eman1200 Posted September 7 Report Posted September 7 OK strange situation......fly to VA for the weekend. Landed plane on Friday, tied down, most DEF turned off the master. Saturday it rained most of the day. Sunday morning I get a call form the airport saying my strobes/navs were on, I was like there is NO WAY 1) I left the master on and 2) no one at the airport noticed until Sunday. So drove up to the plane, sure enough, strobes/master on. Get in the plane, master is most definitely off. Started poking around, pulled the access panel open on the master switch side. Found NO water in there at all, which was our initial guess. Check several places, tried jiggling wires on the master switch, nothing worked, the plane was still getting power as if the master was on. Pulled the inspection panel off for the battery and the only thing we could really come up with is when I pulled the circled fuse, power would cut off: Is it possible this relay is bad? We tried tapping on it, looking for loose wires etc, couldn't find anything. No water in this area from the rain. So pulling that breaker and obviously disconnecting the battery works, otherwise it's as if the master is stuck on. Help! Thanks!
Z W Posted September 8 Report Posted September 8 That's your master relay. It could be bad, or you could have a wiring problem between it and your master switch. I believe a short to ground between the master switch and the relay could cause what you're seeing, in most installations. Don't let the A&P go down to the local auto parts store for a replacement. Spruce sells good FAA/PMA ones. https://www.aircraftspruce.com/catalog/elpages/solenoidx610028.php 3
eman1200 Posted September 8 Author Report Posted September 8 That's your master relay. It could be bad, or you could have a wiring problem between it and your master switch. I believe a short to ground between the master switch and the relay could cause what you're seeing, in most installations. Don't let the A&P go down to the local auto parts store for a replacement. Spruce sells good FAA/PMA ones. https://www.aircraftspruce.com/catalog/elpages/solenoidx610028.phpThat’s the solenoid which is probably less than a year old. We were thinking the relay or the wiring ,like you said.
Z W Posted September 8 Report Posted September 8 @eman1200 My parts catalog calls it a relay. It could also be called a solenoid. Pretty much the same thing. The solenoid with the arrow pointing to it in your photos does not look less than a year old. There is another one mounted on the crossmember that is not present in my plane that looks new, and also like it should have some protection over the terminals to prevent accidental arcing. I suspect someone has done something non-standard with your wiring that may be contributing to your situation.
eman1200 Posted September 8 Author Report Posted September 8 1 minute ago, Z W said: @eman1200 My parts catalog calls it a relay. It could also be called a solenoid. Pretty much the same thing. The solenoid with the arrow pointing to it in your photos does not look less than a year old. There is another one mounted on the crossmember that is not present in my plane that looks new, and also like it should have some protection over the terminals to prevent accidental arcing. I suspect someone has done something non-standard with your wiring that may be contributing to your situation. the aircraft spruce link you provided was for a solenoid. the solenoid pictured above IS less than or around a year old, but it is not what the arrow is pointing to. the arrow in pic 1 points to the inline fuse and the arrow in pic 2 points to the relay, which has been there since I've had the plane. I'm question if the RELAY is part of the problem, not the solenoid.
Z W Posted September 8 Report Posted September 8 For clarity, if your F is wired like my K, and it looks like it is, this one is your master relay/solenoid, mounted on the rear of the battery box where you have to lean way into the tailcone to see / work on it:
Z W Posted September 8 Report Posted September 8 2 minutes ago, eman1200 said: the aircraft spruce link you provided was for a solenoid. the solenoid pictured above IS less than or around a year old, but it is not what the arrow is pointing to. the arrow in pic 1 points to the inline fuse and the arrow in pic 2 points to the relay, which has been there since I've had the plane. I'm question if the RELAY is part of the problem, not the solenoid. The new solenoid in your pictures on the crossmember next to the inline fuse is not powering the main bus, I don't think. It would need to have the big 2 gauge wires going to it. It's powering something else, and I can't tell what.
