Trying to understand my electrical ground issue

[Mooney-Shiner]

Hey, fellows, I'm trying to understand my electrical gremlins to get my speaker to work right. I was testing electric continuity from tray pin to the speaker wire. Good continuity on Speaker HI side (+) and high resistance on the Speaker LO (-) side. I started measuring and shot this video.

Any help is appreciated.    

[MikeOH]

I'm having a little trouble understanding your issue.

What do you mean by 'get my speaker to work right?'  Is it weak, noisy, intermittent, ??

The fact that your ohmmeter reads different with the master on vs. off suggest a ground loop.

You mention that you measured the high side pin to the speaker, but it looks like you just measured the low side (shield) to some random ground on the panel.  I would think you should measure to the return pin on the tray pin, just like you did for the high side.

Total guess, given my lack of understanding, is that the shield on the speaker wire is chafed and is shorting to the airframe somewhere along its length.

[Vance Harral]

You can't use a multi-meter to measure the resistance of components in a circuit with the circuit energized, as you are doing in the video when you got 27 ohms.  Multi-meters measure resistance by passing a small current between the terminals you are measuring across, and observing the voltage drop.  If some external source is already driving current through the same circuit, the multimeter is not going to read correctly.

The 1 ohm measurement you got after you turned off the master switch is the "more correct" resistance measurement.  The reason I put "more correct" in quotes is that when you put your multimeter leads across speaker low and the frame terminal of that port on the panel, you are measuring the total resistance through all paths between those two terminals.  It is tempting to assume the only path between the terminals is the speaker ground wire you're tracing, but there are possibly other paths caused by inadvertent shorts.

The bottom line is, frustratingly, a multi-meter isn't really that great of a tool for determining connectivity of the components in an electrical system.  They can do a good job of telling you when two things which are supposed to be disconnected are actually shorted, but they can't do a very good job of telling you that two things which are supposed to be connected are actually well connected without also being shorted to something else.

 

[PT20J]

I agree with @Vance Harral - you cannot get an meaningful resistance reading with power applied.

I would not expect the cabin speaker to be connected directly to ground. I would expect both leads to go to the audio panel.

What problem are you trying to correct?

[cliffy]

You can show good continuity between the probes with only the multimeter power but the connection could be weak when actual higher power is run through it. 

Also it looks like you are trying to measure something using the outside shielding of the speaker wire. WHY? That only has one function and that is to go to a ground at one end of the shielding only to prevent spurious signals from getting into the radio. It has no function in the operation of the speaker.  One wire goes from your radio to the speaker and the second speaker connection goes directly to a ground at the speaker or through a second wire (not the shielding) back to the correct pin on the radio. Not through the shielding. 

[Mooney-Shiner]

8 hours ago, MikeOH said:

I'm having a little trouble understanding your issue.

What do you mean by 'get my speaker to work right?'  Is it weak, noisy, intermittent, ??

The fact that your ohmmeter reads different with the master on vs. off suggest a ground loop.

You mention that you measured the high side pin to the speaker, but it looks like you just measured the low side (shield) to some random ground on the panel.  I would think you should measure to the return pin on the tray pin, just like you did for the high side.

Total guess, given my lack of understanding, is that the shield on the speaker wire is chafed and is shorting to the airframe somewhere along its length.

Sorry, I should have been more specific. 

The speaker is completely silent and not working right now.

I have pretty good continuity for the High side, but very high resistance(with Master on) on the Low side. I also measured the Low side to the Low side pin return pin on the tray, but I noticed that the Low side pin is connected to the frame (AKA ground). The resistance between the tray Low pin and the ground is 0.8 Ohm (acceptable).

The speaker is not energized when only Master is turned on. It is energized when Avionics switch is turned on, which turns on the Radio/AudioPanel/GPS/etc. But the AudioPanel Speaker Low side pin is connected to the same ground (airframe) that Master uses.

Here is the pinout for the Audio :

[N201MKTurbo]

It looks like speaker low should not be tied to ground. You might need to fix that.

[EricJ]

+1 that speaker low should not be grounded.   The diagram shows a shield which should be grounded, but the speaker high and low should only connect between the speaker and the tray pins.

[cliffy]

Speaker low and pilot phones low go to pin 22   Shielding goes to Ground

[Vance Harral]

5 hours ago, Mooney-Shiner said:

The speaker is not energized when only Master is turned on.

