Radios come on during engine cranking. Normal???

[Dflybldr]

Hello everyone. Just purchased a 1984 M20j. During the cranking phase before firing and when I stop pushing the key in, I hear static in the speakers and see the radio displays flicker/com-on. All this stops after I let go of the key and the radios come on normally when I turn on the radio/avionics master. Does anyone else experience this? Is it normal? Is it a problem?

My Mooney service center is convinced it's bad and it needs to charge me 95$ per hours to trouble shoot it that fix it. 

I had the line boys/girls hook up ground power, cranked it, and saw the same thing. Does this rule out the battery?

Thanks in advance for you responses.

N5739J

[Hank]

Something's not right. I'll leave it to the electrical types to troubleshoot, I hate electrical diagnosis. Wiring is pretty easy, it's all the wrong-way math behind it that makes my head hurt. Mechanical things I can pick apart and out back together, but electrons don't work like that.

My only guess would be improper grounding or a piece of broken insulation somewhere. Good luck!

[jetdriven]

the avionics relay is powered open. pull the avionics master CB it should power up, that effectively what the avionics master does, depowers the relay. . So when you are cranking, the buss voltage drops too low, and the relay un-powers which turns on the radios...

Clean the battery terminals, the grounds at the fire wall, and the battery negative cable lug from the battery.   and make sure you have a good strong battery.  Start there.

[Dflybldr]

Thanks. That's how i understood how it works. I will install the ACS concord RG battery I just bought and make sure I don't' have high resistance connections before I pay them to tell me that's what I need. existing battery is 3.5 years old and sat for a while.

Thanks for the quick reply.

[Yetti]

Looking through the schematics I saw part of the circuit go to the starter solenoid which should provide power to the com relay turning it off when the key supplies voltage to the starter relay. There is a diode in the circuit.  (the comm relay fails to closed)

http://mooney.free.fr/Manuels%20M20J/M20J/Mooney%20Service%20Manuel%20M20J%20Vol.%202%20of%202.pdf

[Dflybldr]

Thanks Yetti, This is exactly what I was looking for. I will have them locate and check the diode. A failed diode would not block the start position from keeping the relay powered open and allow the avionics relay to close (i think). Anyway, It help to point these guys in the right direction and keep them focused. I cleaned, gapped, and rotated by own plugs this weekend and felt pretty darn good about it. I have completed my transition training and hope to solo this weekend.

[carusoam]

Good to make relationships with those two resources...  CFI and A&P.

One will save your money and the other will save your bacon.  Good to have both in your camp.

Best regards,

-a-

[Yetti]

I believe the diode would be there to block the power from avionics switch from engaging the starter solenoid.

[Dflybldr]

Has anyone installed the 940074-501 retrofit kit? My MB4413 relay shows serious signs of abuse. It's strange that I cannot find a SB or SI for it's installation. Only the diode.

The MB4413 is made of unobtainium, and the substitute relays in the kit are on back order. :-(

[Yetti]

did you try Mouser, Digikey and McMaster Carr?

[Ratherbflying]

10 hours ago, Dflybldr said:

Has anyone installed the 940074-501 retrofit kit? My MB4413 relay shows serious signs of abuse. It's strange that I cannot find a SB or SI for it's installation. Only the diode.

The MB4413 is made of unobtainium, and the substitute relays in the kit are on back order. :-(

You will not find MB4413, it is long ago extinct. The good news is, if you carefully clean up the pins on your existing relay (try not to remove too much of the coating when eliminating the corrosion) you can get years more service out of it. If the socket is in a bad state, you can get that off of ebay for $5. (Search for AMF Potter and Brumfield part number 27E122)

The bad news is that the location of the relay and the socket make this a labor hours intensive effort. You would be better off in the long run going to your avionics guy and have him replace the relay with a circuit breaker switch. Your relay failure has already exposed your avionics to some very unkind power surges, and a CB switch will not allow that to happen. The relay design is archaic.

[ArtVandelay]

I believe the diode would be there to block the power from avionics switch from engaging the starter solenoid.

Normally diodes are used to prevent voltage spikes when the relay opens, just in case the RM is on when you try to start the engine. A quick test with voltage meter will tell you if the diode is working.

Edit: looking at it i see what you mean, I don't see this but from wiring diagram I should.

[Dflybldr]

Ok. So I went to visit N5739J in the hospital, I mean Mooney service center today and asked to see the MB4413 relay. It looked bad with signs of overheating on the can (blistered paint). I put it on the bench and it passed continuity and isolation checks. I also applied 12vdc to the coil and all 3 contacts opened as expected. I also checked the contact resistance and they all homed out the same. The socket also passed visual inspection. I will load test the old battery tomorrow and make sure all connections are clean and tight.

I would prefer to figure this out before bypassing with a switch.

Thanks for the clues. 

BTW the San Diego Crownair Mooney service center has been very understanding of a Mooney owner that wants to know and understand their aircraft.

 

 

 

 

[carusoam]

Dfly,

when you get a chance...  Update your avatar info.  It will help others to help you.

Over a few days, they may not remember you have a J.

Best regards,

-a-