Piano Key Switches

doug8082a · Post by **doug8082a** » Mon Jan 06, 2003 3:09 pm

I think I'm having an intermittent problem with my master switch on my '52. Occasionally when I turn it on I don't have power, but after trying it once or twice I eventually do get power. I suppose that after fifty years the switches could stand replacing. Has anyone ever replaced the switches that are actuated by the piano keys in the '48-'52 models (or C120s/C140s for that matter)? Any recommendations on what switches I should use as replacements? How about the o-rings that fit on the back of the piano keys and slide over the switch toggle - any thoughts on where to find replacements for those?

Thoughts and/or ideas appreciated.

Thanks,

Post by **GAHorn** » Mon Jan 06, 2003 7:34 pm

Doug, have you placed a voltmeter on the back of the switch to determine that the switch is indeed the problem? While it likely is, it is also possible that the relay (master solenoid) or that wiring connection defects exist. When you turn your cockpit switch on, it really only provides a ground for the excitation coil in the master solenoid. If that solenoid "clicks" then the cockpit switch is ok. The problem in that case is the solenoid has internal corrosion and is prevented from sending battery power onto the buss. (Internally, the solenoid is a large copper washer that an electromagnet closes onto two large terminals. One terminal comes from the battery and the other goes on to the aircraft buss. Eventually those teminals or the washer become pitted due to arcing/welding flashes which occur each time the switch is closed. That results in a buildup of resistance that eventually results in failure to make contact. Re-cycling the cockpit switch may be simply causing the washer to re-slam into the terminals until that corrosion is broken though and continuity occurs. If so, then replacing the cockpit switch will not cure the problem.)
Even if the master solenoid doesn't "click", it may still be defective. To determine that, you'd have to use the continuity-checker portion of your voltmeter to confirm that the wire leading from the cockpit switch is actually providing a path to airframe-ground when the switch is operated. In other words, you should check for electrical continuity across that master cockpit switch before you automatically assume it to have failed.

doug8082a · Post by **doug8082a** » Mon Jan 06, 2003 8:15 pm

George, Thanks for the tips... I'll check it out the next time at the airport. Admittedly, electrical work is not my strong suit

, so I can certainly use all the assistance I can get!

I'll post back after I check out the switch.

Post by **GAHorn** » Mon Jan 06, 2003 8:54 pm

Ok, here's a simple method to check it out, if you're electron-challenged.

Look at the front of your battery box. There's a solenoid bolted onto it. It has three wires attached to it. One large one comes from the battery. WARNING: Don't touch it with anything like a tool. It's HOT!
Another large one goes to your starter. (When the solenoid operates, that one becomes HOT also! But the starter doesn't operate because it's individual switch is not closed by the pull-cable yet, assuming your airplane is still original.) Don't mess with either large cable.
It's the small wire we're interested in. When your cockpit master switch is OFF, that wire should act like an open circuit. It is a HOT wire, but only small current goes through it. Your master cockpit sw. is at the other end of that small wire, and when the sw. is closed (ON) then that small solenoid terminal is grounded to the airframe. That is what closes the large switch inside that solenoid and sends battery power across from one large terminal to the other,...thereby making battery power available to the starter. (If you look at where that second large cable connects to the starter you'll see a smaller wire that goes on to connect that battery power to the instrument panel and the rest of the airplane's systems.)
So, what you want to do is disconnect that small terminal on the solenoid and then connect one side of your continuity checker to that wire, and the other lead from your continuity checker to airframe ground. Each time you turn you cockpit master on you should see continuity across the checker.
Likewise, if you use a jumper wire, and ground that small terminal on the solenoid, each time you do so, you should hear/feel the solenoid "click" and observe the electrical power come ON in the cockpit. If it doesn't "click" it's a bad solenoid. If it "clicks" but no power goes onto the aircraft...it's a failed solenoid also. (A cockpit dome light would be visible to you out there and prevent a lot of running back and forth to check on that. Each time you ground that small terminal the dome lite should come on simultaneously with the "click". )
Remember tho', it's difficult to check something that's not broken.

If your cockpit switch is operated several times and then it works fine....it'll simply check out fine. You'll need to do this under the same circumstances that you have when things have previously failed.
Good luck.

doug8082a · Post by **doug8082a** » Mon Jan 06, 2003 9:30 pm

Aha! Electrons for Dummies

just what I needed! BTW, at my mechanic's recommendation, I had the solenoid replaced at my last annual (May) since he said that it appeared I had a very old, if not original one, installed. I'll still check the new one out - just because it's new doesn't mean it ain't broke.

Thanks,

Post by **GAHorn** » Mon Jan 06, 2003 11:25 pm

OK, well good. I'm glad it helps. Let me correct something I said in that last msg.
I said that small wire on the solenoid is HOT. More accurately, the small terminal is HOT. (By grounding the terminal the solenoid is activated. Normally this is done thru the cockpit sw.) So when you disconnect that small wire...the terminal is still hot....but the wire is not connected to anything except the cockpit switch. When the switch is closed (turned ON) then that wire is connected to aircraft ground.
So, by operating your cockpit master sw., what you are really doing is providing a path to ground for that solenoid small terminal. Capiche?

russ murri · Post by **russ murri** » Tue Jan 07, 2003 4:35 am

I am having the same problem on my ragwing , not all the time, when I turn on the switch no click, on off , works then . I have a new B and C starter with button instead of pull cable, do I test it the same way? how will I know if it is the switch? Russ

Post by **GAHorn** » Tue Jan 07, 2003 6:44 pm

Yes, test it the same way. The particular starter is not a part of this problem. I only was illustrating how the main buss gets it's electrical power from near the starter motor on the original airplane.