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Master Switch Probs

S

shiney

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

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?.
 
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 :)
 
Radomir said:
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 :)
Click to expand...

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 ?
 
Tap the relay top with a screwdriver, the contactor may be stuck...if it starts to work normally then replace it.
 
shiney said:
I'll try tapping it and let you know the outcome.

Thanks again
Click to expand...

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.)
 
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.
 
Diode Check

shiney said:
what is the way of checking the diode direction ?
Click to expand...

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.
 
They prevent "inductive kick back"

David Clifford said:
My Cessna has them on both the Master and the started solenoids.
Click to expand...

When the coil collapses, like when it is shut off, a reverse polarity spike kicks back. The diode prevents that kick back from harming other components in the system.
 
David Clifford said:
My Cessna has them on both the Master and the started solenoids.
Click to expand...

Just goes to show what I know.

Interesting though - my neighbor, who flies a '64 Comanche, recalls no diodes in his system; what's that, 45 years without them and the sky has not fallen?

That's what makes me wonder about some theories that have evolved over the years. Some guy writes a column or a book and we all blindly jump through the hoop. Where's the data to back up these notions?

Another "cast in concrete" idea is shielded wire to the Mags from the ignition switch. I simply forgot to do it with this installation. And guess what, the inter-phone and com systems have no static interference whatever. I really thought I was going to have to rewire those circuits. There's a single wire from the Mag case to the switch and back to the P-lead terminal, it works.

As you can tell, I have become a skeptic on some of this electric stuff....:)
 
"Good Practices"

David-aviator said:
Just goes to show what I know.

Interesting though - my neighbor, who flies a '64 Comanche, recalls no diodes in his system; what's that, 45 years without them and the sky has not fallen?

That's what makes me wonder about some theories that have evolved over the years. Some guy writes a column or a book and we all blindly jump through the hoop. Where's the data to back up these notions?

Another "cast in concrete" idea is shielded wire to the Mags from the ignition switch. I simply forgot to do it with this installation. And guess what, the inter-phone and com systems have no static interference whatever. I really thought I was going to have to rewire those circuits. There's a single wire from the Mag case to the switch and back to the P-lead terminal, it works.

As you can tell, I have become a skeptic on some of this electric stuff....:)
Click to expand...

Some things are done just as "Good Practices", to avoid the "potential" of a problem, not necessarily to avoid an inevitable problem. Kind of like orienting bolt heads forward. You can put your bolts in the other way, and if the nut never falls off, there is never a problem. There are a lot of "good practices" in aviation and I will agree, many of them are hold overs from a prior era and may not have any basis today. Diodes are cheap insurance, as well as shielded P-lead's.
 
Not resolved

I haven't been able to resovle this problem, I've checked all the wiring and everything is good. It can only be that there is something wrong with the contactor so I'm going to order a new one.

Thanks for all posts, I'll keep you informed!
 
You can check that with an ohm meter

shiney said:
I haven't been able to resovle this problem, I've checked all the wiring and everything is good. It can only be that there is something wrong with the contactor so I'm going to order a new one.

Thanks for all posts, I'll keep you informed!
Click to expand...

You could have a shorted coil. If you have an ohm meter, you can check your coil to ground. You should have a relatively low resistance, not sure but 3 or 4 ohms, but it should not be zero. That would indicate a short which would be consistent with your symptoms.
 
Smoke!

Just a real shot in the dark here - have you charged your battery recently? I once allowed the battery in a truck to go almost completely flat - put my charger on it and let it charge for two days. Charger said that the battery was fully charged. It was, but the polarity was somehow reversed!!! And I was sure that I hooked up the charger correctly. Made smoke when I hooked it up and wiped our several relays, etc.

Other than that remote possibility, its sounds like you have some kind of direct short. Good luck - we'll all be waiting to hear the resolution of your problem. Hope this helps.
 
Now is the time to remove the Master Solenoid...

...from the aircraft and perform a rather simple test.

OBSERVE PROPER SAFETY PRECAUTIONS WHEN PERFORMING THE FOLLOWING TEST.

1. With the solenoid on the bench, hook up a suitable wire from a battery Positive (+) to the BAT terminal of the solenoid.

2. Hook a suitable wire to the solenoid case mounting lugs and connect the free end to the battery NEGATIVE (-) terminal.

At this point, nothing should happen, the solenoid should not be energized.
If the solenoid is energized, it is defective.

3. Now, attach a wire (same size as used in your aircraft) between the negative terminal of the battery and the small terminal post on the solenoid.

At this point the solenoid should be energized, and battery voltage should be present at the output terminal of the solenoid. And none of the wires should be hot.
 
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