Master Relay Failure

[Ventucky Red]

Are these things either they are working or they are not, or, is there a degradation (intermittent failure) over time?

 

[Salty]

Are these things either they are working or they are not, or, is there a degradation (intermittent failure) over time?

yes

*answer is intentionally not clear.

 

[Steamflyer]

You are talking about the solenoid? Presumably battery?

Correct answer was given - yes.

 

[Bell206]

is there a degradation (intermittent failure) over time?

This. Usually it will degrade then fail, but there are several ways to check this.

 

[AV8R_87]

To expand on the above answers, think for a second on how those things operate (generic terms because it applies to more than one kind of relay/contactor):

You apply current to a coil. It creates a magnetic field that pulls a mobile contact against a fixed contact. The mobile contact has some sort of a guide to align it to the fixed contact(s).

So, your failure points are:
1. The coil and its connections. It could be a complete breakage, goiving you a total failure, or it could be a loose/dirty connection that's intermittent enough to drive you crazy.
2. The mobile contact guide: it can get stuck because it's worn/dirty, and it can be a temporary or permanent stickiness, sometimes affected by temperature. Can be temporarily cured sometimes by a bot of percussive maintenance.
3. The contacts can be burnt/dirty. It can completely prevent them from making electrical contact, or it might allow contact to be made sometimes. Even worse, that contact might have a higher than normal resistance, making you think you have a starter or battery problem when trying to crank the engine.
The external connections to the contacts can also be loose/dirty, with similar symptoms.

So yes, plenty of ways for those things to fail in ways that aren't permanent and drive you nuts.

Also, your battery master switch can fail as well. Dirty/burnt contacts on the switch could cause enough of a voltage drop to prevent reliable closing of the battery relay.

 

[GeorgeC]

Also, your battery master switch can fail as well. Dirty/burnt contacts on the switch could cause enough of a voltage drop to prevent reliable closing of the battery relay.

IIRC, there's a diode that's supposed to prevent the arcing that deteriorates the switch contacts, and there is a similar arrangement on the contactor; if either of those fails, then the demise of their corresponding device will be accelerated.

It's easy enough to check the voltage drop across the contactor. Don't forget that the coil itself draws ~10W.

 

[Dan Thomas]

IIRC, there's a diode that's supposed to prevent the arcing that deteriorates the switch contacts, and there is a similar arrangement on the contactor; if either of those fails, then the demise of their corresponding device will be accelerated.

I have never seen a diode across the master switch. Not in any Cessna, anyway. But there is a diode across many master contactor ("relay") coils to prevent voltage spikes entering the entire system on shutdown and maybe ruining electronics. That diode will also protect the switch.

Old master and starter contactors are responsible for a lot of new batteries and starters. The old parts cannon at work, trying to solve a problem. The contactors' copper contacts oxidize so that even low-time contactors are shot because they're so old. All of the starter current is moving through both of those contactors, and it comes to as much as 250 amps or so. If you understand Ohm's Law, you see that a very small resistance in a circuit carrying large amperages will cause a large voltage drop across that resistance, and starting becomes sluggish. This is why trying to measure the resistance of the contacts is difficult, with resistances as small as .05 ohms, so voltage drop measurements are taken instead across those contacts while cranking.

 

[Ed Haywood]

It's such a cheap part, just throw a new one in if it is old and questionable. Call it preemptive maintenance.

 

[GeorgeC]

I have never seen a diode across the master switch.

You are right, I misremembered. There is just the one:

 

[Gary Ward]

It's such a cheap part, just throw a new one in if it is old and questionable. Call it preemptive maintenance.

That's what I did years ago before I had trouble out of them. After 40 years it made sen$e to me. 7 years ago they were much cheaper than they are now.

 

[Gary Ward]

I have never seen a diode across the master switch. Not in any Cessna, anyway. But there is a diode across many master contactor ("relay") coils to prevent voltage spikes entering the entire system on shutdown and maybe ruining electronics. That diode will also protect the switch.

Old master and starter contactors are responsible for a lot of new batteries and starters. The old parts cannon at work, trying to solve a problem. The contactors' copper contacts oxidize so that even low-time contactors are shot because they're so old. All of the starter current is moving through both of those contactors, and it comes to as much as 250 amps or so. If you understand Ohm's Law, you see that a very small resistance in a circuit carrying large amperages will cause a large voltage drop across that resistance, and starting becomes sluggish. This is why trying to measure the resistance of the contacts is difficult, with resistances as small as .05 ohms, so voltage drop measurements are taken instead across those contacts while cranking.

