Time to replace my AV relay in the 252.

[shawnd]

Has anyone replaced their Avionics relay after it fails closed (avionics stay on). For my 252, Mooney provided 940074-503 which replaces the MB4413 original part. See retrofit guide. Plane is in the shop, would appreciate if anyone has pictures of the installation. Tips appreciated as well!

 

[N201MKTurbo]

Are you sure it is the relay? is there current getting to the coil? The relays are usually wired so if there is no coil current, the radios will be on.

[shawnd]

Not quite sure but guessing so.  The MSC replaced the switch as a first test, that didn't remedy the issue. They suggested it was the relay so I now have that. The plane is at a different facility, local avionics shop, getting some avionics installed and I wanted them to tackle this as part of the job.

[N201MKTurbo]

I would have them trouble shoot it before replacing it.

 

[Aerodon]

1 hour ago, shawnd said:

Has anyone replaced their Avionics relay after it fails closed (avionics stay on). For my 252, Mooney provided 940074-503 which replaces the MB4413 original part. See retrofit guide. Plane is in the shop, would appreciate if anyone has pictures of the installation. Tips appreciated as well!

 

I have a replacement relay for you, from a 1996 Bravo.

You better hope it's not the relay, it sure is buried in the panel, behind the circuit breakers.  Looks like the first thing they install.  I can take pictures.

 

Don

 

 

[shawnd]

Sorry if I wasn’t clear - Mooney sent me the (expensive) replacement relay so already have it here. Pictures would be great! Thanks Don

[Aerodon]

There are two avionics relays.  The smaller copper coloured one is the 'avionics control relay' and the larger one is the avionics relay.  For reference the circuit breakers are on the LHS of these pictures.  The square black box is the prop deice timer.  The square silver boxes are RF filters.  You can see the ammeter shunts mounter on the 'inside' side wall.  The avionics relay is also mounted on this inside' side wall.

Would appreciate pictures of the new relay please.

Don

[Fly Boomer]

28 minutes ago, Aerodon said:

There are two avionics relays.  The smaller copper coloured one is the 'avionics control relay' and the larger one is the avionics relay.

There is a small relay controlling a big relay?  Compared to other parts of the electrical system, avionics is a fairly low current draw.  Why the two relays?

[Utah20Gflyer]

3 hours ago, shawnd said:

Not quite sure but guessing so.  The MSC replaced the switch as a first test, that didn't remedy the issue. They suggested it was the relay so I now have that. The plane is at a different facility, local avionics shop, getting some avionics installed and I wanted them to tackle this as part of the job.

I’m not sure I’d trust a shop that replaced a switch without knowing that is the problem, how do they know it’s the relay now?  Maybe it’s a bad ground for the relay?  Could be lots of things other than the relay.  Replacing parts until you find the one that is bad is an expensive way to maintain an airplane.  

[shawnd]

@Aerodon Thank you! The detailed pictures and the guide is amazing. Hopefully others will find your post helpful in the future as well!

I will send photos of the new relay next week when I drive up there again to check in on status. If it comes to it, they sure will appreciate your pictures. Thank you again.

[shawnd]

4 hours ago, N201MKTurbo said:

I would have them trouble shoot it before replacing it.

 

Yeah once they are done with the current work and harnesses are back in and connected, will have them troubleshoot before replacement.

[Aerodon]

15 hours ago, Fly Boomer said:

There is a small relay controlling a big relay?  Compared to other parts of the electrical system, avionics is a fairly low current draw.  Why the two relays?

Agreed, but the large relay is a 'starter size' relay rated at 100A / 28V.  The control relay is a flimsy 'plug in' style.  And there is a diode in the wiring, IIRC there are some serious consequences if this diode wire breaks, I need to go look.

I do not like the way the control relay wiring crosses all the other wires, could easily lead to inadvertent damage.

When I look at the Mooney wiring overall, I think they were trying to be too clever for their own good - more complexity than needed, more intermediate connectors (with nice gold pins). 

 

Aerodon

 

 

 

 

[PT20J]

Is that second relay a M20K-specific thing? There isn't one on my wiring diagram for my 1994 M20J. There is just one avionics relay powered by the avionics master which is powered by the Aux bus.

[Fly Boomer]

4 minutes ago, PT20J said:

Is that second relay a M20K-specific thing? There isn't one on my wiring diagram for my 1994 M20J. There is just one avionics relay powered by the avionics master which is powered by the Aux bus.

I guess I need to pull out my wiring diagrams, and get busy.  The ones I have are pretty bad, so I tend to avoid looking at them.

[MikeOH]

My concern is if the shop is now going to charge you for that relay even if it turns out to be something else? Typically electrical products are NOT returnable (which is logical).