carusoam Posted September 8 Report Posted September 8 Things to remember… Solenoids and relays are names for the same device an electro magnetic switch… the word solenoid is headed for the dustbin of magic words… they last for decades before failing… each time they operate, a tiny spark occurs, releasing a few atoms into the air inside the case… they are slowly wearing from the first day… they have key details in the part name… normally open and normally closed…. NO NC. they are wired in an interesting way with the avionics relay… this provides the avionics to work in the event the avionics relay fails…. It will fail closed. take a look at how your strobes are wired… if your relays are original… you may have three that are very similar… carry a spare of each the NO and NC… Start, master, avionics, battery selector… plus a few more… PP thoughts only, not a mechanic… Best regards, -a-
carusoam Posted September 9 Report Posted September 9 Also… add to your checklist… walking away from the plane… always look back to see the red flashing strobe isn’t operating… could be a memory issue, or a worn relay… Best regards, -a-
Jim Peace Posted September 9 Report Posted September 9 also for those of you with G5s you can get into situation where you turned the battery off then back on to check something then off again and you throw the G5s out of sequence and they will not start the count down and power off...they just stay on....dont ask me how I know..... 1 1
Pinecone Posted September 9 Report Posted September 9 12 hours ago, carusoam said: Things to remember… Solenoids and relays are names for the same device an electro magnetic switch… the word solenoid is headed for the dustbin of magic words… they last for decades before failing… each time they operate, a tiny spark occurs, releasing a few atoms into the air inside the case… they are slowly wearing from the first day… Actually a relay is a solenoid but a solenoid may not be a relay. A solenoid is an electromechanical device that produces a motion from electricity. It can be used for many purposes. If that purpose is to switch electrical power, it is then a relay. But solenoids are used for lots of uses other than switching electricity. 1
Pinecone Posted September 9 Report Posted September 9 12 hours ago, carusoam said: Also… add to your checklist… walking away from the plane… always look back to see the red flashing strobe isn’t operating… could be a memory issue, or a worn relay… Best regards, -a- This is why my Beacon switch is always ON. 1
eman1200 Posted September 9 Author Report Posted September 9 This is why my Beacon switch is always ON.And was also not what happened to me. When I got the call that my lights were on I said there is NO WAY I left the master on. And I didn’t, hence this thread where I specifically mentioned the power is on even when the master is off.
Pinecone Posted September 9 Report Posted September 9 We had a similar issue on my friend's T-34. At an airshow in Mass, overnight was a pretty heavy rain storm. When we got to the airport the next morning, the battery tray was pulled out. The line crew said that in the middle of the rain storm the strobes and nav lights were on. They only way they could stop them was the disconnect the battery. On the T-34, the battery is on a slide out tray that pulling it out, automatically disconnects it. We pushed it back in and all was good. I was flying the plane back to MD for an airshow the next weekend. I launched into an IFR flight. While being vectored for the approach, just as I dropped below the clouds, EVERYTHING went off. And in the T-34, just about everything is electric. I did a manual gear extension and landed without issues. It turns out that the master relay was on the firewall under a small access door. The door does have drain, but it had gotten clogged, so with the heavy rain, the water was dripping on the master relay. I was lucky it held out as long as it did. A few minutes earlier and it would have been a bit dicey as the instruments were electric.
Pinecone Posted September 9 Report Posted September 9 8 minutes ago, eman1200 said: And was also not what happened to me. When I got the call that my lights were on I said there is NO WAY I left the master on. And I didn’t, hence this thread where I specifically mentioned the power is on even when the master is off. I understand what happened to you. I was commenting on the other post about checking for your beacon as you walk away from your plane. 1
carusoam Posted September 9 Report Posted September 9 10 hours ago, Pinecone said: Actually a relay is a solenoid but a solenoid may not be a relay. A solenoid is an electromechanical device that produces a motion from electricity. It can be used for many purposes. If that purpose is to switch electrical power, it is then a relay. But solenoids are used for lots of uses other than switching electricity. Cool! electro mechanical actuators… if your dishwasher is leaking. It has a solenoid valve that is supposed to be NC when the washer is off… (valve closed) similar with the clothes washer… one for hot, one for cold… but, I digress… the Mooney Parts Catalog calls them Relays… shorter, easy to spell, takes a low current signal to quickly switch a high current source… it sort of relays the pilot’s intention… The pilot activates a nice labeled switch on the panel, that activates a big switch close to the device. many pilots don’t know what the panel switch is actually doing… great tech conversation… best regards, -a-
Pinecone Posted September 10 Report Posted September 10 Yes. So, most likely the power comes from the battery to one side of the coil. Then a wire goes from coil to Master Switch, then to ground. If the Master Switch is Off, no current flows so relay is Open. Then you turn on the Master Switch, the relay coil sees current flow and closes relay. Avionics Master Switches work the other way. They are powered from the main bus, so with the Master off, no current to flow. Master On, Avionics Master Off, the power flows from the main bus, through the relay coil to the Avionics Master Switch. And the Relay Opens, so no power to Avionics. When you hit the switch, the current stops, and the relay closes and power goes to the Avionics. This fail safes the Avionics power. Not all aircraft are wired this way. 1
eman1200 Posted September 11 Author Report Posted September 11 Pulled the master switch out, resistance seemed normal. Don’t think it’s the switch at this point. Working on next steps.
