Master Switch Probs

shiney · #1 12-09-2009, 04:21 PM

I?m having a problem with wiring my master switch and battery contactor relay, lots of nasty smells and a very hot ground wire!:

The wiring to the relay is conventional;
* Power in from battery to connection 3 (L/H side)
* Power out to main bus from connection 1 (R/H side)
* Master Switch wired to connection 2 (central connection).

When the battery on/off switch is grounded, the ground wire gets incredibly hot, so hot that I daren?t leave it, also no power to the main bus. I've rerun the wires and and re-read my electric books etc but I can?t think of or see anything obviously wrong with this setup so why am I experiencing this problem?.

Radomir · #2 12-09-2009, 04:48 PM

Diode pointing in wrong direction? Do you have one between battery post and "switch" (small) post? If that one's pointing in wrong direction... it'd be bad

shiney · #3 12-09-2009, 05:09 PM

Quote:

Originally Posted by Radomir

Diode pointing in wrong direction? Do you have one between battery post and "switch" (small) post? If that one's pointing in wrong direction... it'd be bad

I thought that might be the problem but when checking, the diode is wired in such a way that it can only be fitted to the connector one way, i.e. the battery post has a large connection and the switch post has a small connection. The only other possibility is that the diode has been wired wrong to the connectors, what is the way of checking the diode direction ?

Radomir · #4 12-09-2009, 08:32 PM

Best way to check it is to remove it and see if problem persists

Rick S. · #5 12-09-2009, 08:37 PM

Tap the relay top with a screwdriver, the contactor may be stuck...if it starts to work normally then replace it.

shiney · #6 12-10-2009, 02:13 AM

I'll try tapping it and let you know the outcome.

Thanks again

David-aviator · #7 12-10-2009, 06:47 AM

Quote:

Originally Posted by shiney

I'll try tapping it and let you know the outcome.

Thanks again

That probably won't do it.

There's something really screwed up here, the wire coming from the master switch to the relay should NOT be getting hot much less warm. It is a simple connection to ground to get the relay to close. Considering the symptoms, a very hot wire, the wire is being connected to a 12v dc power source either at the master switch or from within the relay with a faulty contactor mechanism or through one of the diodes.

Check the master switch source wire. Be sure it is ground and not something else.

Check the master relay to be sure it is continuous duty and not a starter relay. The master relay is closed by a ground signal, the start relay is closed by 12v dc. Check that what you call pin 2 is NOT shorted to either pin 1 or pin 3. If it is, the relay itself could be bad or one of the diodes is bad or installed wrong.

Get rid of all the diodes for a check of the system. I know there are electric theories why we use them but they are a point of failure.

(I've used them in the past but the present set up does not have them. As near as I can tell, they are not used in the certified world, just here with experimental airplanes.)

Rick S. · #8 12-10-2009, 07:51 AM

Might not...did for me...The relay had stuck somehow and when I went to connect the batteries I got a HUGE surprise. Sparks galore, welding action near the cable terminals. An old hat who was helping me do final assemble, grabbed a screw driver tapped the top of the relay and everything worked fine...go figure.

David Clifford · #9 12-10-2009, 08:13 AM

Quote:

Originally Posted by David-aviator

As near as I can tell, they are not used in the certified world, just here with experimental airplanes.)

My Cessna has them on both the Master and the started solenoids.

JonJay · #10 12-10-2009, 08:15 AM

Quote:

Originally Posted by shiney

what is the way of checking the diode direction ?

Most DVM's or multimeters have a diode check feature. No matter, you can check with a standard ohm meter. Current flows only one way in a diode. Connect the positive side of your ohm meter to the positive side of the diode and you should see very little resistance. Reverse the leads and you should see high or infinite resistance. Also, when power is hooked up, you should get a .7vdc drop across the diode.

Regardless, now that you have "lots of nasty smells", once you figure out where the problem lies, it would be prudent to replace all devices, master switch, relay, and wiring. You never know what you have smoked or weakened.