Master stuck ON

[PandaCub]

Had a very enjoyable flight from Colorado to Hood River, Oregon in my RV-14A (photo 1), but upon landing couldn't get the Master to shut off (photo 2, note ALT FIELD light still on with Master switch down - I promptly disconnected negative battery terminal to prevent drainage).

I assume this is the replacement part I need:

https://shop.vansaircraft.com/cgi-bi...duct=master-sw

But I wonder if anyone else has had this experience and am all ears for any tips/tricks about solving the problem and preventing future occurrences.

[Skysailor]

A master contactor typically only receives a ground from that switch to the small center post. The contactor itself may have failed. Locate the master contactor and remove the small control terminal wire. If connecting the battery negative still brings on power, the contactor itself is the problem.

If the contactor is working correctly without the control wire attached, check the small wire you removed. It will show a ground. Either the switch failed or there is a point along the wire which has shorted to ground.

[Veetail88]

Originally Posted by Skysailor

A master contactor typically only receives a ground from that switch to the small center post. The contactor itself may have failed. Locate the master contactor and remove the small control terminal wire. If connecting the battery negative still brings on power, the contactor itself is the problem.

If the contactor is working correctly without the control wire attached, check the small wire you removed. It will show a ground. Either the switch failed or there is a point along the wire which has shorted to ground.

And if you replace it, DO NOT forget to reconnect the diode.

[Aluminum]

Recent discussion here. See my departure from "standard practice" in post #13.

[Carl Froehlich]

I use two of these in my planes, one for each battery: https://www.hodgesmarine.com/BLUE-SE...D_BwE&click=19

If you shop around you can get them for less than $100.

Note the 0.13A holding current. This is a fraction of the standard master solenoid holding current - and this becomes important as you calculate electrical reserve if you loose the alternator. The standard master solenoid tends to be a major load - just to hold it shut.

I?m not a fan of the Cessna split master/alternator switch - but to each his own.
Carl

[MCA]

The master switch, battery contactor, or wiring between the two could be at fault as mentioned.

I see you have a backup battery. Could that have been left on, or wired in a way that accidentally feeds onto the main bus?

[Southern Pete]

Originally Posted by Carl Froehlich

I use two of these in my planes, one for each battery: https://www.hodgesmarine.com/BLUE-SE...D_BwE&click=19

If you shop around you can get them for less than $100.

Note the 0.13A holding current. This is a fraction of the standard master solenoid holding current - and this becomes important as you calculate electrical reserve if you loose the alternator. The standard master solenoid tends to be a major load - just to hold it shut.

My local distributor is reporting a fire at the Blue Sea warehouse has led to supply difficulties, he is not taking orders at present. Any others finding supply problems?

[bjdecker]

Is this a first time occurance? If so, I'd be really suspicious of the split master switch.

I am not certain about how you have the system wired, but if it's "typical", the ALT warning light will illuminate in the presence of a +12V (or 24V) field current and the absence of B+ voltage from the alternator/regulator.

I see that the Alt & Bat legs are turned off, which would indicate to me that the whole switch has failed. As mentioned elsewhere, disconnect the control wire from the Master relay - either at the back of the switch, or the relay and the relay should de-energize. If so - it's the switch. If not, then it could be the Master Relay has failed "closed"...that doesn't explain the Alt light tho...

B

[PandaCub]

Thanks to all for your replies and sorry for the late follow-up.

I spoke to a factory rep from Lamar Technologies, the firm that makes the master contactor in my RV-14A (the Lamar part was ostensibly an "upgrade" from the standard ES-24115 Master Relay sold by Van's). He said in some cases, if the part gets wet or lives in a damp environment, it can stick on. This explains why I could get it to shut off by whacking it with the handle of a screwdriver.

A couple days ago I replaced the part with a new one and all works as it should now. Lamar even said they'd warranty the defective unit. We'll see. In the meantime I'm happy to not have to disconnect the negative battery terminal anymore!

Now if I can just figure out why the Andair fuel boost pump is so finicky about when it wants to turn on... Always something, eh?