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Good morning Folks,

The electric fuel pump in my bird has developed an auto shut off feature.  I have reached to shut it off several times in the last month only to find it already off.  I thought I was going crazy but it turns out it's been gas lighting me.  I actually saw and heard it pop off climbing through 2500agl a few days ago.  The breaker was clearly popped and needed about 3 mins before it could be reset.  Is this the typical failure mode for a Klixon or should we be digging deeper?

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Could be the pump motor drawing too much current or the switch tripping at too low a current. I would start by measuring the current with an ammeter so you can figure out which and not throw parts at it. Another possibility is a chafed wire shorting to the airframe.

 

To specifically answer your question it is a typical failure mode of the klixon.

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Many of the Klixon swtiches are breaker switches, i.e., it is also a circuit breaker.   So if it is popping into the OFF position it is telling you there is a fault with that circuit.

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38 minutes ago, EricJ said:

Many of the Klixon swtiches are breaker switches, i.e., it is also a circuit breaker.   So if it is popping into the OFF position it is telling you there is a fault with that circuit.

Indeed it’s a breaker switch. Typically a short in the circuit would trip the breaker immediately. In this case the pump runs fine for a few minutes (I turn it on when I receive take off clearance) and then quits at some point during climb out.

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1 hour ago, Shadrach said:

Indeed it’s a breaker switch. Typically a short in the circuit would trip the breaker immediately. In this case the pump runs fine for a few minutes (I turn it on when I receive take off clearance) and then quits at some point during climb out.

Maybe the pump is getting hot and drawing too much current, maybe there's a short somewhere that is intermittent or dependent on something else, maybe the breaker in the switch is getting tired, but I wouldn't ignore that it's trying to tell you something.   I don't know whether the breaker mechanism in the Klixon is thermal or magnetic, but sometimes thermal breakers will trip after accumulating heat for a while...e.g., the circuit has been drawing close to the breaker limit for enough time to trip it.    That might happen if the pump starts drawing more current if it is getting hot, etc.

 

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Can you find another breaker/switch of the same rating for something you can temporarily do without and connect the pump to it to see if it's the pump or the switch?

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Why not just swap it for a different 10A breaker switch, like the beacon or pitot heat?

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1 hour ago, EricJ said:

Maybe the pump is getting hot and drawing too much current, maybe there's a short somewhere that is intermittent or dependent on something else, maybe the breaker in the switch is getting tired, but I wouldn't ignore that it's trying to tell you something.   I don't know whether the breaker mechanism in the Klixon is thermal or magnetic, but sometimes thermal breakers will trip after accumulating heat for a while...e.g., the circuit has been drawing close to the breaker limit for enough time to trip it.    That might happen if the pump starts drawing more current if it is getting hot, etc.

 

I’m certainly not going to ignore it. If I have a problem with the mechanical pump I need the boost pump to work to remain airborne.  The current boost pump is less than a year old.

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2 hours ago, PT20J said:

Can you find another breaker/switch of the same rating for something you can temporarily do without and connect the pump to it to see if it's the pump or the switch?

I can do that and likely will. I have to say that I’m at a point we’re getting underneath the panel is  something I only want to do once per problem...:) I can ask my A&P to do it then I’ll once again have to hear about what it’s like to work in the Mooney foot well.  I already know what it’s like so I don’t want to hear about it...;)

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2 hours ago, Shadrach said:

I can do that and likely will. I have to say that I’m at a point we’re getting underneath the panel is  something I only want to do once per problem...:) I can ask my A&P to do it then I’ll once again have to hear about what it’s like to work in the Mooney foot well.  I already know what it’s like so I don’t want to hear about it...;)

Measure voltage drop across the switch.  As they get old resistance will increase, heat will increase, voltage drop increases.  If you’re seeing more than about.5 volts (drop),  replace it.  Real easy to check with voltage meter.

Definitely better to figure this out first rather than throw parts at it!

And, yeah, .5 volts drop is a guess.  They may or may not start popping with that.

Edited by Ragsf15e

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Great thread participation...

Everyone seems to be saying... it could be one of every step along the way... from experience... :)

1) Klixon switches that get used often do show their wear... they are temperature sensitive mechanical devices that wear with use and current...

2) Mechanically, you can feel the click getting lighter with thousands of operations under their belt. If it is glad to pop off when lightly touched, that is a sign of wear...

3) that click is the thing that the CB and heat have to overcome... the softer the click, the less resistance to turning off...

4) Hmmm. Temperature sensitive CB/switch and hot temperatures on the ground... a high probability of the CB triggering sooner than ever before... and they get warmer as current flows through them... (counter productive sort of)

5) Sure, check the amperage used by the fuel pump. A smart idea...

6) Don’t feel bad if you have to swap out the CB... It has many years on it already...

7) unfortunately, they are like .15 Amu... I looked earlier today for somebody reporting an inverted pitot CB...

8) The pain of Swapping out the CB... is the bus bar connects to a row of CBs at the same time... necessitating removing the bar from all of the CBs to get the one out...

9) Texas Instruments, Sensata, Klixon... all the same corporation...

10) One thing nobody mentioned...

Q: What could possibly bump the toggle switch in flight..? There is an answer for that too...

A: The plastic instrument panel overlay... when the panel shock absorbers are old and worn or missing... have a tendency to bump the switches... check the clearance between the overlay and the switches in the on position...

 

PP thoughts only, not a mechanic...

Best regards,

-a-

 

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