Fuel Pump Capacitors

by Craig E Maiman in 912 / 914 Technical Questions 6 years ago

The manual calls out putting 1uF 100V caps across the terminals of the two fuel pumps. What type of cap have people used and where have they mounted it?

Someone said they didn't think an electrolytic would be a good choice because they're polarized. They thought a ceramic cap right across the terminals would be good.

Thoughts, advice?

Thanks!

Re: Fuel Pump Capacitors

by Bill Hertzel » 6 years ago

Bill Hertzel

13 1.95k 8k

There is typically only one Electric fuel pump.
Section 2400-00-00, Page 16, Paragraph 2.9 calls for a 1uF 25v capacitor.
Electrolytic is acceptable as long as the polarity is observed. Attached the (-) wire to the ground terminal
A Ceramic Cap would be more appropriate for the application and also perform better.
Its purpose is to reduce that electrical noise generated by the brushes within the pump motor.
It should be mounted as close to the brushes as physically possible.
If possible, soldered directly to the Pump Motor would be the preferred location.

Note: On Figure 11, Section 24-00-00 pages 19 & 20 shows capacitors #14 as 1uF.
They should be 22,000uF at 25v as described in section 24-00-00, Page 7.

Bill Hertzel
Rotax 912is
North Ridgeville, OH, USA
Clicking the "Thank You" is Always Appreciated by Everyone.

Thank you said by: Craig E Maiman

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Re: Fuel Pump Capacitors

by Craig E Maiman » 6 years ago

Craig E Maiman

40 123 504

Hi Bill,

Which document are you referring to? The 914 Installation Manual (My plane, a Sling 4, uses the 914, not 912) doesn't have any such section.

The Installation Manual in section 19.4.7 (page 118) says the cap should be 1uF/100V.

BTW, I have two pumps, as one is a backup.
Thanks,
Craig

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Re: Fuel Pump Capacitors

by Bill Hertzel » 6 years ago

Bill Hertzel

13 1.95k 8k

Sorry, I missed the part where you said it was a 914. ;)

A Ceramic Cap as close to the Pumps as possible is the goal.
If you can get them on the actual pumps so much the better.
Otherwise, the back of the connector is the second choice.
Their purpose is to reduce the RF noise of the motors brushes so a foot up the wire harness would not be an effective location.

Ceramic caps are available from ~220uF down to almost zero. Small Caps
Electrolytic Caps are available from ~1uF up to Super Size. Large Caps.
The value we need just happens to be in the overlap range.
Ceramic caps are more rugged and have a lower impedance to RF Frequencies than Electrolytic Capacitors. A Good thing!
They also have higher voltage ratings and are much smaller and less expensive. Also Good!

Any 1uf ceramic cap will be just fine.
Any voltage value more than twice the battery voltage will be fine.
25v would be fine but will be harder to find than the 100v+ varieties.
A 1000v cap would be overkill but would also not cause any problems.
You will be hard pressed to spend over $1 for the caps.
Try Radio Shack if there are still any remaining in your area.
Otherwise, shipping will cost more than a small bag of caps.

The Pump manufacturer should have installed these as part of the pumps if they were diligent.

Bill Hertzel
Rotax 912is
North Ridgeville, OH, USA
Clicking the "Thank You" is Always Appreciated by Everyone.

Thank you said by: Craig E Maiman, RotaxOwner Admin

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Re: Fuel Pump Capacitors

by Craig E Maiman » 6 years ago

Craig E Maiman

40 123 504

I just ordered these: https://www.standardsupply.com/products/m39014-02-1411-mil-qualified-ceramic-capacitors-1uf-100v-10-tolerance-1-1000-hours-failure-rate-radial-lead-5-pieces

And yup, the shipping was as much as the caps... :woohoo:

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