I also rebuilt the switch and installed the diode before I ran into trouble.

 

[Dan Thomas]

I also rebuilt the switch and installed the diode before I ran into trouble.

Yes, but that diode is across the starter contactor coil, not the master, and is there because of the AD against ACS ignition switches. Arcing caused by the coil's voltage spike burns those thin contact cups in the switch, and can cause magneto issues since their contacts are also in that same switch and burned contact debris can interfere with the mag contacts.

 

[Dan Thomas]

It's such a cheap part, just throw a new one in if it is old and questionable. Call it preemptive maintenance.

I started doing that when we bought low-time 172s for the flight school. Some of those airplanes were 30 years old, and after I had had trouble with one of the cranking I found that the contactors were presenting resistance. At the time, the contactors were maybe $25 each. Not now, but still a lot cheaper than having your mechanic replace the battery, and then the starter a couple of times before he figures out that something else is causing the hassles.

 

[Dan Thomas]

In about 2015 Cessna came out with Service Manual revisions that recommended 4-year replacements of the master switch, avionics master switch, and landing light switches. It was good advice, as this stuff is typically run to failure. I have replaced landing light switches in the past; their contacts get oxidized and burned and the resulting resistance causes them to heat up and melt. Sometimes it's oxidation between the wire and crimp terminal on the switch, and the heat conducts from the terminal into the switch. A mechanic, during an annual, should be crawling under the panel and looking at everything, including those terminals. Incandescent landing lights use a lot of current, so lots of potential for heating. Switching to LED lights takes the stress off of the whole circuit including the breaker, switch and wiring.

LED landing lights use about 1/7, IIRC, of the current required for an incandescent light. And you get much more light, too. And much longer life.

 

[3393RP]

A mechanic, during an annual, should be crawling under the panel and looking at everything, including those terminals. Incandescent landing lights use a lot of current, so lots of potential for heating.

Some of the photos posted here reveal wiring behind the panel and under the cowl that have not been inspected or disturbed since someone with $2.95 AutoZone crimpers mashed wires into some cheap terminal connectors and called it good.

More owners need to look at these things. They're in the air flying those planes, not the mechanics.

 

[Dan Thomas]

More owners need to look at these things. They're in the air flying those planes, not the mechanics.

But how many owners have a clue as to what they're looking at? Many---maybe most---don't know what the stuff under the panel does.

There are some pilots with screwdrivers, known by mechanics as the most dangerous things around an airplane. They overtighten everything, ruining the hardware and sometimes damaging the surfaces. And that's just ordinary screws. Imagine them fooling with wires and wire bundles or vacuum hoses and getting them displaced so that the flight controls foul in them.

I used to get some of my students to lay on the cockpit floor (when I had the seats out) and look up under a 172's panel. They were shocked at the amount of stuff under there. None of it made much sense to them.

 

[AV8R_87]

They were shocked at the amount of stuff under there. None of it made much sense to them.

All of that wiring not being marked and not having an updated wiring diagram doesn't help.

 

[Dan Thomas]

All of that wiring not being marked and not having an updated wiring diagram doesn't help.

All of the OEM wiring IS marked, and wiring diagrams are in the service manual. But as you say, the owner usually won't have that manual.



Sometimes the wire bundles have to be untied and separated to find the markings. It can be a lot of work.

 

[Steamflyer]

All of the OEM wiring IS marked, and wiring diagrams are in the service manual. But as you say, the owner usually won't have that manual.

View attachment 145621

Sometimes the wire bundles have to be untied and separated to find the markings. It can be a lot of work.

You'd think an owner would want a copy of the service manual, but sadly that does not seem to be the case for most. A good avionics shop will also mark the wires (those are usually the only ones that change).

 

[Gary Ward]

You'd think an owner would want a copy of the service manual, but sadly that does not seem to be the case for most. A good avionics shop will also mark the wires (those are usually the only ones that change).

I am a first time aircraft owner for 7 years now.
I made sure I got the service manual for my plane when I bought it from my friend. At first he wanted to keep it but I insisted that it was part of the deal.
At work I have 2 four drawer filing cabinets full of truck and car service manuals from the last 40 years of service work on work trucks.Every new purchase the salesman thought we meant owner manual, no we want service manuals which were multiple books and cost some money. After a while they knew we wanted service manuals which added to the cost of the deal.
Now they are useless because all the old stuff is gone.
The new trucks require subscription service and a very expensive computer to work on a new truck. Sadly I leave that to the dealer nowadays. I still do oil changes and grease jobs. Nasty dirty work.
Luckily I can still safely service my 172 using the service manual if you can read and comprehend.
The pages are worn and dirty, just like I like them.

