Radio Master off. Then pull the AUX CB and the radios will power up.

Skip

[bradp]

On 8/2/2020 at 3:21 PM, MikeOH said:

The buss bar is 'hot'; i.e., it is a source of voltage/power.  The multiple wires on the other side of the CB are NOT grounds but feeds to various loads.  Those loads are commonly grounded to the airframe to provide the return path.

Hey @MikeOH - those two wires that I circled In the picture aren’t connected to the circuit breaker lines - apologies if I was not clear. Those are grounds from the left wing (right wire including strobe Nav and pitot heat) and the the ground from the frame of the engine gauge cluster (left wire) that are attached to the mounting screws by a small nut on a couple of the housings first the circuit breaker switches themselves    This is pretty clearly laid out in the service manual wiring diagram as grounds returning to switches.   

When I did my last avionics upgrade I took all ground paths I could find to a grounding bar. Almost all my electrical gremlins with the exception of my dancing strobes and some small alternator whine are gone.  A hangar elf told me that if you take the gauge cluster ground and attach it to a central ground point like a ground bar, they will stop dancing as well. It’s just an observation that there are sources of electrical noise. 

[bradp]

12 hours ago, PT20J said:

A few comments.

1. Mooneys generally have very well engineered power distribution and grounding. Look at the wiring diagram in the Service Manual for details.

2. Electrical buses are BUS not BUSS (even Mooney documentation gets this wrong sometimes -- take it from a EE: Buss means "kiss" -- look it up )

3. If you want to have some fun, bet someone your favorite beverage that you can turn the radios on by pulling a CB. Mooney realized that an avionics master was a single point failure and wired the relay as normally closed. The Radio Master powers the relay open when it is in the OFF position and removes power from the relay when in the ON position. So turn on the Master but leave the Radio Master off. Then pull the AUX CB and the radios will power up.

Skip

I found that out the hard way a few years ago skip.  
 

 

[bradp]

@carusoam

Anthony sometimes a picture is worth 1000 words 

[chriscalandro]

Looks like you have a pretty good mess going on there. 
 

good luck!

[bradp]

14 minutes ago, chriscalandro said:

Looks like you have a pretty good mess going on there. 
 

good luck!

It’s actually really well organized.  Mooney did a good job of labeling every wire and connector.  Avionics shops over 40 years - not so much. You can look up any wire in the original airframe and know where it is, what it does, where it’s coming from and where it’s going to. 

[RLCarter]

1 hour ago, bradp said:

 

 

When you say “CB frame screw” are you talking about the screw(s) that holds the CB panel that the CB’s are mounted to? PITA but can you get a picture of where the ground wire is terminated at the CB?

[chriscalandro]

1 hour ago, bradp said:

It’s actually really well organized.  Mooney did a good job of labeling every wire and connector.  Avionics shops over 40 years - not so much. You can look up any wire in the original airframe and know where it is, what it does, where it’s coming from and where it’s going to. 

Dude,

You have 6 incorrectly terminated wires going to a single CB.  What do these wires go to?

 

That wire from the Guage cluster should not be going to a CB switch screw....

Edited August 5, 2020 by chriscalandro

[bradp]

@chriscalandroThis is the OEM design. It hasn’t been touched.  This is not my circuit breaker panel. It’s the circuit breaker switches (representative example photo attached, not my actual panel) 

[carusoam]

Brad,

Thanks for the added details.

Best regards,

-a-

[bradp]

8 hours ago, RLCarter said:

When you say “CB frame screw” are you talking about the screw(s) that holds the CB panel that the CB’s are mounted to? PITA but can you get a picture of where the ground wire is terminated at the CB?

Here’s a picture of “a” switch assembly found on the internet, not attached to any particular n-number of course, where this grounding was inadvertently discovered when the brass screw was broken off during removal.  Clearly showing the mounting of the grounds versus the lines. 
 

If you look up the wires attached to the yellow ring terminal, they are all grounds from the left wing  The wire attached to the red terminal is from the engine gauge cluster  

[Rotorhead]

I've been attempting to track down the reason my pitot heat has stopped working. I believe this dumb ground is my issue. As picture, you can see one of the nuts backed out and the ground came off with the screw still in place. I had a feeling this was the issue but didn't want to hook the wire back up to the wrong place. After looking at schematics and this forum topic, I have way more confidence this should work now. Fingers are crossed since I have to make a return trip over the Rockies about 800nm away!