carusoam Posted Monday at 11:39 PM Report Posted Monday at 11:39 PM On 9/10/2025 at 11:40 PM, eman1200 said: Pulled the master switch out, resistance seemed normal. Don’t think it’s the switch at this point. Working on next steps. Switches often wear… they are designed with an over center bump that keeps the either opened or closed… as they wear, they can easily be bumped and switch positions… I haven’t seen one physically switch, but not change the open/close status… in this case… it sounds like the switch is operating correctly. But, the next device in line is probably not… master switches are not as complex as the CB switches that carry more load… PP thoughts only, not a mechanic… best regards, -a-
eman1200 Posted Tuesday at 02:41 AM Author Report Posted Tuesday at 02:41 AM I'm looking for a part number/replacement for this puppy: pretty difficult for me to find it in the svc/maintenance manual but I'm wondering if a Sky-Tec Sts-M12 Battery/Master Solenoid By Lamar would suffice, other than how it would be mounted?
47U Posted Tuesday at 05:03 AM Report Posted Tuesday at 05:03 AM 2 hours ago, eman1200 said: I'm looking for a part number/replacement for this puppy: We’re still referencing the master relay, correct? Mx Manual 106, pg 9-129. Original installation by the factory was manufactured by Cutler Hammer, pn 6041-H-231. Several used examples on controller.com, and BAS lists them on their website, also. https://www.controller.com/parts/search?PartNumber=6041-H-231&SearchType=Start I think Spruce sells TSO’d replacements… if you’re looking for a new one. 1
eman1200 Posted Tuesday at 05:16 AM Author Report Posted Tuesday at 05:16 AM We’re still referencing the master relay, correct? Mx Manual 106, pg 9-129. Original installation by the factory was manufactured by Cutler Hammer, pn 6041-H-231. Several used examples on controller.com, and BAS lists them on their website, also. https://www.controller.com/parts/search?PartNumber=6041-H-231&SearchType=Start I think Spruce sells TSO’d replacements… if you’re looking for a new one.Thanks! 1
Z W Posted Tuesday at 11:37 AM Report Posted Tuesday at 11:37 AM When I went through this, the old original OEM part number relay was no longer available anywhere. There is some kind of advisory circular that allows A&Ps to install relays that don't match the part number, and arguably to install relays that are not TSO'd nor designed and intended for aircraft use. Someone in the distant past had used relays with NAPA auto parts stickers on them for my shore power, master, and starter relays. I discovered it when they started failing in interesting ways - the starter relay failed closed, resulting in the prop starting to move when you turned on the master. Modern TSO'd units should not fail like that. The answer is to use the TSO'd relays from Spruce. The mounting holes lined up just fine on mine, replacing an older style like the one in your photos. If not, you or your A&P might need to drill a couple new holes in your battery box, it just mounts with a couple of screws/bolts. Good time to order new mounting hardware while you're getting a shipment in from Spruce. Good luck. 1
eman1200 Posted Tuesday at 08:17 PM Author Report Posted Tuesday at 08:17 PM for the record, are there people who have replaced the oem master with the Lamar sts-m12? from the specs: "Silver Contacts provide 300A in-rush & 150A continuous-duty capability Much higher than OE units which is higher in-rush but fewer continuous. I think other than starting the plane this should be more than enough but just wanted to check.
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