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201 Schematic / Bad Avionics Relay?


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This is a two-parter.

1. I had a set of electrical schematics that covered our serial number (1989 M20J, 24-3105), but I can't find them now. I'm pretty sure I found them the last time just Google searching. Can someone point me to where I could find that again?

2. Our avionics master switch is no longer turning off our avionics. Any time the master is on, the avionics are on, regardless of avionics switch position. This issue has shown up twice when shutting down the plane over the last couple months. In the last 2 days (about 4 flights), it has been on solid. Seems likely to be the avionics relay. Should this relay be pretty easy to get a hold of? Any suggestions for other likely culprits? 

 

 

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Generally avionics relays are "fail engaged".... and when energized remove power from the radio bus.  

When you turn on the master relay, the radio master immediately "removes" the power from the radio bus. (if the switch is in the correct position). 

http://mooney.free.fr/Manuels M20J/M20J/Mooney Service Manuel M20J Vol. 2 of 2.pdf

What I see is that if either the starter is engaged, or the switch commands it, the relay should close and "remove" power...   It looks like there are 3-4 paths through the relay so that if any one of the contacts fails it will still transfer power.  

So, if the master cuts out when you hit the starter.. then it is the switch... if not.. then the relay..   you can hit the relay with  + voltage and it should click.   Also check the connection from the relay to ground.. if that is broken then it won't activate with either the starter or the switch.   really easy to debug with a volt meter.
 

 

Edited by PaulM
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The POH should have the relay in the electrical drawing as well...

Finding the relays can be a challenge... Expect that they are in the tail near the battery.  

 

While on the ground, engine off... test the voltages, and/or continuity

Master off, Avionics off... avionics relay should be closed... but no voltage...

master on, avionics off... avionics relay should be open... still no voltage...

master on, avionics on... avionics relay should be closed... voltage available to the avionics...

 

The relays when working give a nice solid click or thunk.  It might require two people to throw the switch and listen for the thunk...

 

All of this reverse logic is to allow the avionics to work when the master relay fails...

 

PP thoughts only,

-a-

 

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Got to spend some time with the plane and a multimeter this morning. I thought I had found the avionics relay, but after some closer investigation, I was wrong. There are two large relays in the tail. One is the master relay. The other looks identical to the master relay, but seems to cut/supply connection to the hot lead of the ground power plug. I'm guessing it activates when the master does, but I'm not sure. I need to study the schematic more closely. I'm curious about the ground power relay and what it does.

On to the multimeter results:

-Master on, avionics switch off - 28VDC at both terminals of the avionics switch

-Master on, avionics switch on - 28VDC on the input side of the switch, 0VDC on the output side

 

The switch seems to be working correctly. With my head under the panel, I can hear a relay shifting when turning the avionics switch on and off. The relay sound is coming from behind the panel. The relay sound isn't quite as "sharp" sounding as I'm used to hearing. 

At this point, I basically ran out of time. With some more hunting, I can probably find the avionics relay. Does anyone know off hand where it is in a 1989 J model? Getting the multimeter on it and confirming that it is or isn't working is probably my next step.

 

Thanks for all the input so far.

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40 minutes ago, AaronDC8402 said:

Got to spend some time with the plane and a multimeter this morning. I thought I had found the avionics relay, but after some closer investigation, I was wrong. There are two large relays in the tail. One is the master relay. The other looks identical to the master relay, but seems to cut/supply connection to the hot lead of the ground power plug. I'm guessing it activates when the master does, but I'm not sure. I need to study the schematic more closely. I'm curious about the ground power relay and what it does.

The GPU plug includes 3 pins..   "engage (28V)" "+v(28)" and GND...  it is designed that the larger +/GND pins engage first.. and then once the "engage" pin mates it triggers the relay that connects the +(28) to the battery bus..  by this time the +28V connections already has good solid contact. 

The relay is there so that the socket doesn't normally have any voltage on it... you need the plug to connect it to the system. 

 

Quote

On to the multimeter results:

-Master on, avionics switch off - 28VDC at both terminals of the avionics switch

-Master on, avionics switch on - 28VDC on the input side of the switch, 0VDC on the output side

switch is fine.... so you have to check that the relay is actually triggering, and that the contacts haven't been welded. 

Edited by PaulM
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I had the identical problem which took hours to diagnose, but finally got it resolved. Identical symptoms, when I turned the master on all of the avionics powered up even though the avionics master was off. When my plane was AOG I told the shop to look into the avionics master problem because I lived with it for six months and wanted a final resolution. They initially reported back that they bench tested the relay and it was in good shape. When they reinstalled it the problem continued. It wasn't until they traced the wiring that they found wires were coming out of a crimp. Once they re-crimped, it worked perfectly and has ever since. Unfortunately I can't tell you where the crimp was located.

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The mystery has been solved. Well, the mystery of the relay's location at least has been solved. It was behind the circuit breaker panel. It couldn't have been much harder to get to. It's on top of a "shelf" in a way that you can't see it at all from under the foot well. It's deep enough in there, that you can only barely see it with the glareshield removed.

The relay is a Potter & Brumfield MB-4443-1. Seems a little difficult to get a hold of, so we're going to start with cleaning the contacts. They did have some corrosion on them. You could still hear it clicking when you flip the switch, so the solenoid portion was still working. Just not enough to pull the contacts open.

We checked the connections at the relay, and they seem well connected.

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I had the exact same problem in my 1988 J. The relay part number is the same. I found one for $600 with no guarantee it would work. The local Mooney service center advised making sure none of the circuit breaker terminals were coming into contact with the buss or the relay. In my case, we found corrosion on the relay terminals. The relay bench tested fine. Since cleaning up the terminals and making sure there were no other contacts, it has been fine. Fingers crossed because the kit from Mooney to fix this costs $1200 to $1400, not including installation.

JB

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http://mooney.free.fr/Manuels M20J/M20J/Mooney Service Manuel M20J Vol. 2 of 2.pdf

The schematics

You might note that there is a wire from the starter solenoid with a diode that will turn off the radios if when the starter is engaged.  The diode is sky wired into the wire at the relay.

Someone was able to use the other side of the relay when one side burned up.

They fail closed.  ie they are powered to turn off.

There is another relay up there for the landing light in some planes.

Strangely enough a 5 pin bosch relay also has a closed when not powered side to it.

I think Mooney undersized the radio relay based on the amperage of the CB it is supposed to handle.

Someone just yanked mine out of the circuit because they though it not important.

 

IMG_20180217_082130.jpg

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