Thanks @bradpfor starting this topic, it may have literally saved me a bunch of time and money!!

 

Edited September 23, 2020 by Rotorhead

[carusoam]

RH,

Can you post the pic again...

Send it to me, I can post it for you...

 

I see what you are saying now... 

It is still better to get fixed before departing on a long flight... in case a few other things may have had their electrical flow interrupted the same way...

Expect that the pitot heater’s ground is much closer to where the pitot heater is...

I think i might be misunderstanding what you have typed... OR The pitot tube heat is always getting power supplied, and the ground gets switched at the end to allow the power to flow... There are a few circuits that do it this way... but pitot heat is a lot of electricity in heavy wires and would be less safe to do this way... 

You might have a power supply wire that has come loose...

What else blinked the time you thought the pitot heat was causing a problem?  See what else goes to the  pitot heat CB...

PP thoughts only, not a mechanic...

Best regards,

-a-

[Jerry 5TJ]

When I dug into the avionics harness on my C around 2012 I found it was wired up every which way except well.  I removed it all and started over.  The new harness terminated grounds in a single point using this grounding block 

Grounding Connector

It made for a neater and electrically quieter harness.  

[Rotorhead]

8 hours ago, carusoam said:

RH,

Can you post the pic again...

Send it to me, I can post it for you...

 

I see what you are saying now... 

It is still better to get fixed before departing on a long flight... in case a few other things may have had their electrical flow interrupted the same way...

Expect that the pitot heater’s ground is much closer to where the pitot heater is...

I think i might be misunderstanding what you have typed... OR The pitot tube heat is always getting power supplied, and the ground gets switched at the end to allow the power to flow... There are a few circuits that do it this way... but pitot heat is a lot of electricity in heavy wires and would be less safe to do this way... 

You might have a power supply wire that has come loose...

What else blinked the time you thought the pitot heat was causing a problem?  See what else goes to the  pitot heat CB...

PP thoughts only, not a mechanic...

Best regards,

-a-

It's actually in reference to this post.  

 

[Rotorhead]

False alarm, still no dice. =( Pitot heat still isn't working.

[carusoam]

Do you still get the low voltage warning when you try the pitot heat?
 

There are two ways to test the pitot heat...

1) Watch for the voltage drop.

2) Check it physically to feel it warming up.

Use caution, if you have a leak of electricity... touching anything related to the pitot heater, may be a bad idea...

The pitot heat gets hot enough to mess up your skin when it is working normally...

Since the system is in question, do both tests... to verify the electricity is going to the pitot heater, and it is getting hot....
 

PP thoughts only, not a mechanic...

Best regards,

-a-

[neilpilot]

28 minutes ago, carusoam said:

There are two ways to test the pitot heat...

1) Watch for the voltage drop.

2) Check it physically to feel it warming up.

3) When you switch heat on, look for a deflection in your mag compass.  

[MikeOH]

I'd pull the access cover near the pitot.  Take off the wire(s) (not sure if the return path is through the aircraft frame).

Then, use an ohmmeter to make sure the pitot has some resistance (>1 ohm, <5 ohm, rough numbers!).  Next, use a voltmeter to measure across the feed wires (or, hot wire to ground) with the pitot heat switch on.  Should see battery voltage.

If not, check for voltage on both sides of the pitot switch. If no voltage on the pitot side, the switch is bad.  If no voltage on the source side, check the CB on both sides.  (you may just have a CB/switch).

That should narrow it down to a wiring versus failed component problem.

[Warren]

You can start at the breaker switch.  Start with the breaker switch off and the master turned on.

1.  Make sure you have battery voltage on the supply side (bus).  And make sure the connection is good, screw tight.

2.  Measure resistance to ground from the output side (verify the pitot is connected, has reasonable resistance and has a ground).  Check the screw is tight.

3.  Turn on the breaker to ensure you are getting battery voltage on the output side.

This will quickly narrow down where to troubleshoot next.