 

[ateamer]

There are some pilots with screwdrivers, known by mechanics as the most dangerous things around an airplane. They overtighten everything, ruining the hardware and sometimes damaging the surfaces. And that's just ordinary screws. Imagine them fooling with wires and wire bundles or vacuum hoses and getting them displaced so that the flight controls foul in them.

When I bought my RV-8 (I was the third owner), every single inspection plate and fairing screw was hogged out. Some of them I best advice I have gotten was from an A&P who said to never use a power driver to tighten screws.

 

[Gary Ward]

This handheld long shank snap on Phillips #2 is the best I have ever used. I have have wore out a few in the past.The long shank provides more leverage and accessibility to some screws .

 

[Domenick]

I am a first time aircraft owner for 7 years now.
I made sure I got the service manual for my plane when I bought it from my friend. At first he wanted to keep it but I insisted that it was part of the deal.
...
Luckily I can still safely service my 172 using the service manual if you can read and comprehend.
The pages are worn and dirty, just like I like them.

If you are so inclined, the 172N POH, Parts, and MX manuals are also available as PDFs.

Dropbox

www.dropbox.com

 

[3393RP]

But how many owners have a clue as to what they're looking at? Many---maybe most---don't know what the stuff under the panel does.

There are some pilots with screwdrivers, known by mechanics as the most dangerous things around an airplane. They overtighten everything, ruining the hardware and sometimes damaging the surfaces. And that's just ordinary screws. Imagine them fooling with wires and wire bundles or vacuum hoses and getting them displaced so that the flight controls foul in them.

I used to get some of my students to lay on the cockpit floor (when I had the seats out) and look up under a 172's panel. They were shocked at the amount of stuff under there. None of it made much sense to them.

I find it a little odd and a bit unsettling that many pilots don't make efforts to learn what's going on under the cowl and panel. It's not particularly difficult to gain an understanding of a single engine aircraft's operation principles, and knowledge of those systems could prove critical when something is making preparations to roll over and die.

I think every pilot should be comfortable with removing a cowling and performing a basic inspection of the engine and accessories, and do so regularly. An occasional look under the panel for abraded wiring and other obvious issues doesn't require an A&P level of knowledge.

 

[WDD]

Are these things either they are working or they are not, or, is there a degradation (intermittent failure) over time?

Another POV. My contactor started acting up, and it turned out to be a bad battery. New battery, all the problems went away.

Bonus: I have the service manual for my plane (182 P) that wife and I bought last year. Agree 100% that it pays to dig into it. For example, only last year did I find out there was even a thing called a solenoid contactor / master relay. Couldn't figure out what the diode was doing there, and and today I found out. Learn something new every day.

 

[Bell206]

I find it a little odd and a bit unsettling that many pilots don't make efforts to learn what's going on under the cowl and panel.

I found its been a growing trend among Part 91 owners the past 20 years which I especially noted with my last owner assist clients. I think the main driver has been a loss of mechanical ability, need, and understanding simply due to things in general dont break anymore so the same mindset is applied to their legacy aircraft. However, when compared to any other form of transportation its become the norm as well. For example, people wait for their car to tell when to change oil or dont look under the hood until the engine service light is on.

 

[Ventucky Red]

Another POV. My contactor started acting up, and it turned out to be a bad battery. New battery, all the problems went away.

Bonus: I have the service manual for my plane (182 P) that wife and I bought last year. Agree 100% that it pays to dig into it. For example, only last year did I find out there was even a thing called a solenoid contactor / master relay. Couldn't figure out what the diode was doing there, and and today I found out. Learn something new every day.

Although my plane had been neglected for the past few years before I took ownership, the previous owner had a full-service manual with all the updated bulletins. I have been studying these and running the diagnostics as outlined

What I have is an intermittent problem that leads us to believe there is a short or a loose / corroded ground somewhere. Seeing that we need to get under the hood/panel, we are going to change out the master switch, alternator field circuit breaker, and the bus breaker while we are in there < for less than $100 in parts that are well over 45 years old.

I think every pilot should be comfortable with removing a cowling and performing a basic inspection of the engine and accessories, and do so regularly. An occasional look under the panel for abraded wiring and other obvious issues doesn't require an A&P level of knowledge.

100% as my father taught me "If you going to play with them, better know how to fix them." I know there are limitations as to what we can do, but knowing what to look for and what is required to fix it is a must.

