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Throttle Microswitch - Fuel Boost Pump


carqwik

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Boost pump light didn't come on with full throttle on takeoff today.  Manually turned on the boost pump and got the light.  Does that mean the microswitch in the throttle is inop?

(And that's a second little switch that needs work...the cowl flap indicator micro switch  (or potentiometer?) is dead too.  Died right after annual last year.) 

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I had the same problem with the throttle boost pump micro switch. The switch arm has a small roller on it that rides on the throttle cable, and mine had developed a flat spot, presumably due to lack of lubrication/rotation. We rotated the roller to allow it to provide adequate travel on the switch, and I'm monitoring it for wear. I'll be replacing it at some point.

Backup is to manually turn the boost pump on for takeoff, a practice the previous owner had employed as a matter of course (didn't trust the "automation").

Cheers,

Rick

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Hate to show ignorance, but where is the micro switch?  Do I have to remove a panel to get to it?  I bet mine is similarly worn as yours...perhaps simply rotating the roller will make a difference.

And yes, I manually turned on the boost pump...did that right before rotating after not seeing the idiot light illuminate.  Thankfully, the boost pump still works!  (It should..it got overhauled last year!)

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My bad, it's on the throttle cable on lower right side of the engine. It's a fairly large switch housing (mine is white) attached to the cable shield, with the usual thin metal switch arm with the roller riding on the cable. Not TOO hard to find, but unfortunately does require removal of the lower cowl. I'll look to see if I have a picture that shows it, and update this later.

Cheers,

Rick

 

I couldn't find a good picture, but here's the drawing from the parts catalog. #13 and #14 are both identified as micro switches, with #14 being called out as the boost pump switch. You'll know it when you see it. Let me know if this doesn't get it for you.

IMG_0068.PNG

IMG_0069.PNG

Edited by Junkman
Clarification
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  • 3 weeks later...
  • 2 months later...

Just as a follow up...

Plane came out of annual at Top Gun today.  Neither the mechanic nor Mark Rouch (owner of Top Gun) could replicate the problem...the boost pump came on and the annunciator lit up at full throttle.  They did clean it up a bit or so (tighten some things he said) but it always worked for them.

Of course, that's great...a gremlin that disappears when the plane goes to the shop.  OTOH - the gear failed to retract in the shop!  Needed a new "up limit switch" which fixed this problem - yet I never had a problem with the gear coming up ever before!!!  The gremlin jumped from the throttle microswitch to the gear limit switch!! 

Well if there's any blessing in the this, I got nice tailwinds of 35 to 70 knots the entire way back from Stockton today. 

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  • 3 years later...

This is a great post. I was just looking at some troubleshooting for the boost pump not coming as it should with full throttle. I did find two pictures, there are two different switches listed in my parts manual. I loathe going back in there because I know what a beast it is to get the lower cowl off alone, so I might just do the back up plan which is to put the boost pump on during takeoff. I also figured out while troubleshooting that the boost pump CB does nothing so I get to play my second favorite game "identify what circuit I go to!".

I am getting 30PSI from the engine pump and 53ish from the boost pump.

 

Screen Shot 2021-07-03 at 5.40.56 PM.png

11SM244-T.png

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19 hours ago, PilotX said:

This is a great post. I was just looking at some troubleshooting for the boost pump not coming as it should with full throttle. I did find two pictures, there are two different switches listed in my parts manual. I loathe going back in there because I know what a beast it is to get the lower cowl off alone, so I might just do the back up plan which is to put the boost pump on during takeoff. I also figured out while troubleshooting that the boost pump CB does nothing so I get to play my second favorite game "identify what circuit I go to!".

I am getting 30PSI from the engine pump and 53ish from the boost pump.

 

Screen Shot 2021-07-03 at 5.40.56 PM.png

11SM244-T.png

My fuel pressure at any other setting besides idle is 40 psi.  I can’t speak whether 30 is in the right range but thought I’d give you a frame of reference.

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I was looking through my flight logs and it is 30 in cruise. When the boost pump comes on meaning I turn it on it bumps up to 50. Both of those are within the acceptable range from the POH. I know we spray tons of fuel in there for cooling on takeoff my temperatures are not bad.

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On 12/21/2017 at 4:53 PM, carqwik said:

Just as a follow up...

Plane came out of annual at Top Gun today.  Neither the mechanic nor Mark Rouch (owner of Top Gun) could replicate the problem...the boost pump came on and the annunciator lit up at full throttle.  They did clean it up a bit or so (tighten some things he said) but it always worked for them.

Of course, that's great...a gremlin that disappears when the plane goes to the shop.  OTOH - the gear failed to retract in the shop!  Needed a new "up limit switch" which fixed this problem - yet I never had a problem with the gear coming up ever before!!!  The gremlin jumped from the throttle microswitch to the gear limit switch!! 

Well if there's any blessing in the this, I got nice tailwinds of 35 to 70 knots the entire way back from Stockton today. 

cold solder joint on micro switch

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