I think you meant to say the speaker is not supposed to be energized when only the Master is turned on.  The fact you see different resistance readings on your meter with the master on vs. off tells us that something in that circuit actually is energized when the Master is on, and that's a clue about the problem.

Looking more closely at your video this morning, the thing you have connected to with your alligator clip near the speaker appears to be shield material, not a signal wire, as others have mentioned.  Here's a screen capture:

Your red alligator clip is connected to braided shield material here.  Shielded cable comes in different flavors.  The flavor you want for audio has two insulated signal wires in the innermost layer, and a braid of shielding mesh around the two signal wires.  In the photo above, you appear to have the single-conductor flavor, which has only one signal wire in the center, and a braid of shielding around it.  It looks to me like someone who did not understand audio wiring has wired your speaker incorrectly, with multiple problems.  First, they are trying to use the shield material as signal low, which is not appropriate - either they don't understand how this works, or they didn't have any two-conductor shielded cable and hacked it; and (2) they have connected that shielding material to airframe ground instead of (or in addition to) the audio ground of your audio panel.

Note that this incorrect wiring can "work" if there is not much electrical noise in the system, and the amplitude of the audio signal sent to the speaker is high.  You might actually hear some audio through it if you turn the speaker volume way up, which is maybe what happened in the shop when this was installed.  But it is not correctly wired.

If you look at the wiring diagram, all the paired audio signals have a dotted-line ring around the signal wires.  That ring is the shielding material.  Here's a screen shot of the shield coming off the speaker (which is adjacent to and tied to the shield wire of the Pilot Phone group, that's why there are two rings here):

In other words, there are three connections for every audio group: two are the audio (+) and (-) signals, the third is the shield.  If you look at the connection dot coming off the shield, it traces to Pin 1 of the audio panel in your wiring diagram.  Again, Pin 1 is audio ground, not airframe ground.  Airframe ground is represented by the three-pronged fork at Pin Z.  Note that they are not connected to each other:

People who don't know what they're doing sometimes think "ground is ground", and wind up inadvertently or deliberately shorting audio and airframe ground together.  It appears to me that's what has been done in your airplane, given that you can measure low resistance between the (-) terminal of the speaker and your airframe.  Again, this can sorta "work", but it usually injects all kinds of noise into your audio, and in the worst case dampens the audio signal to effectively zero.

In summary, based on what you've shown us here, I think you need to replace the wire that runs from the audio panel to the speaker.  Throw away the one-conductor shielded cable, and replace it with two-conductor shielded cable, with the two conductor wires connected to pins W and 22, and the shield connected to pin 1.

 

 

[Aerodon]

27 minutes ago, Vance Harral said:

 

In summary, based on what you've shown us here, I think you need to replace the wire that runs from the audio panel to the speaker.  Throw away the one-conductor shielded cable, and replace it with two-conductor shielded cable, with the two conductor wires connected to pins W and 22, and the shield connected to pin 1.

 

 

Yes, I am told not to wire up modern audio panels the way Cessna and Mooney used to do it.  Do it like the PS Engineering shows it.

[Vance Harral]

1 minute ago, Aerodon said:

the way Cessna and Mooney used to do it

This is a good point.  With regard to the shop that installed the audio panel, that single-conductor shielded wire to the speaker may be a case of, "we re-used the factory-installed cable even though it's not correct for this audio panel 'cuz it sorta works"; as opposed to, "we don't know what we're doing and installed the wrong kind of wire".

[Aerodon]

10 minutes ago, Vance Harral said:

This is a good point.  With regard to the shop that installed the audio panel, that single-conductor shielded wire to the speaker may be a case of, "we re-used the factory-installed cable even though it's not correct for this audio panel 'cuz it sorta works"; as opposed to, "we don't know what we're doing and installed the wrong kind of wire".

It has something to do with the way that the audio amp works in modern audio panels.  And it's quite hard to do because now you have to open up the ceiling to get access to the speaker.

[MikeOH]

@Mooney-Shiner

Is this a NEW problem?  Or, have you always used headphones and just noticed the speaker doesn't work?

As others have said, the speaker isn't wired correctly per the schematic.  But, it is not clear if you are troubleshooting a new problem or an issue that has always been present.

If it is a new problem, I would disconnect the high side wire at both ends (speaker and tray) and check for a short between the high side wire and ground.