As for my electrical problem, in a way this is a blessing in disguise as it is forcing to learn about the system...

 

[Domenick]

Another POV. My contactor started acting up, and it turned out to be a bad battery. New battery, all the problems went away.

Bonus: I have the service manual for my plane (182 P) that wife and I bought last year. Agree 100% that it pays to dig into it. For example, only last year did I find out there was even a thing called a solenoid contactor / master relay. Couldn't figure out what the diode was doing there, and and today I found out. Learn something new every day.

Just arrived yesterday. Serendipity, karma, or what?

Dropbox

www.dropbox.com

 

[Ed Haywood]

As for my electrical problem, in a way this is a blessing in disguise as it is forcing to learn about the system...

100% the right mindset.

When you are happy with your electrical system, start learning the other systems. Fuel, oil, ignition, pitot/static, flight controls, engine controls, etc. Do all annuals as owner assist, and ask your A&P to let you do as much as possible under supervision.

You will be a much more confident pilot, and your aircraft will be safer.

 

[catmandu]

When I bought my RV-8 (I was the third owner), every single inspection plate and fairing screw was hogged out. Some of them I best advice I have gotten was from an A&P who said to never use a power driver to tighten screws.

Meh, every opinion in moderation.

I start screws by hand to avoid cross threading. I use a JIS #2 Phillips bit in a power driver with the clutch set very low to drive them. I tighten them to spec with a JIS #2 Phillips screwdriver. I have a few bags of every needed screw size, and replace any fastener that starts to look less than “like new” which is an exception to my mantra of owning a Go Plane vice a Show Plane.

 

[Ed Haywood]

Meh, every opinion in moderation.

I start screws by hand to avoid cross threading. I use a JIS #2 Phillips bit in a power driver with the clutch set very low to drive them. I tighten them to spec with a JIS #2 Phillips screwdriver. I have a few bags of every needed screw size, and replace any fastener that starts to look less than “like new” which is an exception to my mantra of owning a Go Plane vice a Show Plane.

Same here. The Milwaukee 12v drivers have a very good clutch and speed control.

 

[Bell206]

Some of them I best advice I have gotten was from an A&P who said to never use a power driver to tighten screws.

And still good advice. The main issues I ran across were a number of owners only a had a cordless drill (vs driver), used worn screwdriver bits, and the use of excessive speed which would gall nutplate threads. Easy money for me to fix in most cases. Regardless, outside of certain helicopters and cabin class airplanes with large removable panels, never found the need to use a power driver over a good speed handle even for removals on small Cessna or Piper singles.

 

[Ventucky Red]

100% the right mindset.

When you are happy with your electrical system, start learning the other systems. Fuel, oil, ignition, pitot/static, flight controls, engine controls, etc. Do all annuals as owner assist, and ask your A&P to let you do as much as possible under supervision.

You will be a much more confident pilot, and your aircraft will be safer.

Thanks for the encouragement Ed. As for the other systems, well lets say electrical, next to avionics is the last on the list.

Our engines, to me are nothing more than VW engine... I have rebuilt many of those with few having some Gene Berg enhancements... We were saying/playing Zoom-Zoom long before Mazda coined it.

 

[GeorgeC]

I am a first time aircraft owner for 7 years now...

I'm in the same boat. Had you asked me in 2018 if I'd have my head under the panel, I would have said no way, there be dragons.

Oh, and thanks for the reminder; I treated myself to a paper copy of the service manual

But how many owners have a clue as to what they're looking at? Many---maybe most---don't know what the stuff under the panel does.

There are some pilots with screwdrivers, known by mechanics as the most dangerous things around an airplane. They overtighten everything, ruining the hardware and sometimes damaging the surfaces. And that's just ordinary screws. Imagine them fooling with wires and wire bundles or vacuum hoses and getting them displaced so that the flight controls foul in them.

I am sure I have stripped a few nutplates along the way, but you gotta start somewhere. To be fair, I've also seen hamfisted stuff done by techs that should have known better. There are shops who went the extra mile and caught things that saved my bacon, and there are shops who overlooked things or actively dorked things up. Nobody's perfect; only the trend line matters.

I am neither mechanically inept nor mechanically gifted, so I am careful to know my capabilities and limitations. The engineer's curse of needing to know "why?" goes a long way, though, and I have a decent intuiton of "I'm pretty sure that's not supposed to look/sound/feel like that".

In no small part due to folks on this forum, I'm a better owner, and my plane is in the best shape it has been in decades.