Master or Master Relay?

[Flash]

Symptoms: Last few times I tried to start my M20J, I turned on the master switch and either (1) my No. 2 GI-275 came on as an attitude indicator, with no power to the No. 1 GI-275 or the engine monitor, meaning I wasn't getting electrical power from the battery to the panel (the No. 2 GI-275 has a backup battery that, apparently, is working as designed!), or (2) everything worked normally. When it worked, the plane started and ran fine.

Is this probably the master switch itself (try a little contact cleaner and hope this solves things) or the master relay? Any easy way to diagnose?

[Z W]

Sounds like the relay or a bad connection. Your master switch operates by grounding a pole on the relay. You could test the relay and the switch by manually grounding it and seeing if it clicks, if it hasn't come on with the switch thrown.

Good time to check and clean the main ground connection between the battery and the airframe, and all of the wires and terminals in the main power system. Recommend using a multimeter to verify you have 12/24v where you're supposed to. Low battery voltage could be contributing.

My battery solenoid part number was no longer available. Spruce sells good replacements from Lamar for less than $100. I now keep a spare on the shelf in the hangar. A continuous duty one can also be used as a starter solenoid in a pinch so makes a good spare.

https://www.aircraftspruce.com/catalog/elpages/solenoidx610028.php

[Pinecone]

Why would your #2 come on unless there is some power getting to the bus.

Does anything else work, like lights?

[Flash]

2 hours ago, Pinecone said:

Why would your #2 come on unless there is some power getting to the bus.

Does anything else work, like lights?

No. 2 GI-275 has an internal battery that provides at least an hour of attitude indicator if the No. 1 fails. Thus, having it come on as an AI shows that it is working as it should but does not suggest that any power is flowing from the main battery.

[MikeOH]

2 hours ago, Flash said:

No. 2 GI-275 has an internal battery that provides at least an hour of attitude indicator if the No. 1 fails. Thus, having it come on as an AI shows that it is working as it should but does not suggest that any power is flowing from the main battery.

This does not make sense to me.  The #2 GI-275 is NOT on when the master is off!  So, as @Pinecone asks, if it comes on via its battery, it must be sensing some power due to the master being turned on.

[Yetti]

So it really depends on how the 275 are wired in.

Also it could be the Comm Bus Relay.

The Comm bus relay will fail to being closed.   Which means when you turn on the master it opens the Comm bus relay and stops power from flowing to the comm bus.  You may want to check where the Comm switch was on or off.

Sometimes you would want one gauge wired to the master bus and the other wired to the Comm buss.   This is more in line with an engine monitor that you want to come on with the master so you can watch the engine start on the EMS.

Lastly there is a circuit that turns off the Comm bus when the starter is engaged

I would go with you forgot to turn off the comm panel switch.

The other thing could be that a battery failed in one of the 275s and it used to stay on via battery when the starter switch was turned on.

 

[Pinecone]

23 hours ago, Flash said:

No. 2 GI-275 has an internal battery that provides at least an hour of attitude indicator if the No. 1 fails. Thus, having it come on as an AI shows that it is working as it should but does not suggest that any power is flowing from the main battery.

Except what is triggering it to turn on????

When my back up G-5 loses power it asks if I want it to stay on or shutdown.  If off, it comes one when it gets power.

[EricJ]

21 minutes ago, Pinecone said:

Except what is triggering it to turn on????

When my back up G-5 loses power it asks if I want it to stay on or shutdown.  If off, it comes one when it gets power.

They should talk to each other.   If they are like G5s, when the PFD fails (maybe from loss of power if there is no battery?), the HSI reverts to an AI.   I don't know whether it is supposed to turn on if it's off, since that would imply that there's stuff active in the G5 (or GI-275) that is monitoring the serial link even when it is "off", but stranger things have been done in the Garmin universe.

[Utah20Gflyer]

A multimeter should be able to answer your question definitely.  
1. check voltage at master switch input

2. Check voltage at master switch output with switch in the on position.  
 

3. Check voltage at relay input

4. Check voltage at relay output

Also verify all wiring connections are tight while you do this.  If there is anything else between the battery and the bus like a fuse/breaker then check before and after that too. 
 

This will tell you where the problem is. Pretty easy.  
 

Your back up GI 275 won’t start up in back up mode unless it gets some power initially to tell it to be on in reversionary mode.  

 

[Pinecone]

7 hours ago, EricJ said:

They should talk to each other.   If they are like G5s, when the PFD fails (maybe from loss of power if there is no battery?), the HSI reverts to an AI.   I don't know whether it is supposed to turn on if it's off, since that would imply that there's stuff active in the G5 (or GI-275) that is monitoring the serial link even when it is "off", but stranger things have been done in the Garmin universe.

But it needs a signal to turn on.  

[Yetti]

19 hours ago, Pinecone said:

But it needs a signal to turn on.  

The Com bus actually comes on (provides a signal} before the Comm relay can turn it off when the master relay comes on.    It's momentary.    This is probably why we could deduce at least 1 of the 275s are wired to the comm buss.    I started with it would help to know how they are wired.

[Pinecone]

1 hour ago, Yetti said:

The Com bus actually comes on (provides a signal} before the Comm relay can turn it off when the master relay comes on.    It's momentary.    This is probably why we could deduce at least 1 of the 275s are wired to the comm buss.    I started with it would help to know how they are wired.

OK, but you say the Com relay turns off when the Master Relay comes on, so the Master is powering up.