Just me, but I'd want the old relay so that I could confirm it actually failed.  I have zero use for the 'shotgun approach' to troubleshooting; even worse when YOU have to pay for their scatter shots

[Aerodon]

7 hours ago, PT20J said:

Is that second relay a M20K-specific thing? There isn't one on my wiring diagram for my 1994 M20J. There is just one avionics relay powered by the avionics master which is powered by the Aux bus.

The M20K has a small avionics toggle switch, the kind you have to pull to toggle so you don't inadvertently bump it off.  And like I said previously, it is a monster avionics relay, so I can see why they did a control relay.  But then there are multiple points of failure.  (And no simple 50A breaker switch for emergency power like Piper did).

I looked at the M20R wiring, and as far as I can see it has a 3A rocker avionics switch, instead of the control relay.  I'm in the process of installing one of these rocker switches as part of the panel upgrade, so I may as well remove the control relay to make it more reliable.  One less CB in the RHS panel too.

 

 

Aerodon

 

[Aerodon]

The more I look at this relay, the less I like it.  It weighs 1lb, has 5 Ohm resistance, so draws 5A to keep the avionics off just when you really don't want to be drawing current from the battery before and during startup.

I have seen several aircraft with just a high quality CB switch (Cessna) and sometimes a 'hooded' second CB switch (Piper) for backup.

Aerodon

 

 

 

 

[Aerodon]

On 1/12/2024 at 3:20 PM, shawnd said:

Has anyone replaced their Avionics relay after it fails closed (avionics stay on). For my 252, Mooney provided 940074-503 which replaces the MB4413 original part. See retrofit guide. Plane is in the shop, would appreciate if anyone has pictures of the installation. Tips appreciated as well!

 

Shawn,

The way the avionics relay is wired up, if you inadvertently try to start while the avionics are on, the starter switch should also turn the avionics off.

I looked at my CB panel assembly again, it would appear that I do not have the original avionics relay, I can see where this relay was removed and replaced by the 100A solenoid (as shown here), probably as per the service bulletin kit you have received.

Aerodon

 

 

 

 

 

[shawnd]

18 hours ago, MikeOH said:

My concern is if the shop is now going to charge you for that relay even if it turns out to be something else? Typically electrical products are NOT returnable (which is logical).

Just me, but I'd want the old relay so that I could confirm it actually failed.  I have zero use for the 'shotgun approach' to troubleshooting; even worse when YOU have to pay for their scatter shots

Agree, but its too late. I believe they did do some troubleshooting - my guess is listening to the relay to hear the clicks. At the end of the day, I am thinking of buying the relay as keeping Mooney afloat :-)

[shawnd]

14 hours ago, Aerodon said:

Shawn,

The way the avionics relay is wired up, if you inadvertently try to start while the avionics are on, the starter switch should also turn the avionics off.

I looked at my CB panel assembly again, it would appear that I do not have the original avionics relay, I can see where this relay was removed and replaced by the 100A solenoid (as shown here), probably as per the service bulletin kit you have received.

Aerodon

Interesting. I definitely had times when the avionics stayed on and wouldn't shut off through engine start **cringe**.  Agree, the part I received looks like your solenoid. I will send pictures this Friday when I head up to the shop again.

[Aerodon]

OK, so you probably need to look at two things:

1) if you have the big round solenoid type relay, your plane has probably already been upgraded.  From the wiring diagram the original relay was a multi pole relay with the terminals 'paralleled' for higher current.  So if you have that, go ahead and get it changed.

2) There is something wrong with your control relay.  These are inexpensive and plug in and out.  It could be the relay, or broken wiring, or bad contacts with the base of the relay.  If the avionics are not turned off by either the switch or the 'engine start', then it is a problem with the ground wire to the relay.  The ground wire is short, it goes from the relay to the grounding block nearby.  

Attached are pictures of the control relay, part number  W67CSX-3.

Aerodon

 

[shawnd]

Thanks @Aerodon will have them check it. I am quite certain I have the original relay from a quick peek last week. The other symptom that I commonly observed was that during shutdown, the avionics wouldn't shut off after the first try at flipping the switch, typically took 4-5, maybe more to eventually turn off.

I decided to dig in to understand it a bit more, attached you will find the

1. parts list for my 252's Avionics relays
2. schematics with the interesting areas highlighted and connections traced.

 

 

Schematics
M20K-AVRelaySchematics.pdf

 

Update

1/15 - Fixed avionics switch trace to GND bus. Thanks @Aerodon

 

 

[Aerodon]

Almost correct, the ground from the avionics switch goes to the main grounding bus, not the alternator ACU.

[shawnd]

3 hours ago, Aerodon said:

Almost correct, the ground from the avionics switch goes to the main grounding bus, not the alternator ACU.

Haha - that's what I get for doing things late night. You are absolutely correct - the terminal numbers match now. Updated the PDF in the post above.

[shawnd]

Here are the pictures as promised, got the old relay installed which will be swapped out for the solenoid. (Instruction PDFs in next post)