Bad Master Switch?

[jsalley]

Went to pick up my 12iS at the paint shop today. After a normal startup, the radio wouldn’t come on, although the transponder, gps, and autopilot all worked normally. Shut down the engine as usual, but when I turned off the master switch, the EFIS’s, cooling fans, and elevator trim all remained energized. Working the master switch on and off had no effect (although the green indicator light on the switch worked normally). Went to restart the engine to taxi to the FBO, but the ignition and fuel pumps would turn on only with the emergency EMS switch activated. Had to pull the start/trim fuse to power everything off. A&P looked at it, removed the ignition module panel, and checked continuity of the master switch. He thinks the switch may have failed internally. New one is ordered, but it doesn’t make sense to me that the radio won’t power on when the other avionics will, given that the radio is powered by the same avionics switch and bus as the autopilot and transponder, and that the fuel pumps won’t power up without the emergency EMS switch on.

Thoughts? Tony and Nate, I’m listening!

 

[Bob Y]

Regarding the radio, I know this will be a stupid question, but are you SURE the knob on the radio wasn’t turned off? Possibly during the painting process?

As to your other issues, it does sound like contacts on the master are welded shut. Remember that if the master isn’t cycled, you won’t be able to re-start the pumps or ignition after a shutdown, so why turning on the EMS switch allowed them to come back on line. The start computer in the ignition module is looking for a voltage drop to tell it to re-energize the pumps and ignition. Same as during an air start, so those symptoms are explainable.

 

[jsalley]

Thanks Bob - knob was the first thing I checked. I think you’re right about the start module preventing the pumps from running - I hadn’t thought of that. The switch had a funny sound when next to my ear - A&P thought it sounded like a displaced internal spring, which could well cause the contacts to stay in continuity. Still doesn’t make sense about the radio, though. And I pulled the center instrument panel and radio to check the connectors for bent pins, but all looked normal. No change when replaced.

 

[Bob Y]

Well, if you’re sure the xponder was coming on, I’m at a loss. I’m not aware of any internal fuses in the 200B. I don’t suppose you have any 12 friends near you with a 200 that you could swap radios to check, do you?

 

[jsalley]

Well, if you’re sure the xponder was coming on, I’m at a loss. I’m not aware of any internal fuses in the 200B. I don’t suppose you have any 12 friends near you with a 200 that you could swap radios to check, do you?

Good thought, but the plane is stranded 3-1/2 hours away. The A&P up there is going to replace the master switch (new one should be in hand in 2 days), and we’ll reassess the radio issue after that.

 

[Bob Y]

Don’t know where it is now, but maybe if you post on the RV-12 page asking if someone with a 200 or 200B in the area might be willing to do a quick swap, someone may offer to help? Just another thought.

 

[jsalley]

Update: Master switch replaced today. The radio now works (coincidental? I wonder…), but master switch still doesn’t function to deenergize anything when turned off. A&P that installed the switch now thinks it might be a stuck or internally frozen solenoid, although I could hear it click when the fuse was pulled and replaced. Spoke to Builder Support at Van’s, and they agreed that it sounds like a bad master solenoid, which they’ve seen before. Hopefully that will take care of it. Fingers still crossed…..

 

[RV7A Flyer]

Update: Master switch replaced today. The radio now works (coincidental? I wonder…), but master switch still doesn’t function to deenergize anything when turned off. A&P that installed the switch now thinks it might be a stuck or internally frozen solenoid, although I could hear it click when the fuse was pulled and replaced. Spoke to Builder Support at Van’s, and they agreed that it sounds like a bad master solenoid, which they’ve seen before. Hopefully that will take care of it. Fingers still crossed…..

This seems like something a simple voltmeter should be able to check?

 

[jsalley]

This seems like something a simple voltmeter should be able to check?

That’s the next step.

 

[RV7A Flyer]

My point was perhaps a bit too subtle. Your mechanic doesn't seem to be troubleshooting so much as shotgunning:

"He thinks the switch may have failed internally."
now he "thinks it might be a stuck or internally frozen solenoid".

He should have checked things with a VM first before starting to change things out. He's probably right this time, those solenoids are known to fail (much more often than switches as far as I can tell). That's where I'd have started, at least...power on, check voltage at the solenoid. Power off, check it again.

 

[jsalley]

My point was perhaps a bit too subtle. Your mechanic doesn't seem to be troubleshooting so much as shotgunning:

"He thinks the switch may have failed internally."
now he "thinks it might be a stuck or internally frozen solenoid".

He should have checked things with a VM first before starting to change things out. He's probably right this time, those solenoids are known to fail (much more often than switches as far as I can tell). That's where I'd have started, at least...power on, check voltage at the solenoid. Power off, check it again.

I don’t disagree, but because of logistics I’m stuck with what I’ve got.