FlyDave Posted February 20, 2011 Report Share Posted February 20, 2011 Hi all, I have an 1982 M20J. It has a radio master switch (right next to the master power switch) and it also has another switch/CB under the panel that is a bypass to the radio master in case the radio master dies. Here is the scenario I experienced last week: 1. Master & Radio master OFF, bypass switch in normal position as it has been for 200+ hours. I turn on the master switch (radio master still off) and all avionics come on. I change position of the bypass swtich with no change. I turn the master switch off, wait a minute, turn the master back on and master power is up but avionics are off. I turn the radio master on and all avionics come up normally. Everything seems normal and I go flying. 2. 2 hours flying time and 3 starts/cycles of all switches and everything appears normal. On downwind departure the avionics master switch pops (it's a CB) and avionics all quit. I turn the avionics master back on and everything comes up. I land 20 minutes later with everything normal. I fly again a few days later and everything seems fine. A couple of scenarios come to mind. On #1 it would seem that the avionics master relay was stuck closed and energizing/de-energizing it cleared the connection between the contacts. But that could also have been the switches. On #2 it would seem that the radio master switch failed. Either could also be a short somewhere but I'm thinking more along the lines of a failed radio master relay. Has anyone replaced the radio master relay in an early 1980's M20J? The relay is a Potter Brumfield MB4413. I haven't been able to find one and Wilmar says the're not made any more and the retro-fit kit is $800 and (for a normal mechanic) a full day of work. Does anyone have a line on a replacement for this $50 relay that is not (ouch!) $800? Thanks, Quote Link to comment Share on other sites More sharing options...
Magnum Posted February 20, 2011 Report Share Posted February 20, 2011 It is the radio master relay. I had the same problem. It is wired "backwards": When the avionics master switch is in the OFF position, the switch is actually closed and the relay gets power, and the avionics is off. So when you switch the battery master on and the relay "hangs", the avionics will be on. Quote Link to comment Share on other sites More sharing options...
richardheitzman Posted February 20, 2011 Report Share Posted February 20, 2011 relays do stick and they do break down with time. I am not familiar with a "bypass" switch and never heard of it installed in any aircraft I have ever worked on. That seems to just complicate things. Power on, Master on, relay is energized and flows power to main buss. Master off, and relay is de-energized. I don't recall if the master switches power or a ground so if you want my opinion, here it is: First check all grounds in the system. Make sure your Avionics relay is grounded and all contacts are clean and free of corrosion. You are jumping to conclusions without taking a look. After everything is clean, take a look at this "bypass" switch. That sounds fishy to me. If everything is OK, but still failing, change the Avionics Master switch FIRST. That swich is what is controlling the relay On/Off. If you have changed the switch and it is still failing then start looking for a relay to put in. Just because you can't get it new does not mean there are no serviceable ones out there. Before you change the relay make damm sure all the wiring in the charging/electrical/avionics systems are good. (Clean, no corrosion, intermittent shorts) paying CLOSE ATTENTION to all feed throughs and areas of chaffing. Good Luck Quote Link to comment Share on other sites More sharing options...
FlyDave Posted February 20, 2011 Author Report Share Posted February 20, 2011 Thanks guys! Richard - I'll check/clean all connections working with my mechanic. The relay is on the circuit breaker panel tray. It's a real rats nest trying to get that tray out without pulling and damaging other connections. So I need a certain amount of supervision. I have a line on a replacement avionics master switch ($180 - yeeeowww) so I'll do that first. Magnum - what relay did you replace the failed one with? Quote Link to comment Share on other sites More sharing options...
Magnum Posted February 22, 2011 Report Share Posted February 22, 2011 Quote: FlyDave Thanks guys! Richard - I'll check/clean all connections working with my mechanic. The relay is on the circuit breaker panel tray. It's a real rats nest trying to get that tray out without pulling and damaging other connections. So I need a certain amount of supervision. I have a line on a replacement avionics master switch ($180 - yeeeowww) so I'll do that first. Magnum - what relay did you replace the failed one with? Quote Link to comment Share on other sites More sharing options...
Magnum Posted February 22, 2011 Report Share Posted February 22, 2011 I just checked the electric schematics for my '87 J. The Avionics master switch is connected to a "Avionics Control Relay", and this activates the Avionics Master Relay. When the Radio Master is in the OFF position, the switch is actually closed, the control relay and the master relay engage and power is cut to the avionics bus. Radio master ON cuts the power to the control relay and this cuts power to the avionics master relay and the avionics bus gets power. Maybe only the avionics control relay is not working. Quote Link to comment Share on other sites More sharing options...
FlyDave Posted February 24, 2011 Author Report Share Posted February 24, 2011 Quote: FlyDave 1. Master & Radio master OFF, bypass switch in normal position as it has been for 200+ hours. I turn on the master switch (radio master still off) and all avionics come on. I change position of the bypass swtich with no change. I turn the master switch off, wait a minute, turn the master back on and master power is up but avionics are off. I turn the radio master on and all avionics come up normally. Everything seems normal and I go flying. 2. 2 hours flying time and 3 starts/cycles of all switches and everything appears normal. On downwind departure the avionics master switch pops (it's a CB) and avionics all quit. I turn the avionics master back on and everything comes up. I land 20 minutes later with everything normal. I fly again a few days later and everything seems fine. Quote Link to comment Share on other sites More sharing options...
FlyDave Posted March 2, 2011 Author Report Share Posted March 2, 2011 We replaced the relay this weekend. I flew it Monday (1 45-minute flight) and yesterday ( 4 flights/4 airports) and all appears normal. Hopefully the issues are resolved. Also, the switch that I though was the avionics bypass was actually the power switch/CB for the pneumatic door seal. If you don't have a pneumatic door seal, get one! It quiets the airplane down dramatically (no wind noise or whistling in the cockpit). Quote Link to comment Share on other sites More sharing options...
daverob Posted July 11, 2013 Report Share Posted July 11, 2013 Ladies and Gents,, where can I get a replacement for mb-4413 as it seems they are no longer available. Dave   I have to tap mine occassionally to get it to work. Quote Link to comment Share on other sites More sharing options...
flyboy0681 Posted July 11, 2013 Report Share Posted July 11, 2013 About a year ago the radio relay was stuck in the "on" position in my J. After researching the problem on this site I told my avionics shop to first check the relay for carbon buildup before proceeding with costly research. Sure enough that's what it was and it turned out to be a relatively cheap repair. I said "relatively" because as mentioned, it's not easily accessible. Quote Link to comment Share on other sites More sharing options...
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