1970 M20F - Mysterious Black Box

[ProtoFly]

Recently acquired 1970 M20F.  First annual in progress, owner (me) assisting, and trying to go through lots of things to ensure....well....safety.

The plane has had a couple different autopilots, none of which are installed now, but yay, wiring is still there, so in an attempt to clean things up, I'm going through and trying to remove unused bits.  The 'last' autopilot that was removed was a Brittain B6, from what I can tell.  Whoever 'uninstalled' took the stuff out of the panel, along with any control units, but left all the wiring behind, zip tied to the rest.  While tracing wires around, I discovered a bunch of wires that are just 'capped', along with some 'not capped', that go into a black box immediately behind and below the glove box, but below the Shower of Sparks unit.  No label on the box on the visible sides, but about 6 wires that look to be attached, have small bits of blue masking tape with faded lettering.

ptt+, ptt-, gear, over volt + (light), as examples.

Going through the wiring diagram, and I don't see anything 'factory' that would fit the description, so wondering if the collective wisdom here can maybe explain what the box is.

Pic of the plug and box:



 

 

 

[ProtoFly]

To add to this, a pressure switch attached to the pitot line, with all but two wires disconnected.  Wondering what this is trying to do, hanging there in space.

My A&P IA hasn't seen either device before, but is all in favor of removing no-longer-used wiring and crap.

[ProtoFly]

And... I think the VEP 654 switch is SUPPOSED to be for avoiding the landing gear coming up on the ground, but I'm not sure it's wired in at all...

So, that leaves the 'black box'...

 

[EricJ]

24 minutes ago, ProtoFly said:

To add to this, a pressure switch attached to the pitot line, with all but two wires disconnected.  Wondering what this is trying to do, hanging there in space.

My A&P IA hasn't seen either device before, but is all in favor of removing no-longer-used wiring and crap.

That's the landing gear airspeed safety switch.   If you have a red override button on your instrument panel, this is the gear safety and has been installed to replace the squat switch.    It's a better system than the squat switch in most regards.   It only needs two wires since it is just a continuity switch.

[wburger1]

48 minutes ago, ProtoFly said:

To add to this, a pressure switch attached to the pitot line, with all but two wires disconnected.  Wondering what this is trying to do, hanging there in space.

My A&P IA hasn't seen either device before, but is all in favor of removing no-longer-used wiring and crap.

Looks like the safety gear up switch and it needs to be there.  

[ProtoFly]

28 minutes ago, EricJ said:

That's the landing gear airspeed safety switch.   If you have a red override button on your instrument panel, this is the gear safety and has been installed to replace the squat switch.    It's a better system than the squat switch in most regards.   It only needs two wires since it is just a continuity switch.

Makes sense.  However, no override switch that I can find.  More fun to be had.

[PT20J]

Is the VEP switch just hanging there? I cannot see how it is attached. The two mounting methods I am familiar with is attached to the rear of the airspeed indicator or attached to a bracket near the airspeed indicator. 

[ProtoFly]

15 minutes ago, PT20J said:

Is the VEP switch just hanging there? I cannot see how it is attached. The two mounting methods I am familiar with is attached to the rear of the airspeed indicator or attached to a bracket near the airspeed indicator. 

Yeah, literally just hanging there.  It's a bit of a mess back there...

[EricJ]

1 hour ago, ProtoFly said:

Yeah, literally just hanging there.  It's a bit of a mess back there...

It needs to be secured to something.   There's a lot of stuff back there, including some steel fuselage tubes, that are suitable to secure that.

[DCarlton]

8 hours ago, ProtoFly said:

To add to this, a pressure switch attached to the pitot line, with all but two wires disconnected.  Wondering what this is trying to do, hanging there in space.

My A&P IA hasn't seen either device before, but is all in favor of removing no-longer-used wiring and crap.

I’m not a mechanic not an expert but I’m on my third airspeed safety switch.  If I remember correctly the original Duke switches only had two wires.  Some models of the newer VEP switches have four wires.  As previously noted, you only need two wires. The other two could be used to trigger a Hobbs meter for example.  The schematics for the VEP or World Magnetics switches are available on line.  Do not remove it. You need it.  Check operation during your annual gear swing.  They can be adjusted.  Check your wiring diagrams and the specs for the minimum airspeed that’s supposed to allow the gear to come up. You’ll be applying careful pressure to the pitot tube to trigger the switch during the gear swing while watching the airspeed indicator.  I do not have an override switch on my 67F panel.  

[Jsno]

I am doing something similar to my 1969.  Are the wires marked with tape going to the black box?  It looks like something for the auto pilot.  Before removing anything you should trace the wires and study your wiring diagrams in the MM manual to underStand what they are for before removing anything.

