Something I learned about electric fuel pressure senders

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PilotDAR
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Something I learned about electric fuel pressure senders

#1 Post by PilotDAR » Mon Jan 11, 2016 10:41 am

My carburetted, fuel pumped Lycoming O-360 was showing lower and lower fuel pressure on the electric engine cluster indicator. It was time to investigate. 380 hours total time on the engine, accessories and electric fuel pump. The fuel pump put out the required 4 PSI, as independently measured. The cockpit indicator seemed to be working, so I did a few electric checks with a variable resistor, and it seemed fine. I removed the sender (a Mitchell/VDO, crimped closed type, which is the one specified for the indicator) and tested it. It too seemed to be working, I got a resistance change based upon pressure change. But, after doing some comparison, the resistance "sent" by the sender was not quite correct for the indicator for the pressure to indicate. I did a PSI per Ohm chart for each, and it did not line up.

So, I figured a sender failure, and uncrimped it to learn. What I found was that the wiper which follows along the wires of the resistance coil, had worn so badly that it spanned many wires, so shorted them together. No harm to anything, but it explains the lower and lower fuel pressure indication over time. 380 total time is not a lot, but the sender is threaded directly into a "T" fitting on the carb, so it vibrates with the engine, and just wore itself out. The oil pressure sender seems to be the same type of unit, albeit with a different pressure range, so I will be more aware of apparent changes in engine oil pressure over time too...
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fish4life
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Re: Something I learned about electric fuel pressure senders

#2 Post by fish4life » Mon Jan 11, 2016 10:51 am

Good information, I'm glad we still have guys like you posting valuable stuff.
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cgzro
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Re: Something I learned about electric fuel pressure senders

#3 Post by cgzro » Mon Jan 11, 2016 10:55 am

Saw similar problems with servos that used a potentiometer as a feedback device to indicate position.
Vibration caused the wiper to eat onto the rsesistive surface the result being worse and worse oscillations as the servos jitter around hunting for a neutral feedback.

This was in giant scale Rc planes where e vibration from big single engine gasoline engines was a nightmare. i got very good at removing old potentiometers every few months and soldering in new ones. Eventually they went to optical feedback i think.
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ahramin
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Re: Something I learned about electric fuel pressure senders

#4 Post by ahramin » Mon Jan 11, 2016 3:45 pm

I always mount oil and fuel pressure senders on the firewall and run lines to them.
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