[ProtoFly]

6 hours ago, Jsno said:

I am doing something similar to my 1969.  Are the wires marked with tape going to the black box?  It looks like something for the auto pilot.  Before removing anything you should trace the wires and study your wiring diagrams in the MM manual to underStand what they are for before removing anything.

Given that the autopilot was removed, and I can find no reference in the wiring diagram for this black box, I'm inclined to agree with you.  And yeah, wires remaining are marked with blue masking tape, barely legible.

im going through all the wiring, eliminating anything to do with the autopilot, so as to make room for a new one maybe in the spring.  I expected the first annual to be lengthy and cost more $$, but end up with a better plane.

[Jsno]

From the wires going to it I wonder if it is some kind of audio alarm system.

[PT20J]

If you are going to do much panel upgrading along with the autopilot, it will be easier to remove old wiring when everything is opened up.

[ProtoFly]

1 hour ago, PT20J said:

If you are going to do much panel upgrading along with the autopilot, it will be easier to remove old wiring when everything is opened up.

It is as upgraded as I can reasonably afford, short of an autopilot.  Has two G5's, GNS530w, EDM-830.  Overall I'm happy with the panel.  Which is why I'm trying to get rid of all the extra stuff...

[DCarlton]

35 minutes ago, ProtoFly said:

It is as upgraded as I can reasonably afford, short of an autopilot.  Has two G5's, GNS530w, EDM-830.  Overall I'm happy with the panel.  Which is why I'm trying to get rid of all the extra stuff...

Again, no expert here, just experienced with my M20F.  Problem with removing old stuff, is (a) it's risky; cut or remove one wrong wire and you'll spend hours finding the problem (b), you'll do everything possible to clear out the old wiring, and tidy everything up as best as possible; then look up after all the work and still see a rats nest.  But I did it too.  

[MikeOH]

Another opinion to 'leave well enough alone'...sure it's tempting to make it all neat and clean and save a couple of pounds.  But, for me, the risk of mucking something up would stop me.

[Aerodon]

The gold box is an airspeed switch (actually two switches) that can be used for gear, transponder, timer on, traffic systems, hobbs etc, anything that needs on when the airspeed is alive.

Keep us out of suspense, follow the wires on the black box.

 

Aerodon

 

 

[Shadrach]

On 12/21/2024 at 3:31 PM, ProtoFly said:

Recently acquired 1970 M20F.  First annual in progress, owner (me) assisting, and trying to go through lots of things to ensure....well....safety.

The plane has had a couple different autopilots, none of which are installed now, but yay, wiring is still there, so in an attempt to clean things up, I'm going through and trying to remove unused bits.  The 'last' autopilot that was removed was a Brittain B6, from what I can tell.  Whoever 'uninstalled' took the stuff out of the panel, along with any control units, but left all the wiring behind, zip tied to the rest.  While tracing wires around, I discovered a bunch of wires that are just 'capped', along with some 'not capped', that go into a black box immediately behind and below the glove box, but below the Shower of Sparks unit.  No label on the box on the visible sides, but about 6 wires that look to be attached, have small bits of blue masking tape with faded lettering.

ptt+, ptt-, gear, over volt + (light), as examples.

Going through the wiring diagram, and I don't see anything 'factory' that would fit the description, so wondering if the collective wisdom here can maybe explain what the box is.

Pic of the plug and box:



 

 

 

 

Is the box mounted on the pilot’s sidewall?  That is typically where the Brittain PC controller (at least I think that’s what it is) shown below is mounted when installed on a Mooney. That Box looks a bit more modern but if it is installed in the same place, it is likely a remnant of the last Brittain AP.

[dkkim73]

The VEP switch, in my plane, has one micro-relay as part of the gear safety circuit (in lieiu of a squat switch as noted) and the other 3-wire circuit switching the stall vane heater between low and high (ground and flight), through the combined pitot heat circuit. Might be FIKI only in this model, not sure. 

But it sounds like there's some variation out there so you should figure it out carefully before eviscerating. 

That last black thing? Not sure. Probably a remote unit for a turbo encabulator or something like that. 

[ProtoFly]

On 12/30/2024 at 1:37 PM, Shadrach said:

Is the box mounted on the pilot’s sidewall?  That is typically where the Brittain PC controller (at least I think that’s what it is) shown below is mounted when installed on a Mooney. That Box looks a bit more modern but if it is installed in the same place, it is likely a remnant of the last Brittain AP.

It's a roughly 3"h x 4"w x 5"L black box on the passenger side.  Most of the wires to it are clipped and capped.  I'm thinking about unplugging it and seeing what (if anything) doesn't work.  On the ground, of course.

Ill also pull it out and see if there's any labels underneath it....