RV-12 fuel sender

[rgmwa]

Yesterday while taxying out in the RV-12, the fuel level reading on the Dynon showed zero. I knew there was plenty of fuel in the tank, which the Moeller confirmed, so I took off. After takeoff, the gauge decided there was fuel in the tank after all, and slowly got to a reasonable reading. Then it dropped out for a short while later in the flight after some steep turns (maybe coincidental), but then came back up to level. The sender response has always seemed a bit sluggish. Any ideas why this might be? I don't particularly want to pull the tank or replace the sender, but obviously may have to. The sender was installed as per the plans and the float arm worked smoothly at the time.

[Mich48041]

A ground wire should be attached to the sender mounting plate, not the tank access cover or tank.
Short the sender center screw to ground to see if the fuel level goes to full. If it does, then there is nothing wrong with the aircraft wiring.
Disconnect wire from center screw, then measure resistance from sender mounting plate to center screw. On my RV-2, the resistance is :
FULL = 30 OHMS
EMPTY = 225 OHMS

[rgmwa]

Thanks Joe. I'll try that next time I'm down at the hangar.

[dbhill916]

Originally Posted by rgmwa

Thanks Joe. I'll try that next time I'm down at the hangar.

Hi rgmwa,

I'm getting exactly the same problem you described. The fuel level suddenly drops to an unrealistic value and may/may not climb back towards something that makes sense. It has never reported more than was in the tank, but definitely unreliable for lower values.

Since high resistance is interpreted as a low fuel value, I'm assuming I'm dealing with an intermittent open contact / wire somewhere.

https://www.dropbox.com/s/sllm3fyd7b...44.01.JPG?dl=0

Thanks,
-dbh

[rgmwa]

May not be quite the same. Mine doesn't drop suddenly or fluctuate rapidly, just reads zero or some value well below what's actually in the tank on startup, and over the next 20-30 minutes may struggle up to somewhere near the correct value, after which seems to work OK. The plane is down for its first annual at the moment, so this is still on the to-do list.

[DaleB]

After I had my tank out, rebuilt it, applied all the SBs and re-painted I had a zero reading from the sender. I don't know if there has been a modification to add a ground wire for the sender since my kit was produced, but mine has a single wire going to the sender. No ground other than through the tank and airframe. Well, with all the tank sealant and epoxy primer... no ground. I had to add a ground wire from the sender mounting plate to a convenient grounding point. I used one of the ELT antenna mounting plate screws. It's been rock solid since then.

A complete lack of ground will cause an empty reading. A flaky, unreliable or partial ground will cause readings somewhere between true and empty, and might be intermittent. Readings are averaged out over at least several seconds, so if you have a ground that's intermittent it may cause partially low readings at times.

[DHeal]

If you only have one wire attached to the fuel sender, do as Joe suggests -- install a dedicated ground wire from the fuel sender base to a convenient airframe ground location. The original design had only the positive wire and relied upon the tank itself for ground -- frequently, overzealous use of sealant interfered with this grounding resulting in erratic fuel quantity readings. Be sure to get a good clean ground on the fuel sender base mounting screw. This has worked well for me -- YMMV.

[rgmwa]

Originally Posted by Mich48041

A ground wire should be attached to the sender mounting plate, not the tank access cover or tank.

Thanks Joe. It's taken me a while to get around to this for various reasons, but adding a ground wire today fixed the problem. The Dynon had been slowly becoming less and less reliable until it pretty much gave up altogether. Now, it matches the Moeller again like it did when I first completed the PAP.

[Mich48041]

rgmwa, You are welcome.

[dbhill916]

Originally Posted by DHeal

If you only have one wire attached to the fuel sender, do as Joe suggests -- install a dedicated ground wire from the fuel sender base to a convenient airframe ground location. The original design had only the positive wire and relied upon the tank itself for ground -- frequently, overzealous use of sealant interfered with this grounding resulting in erratic fuel quantity readings. Be sure to get a good clean ground on the fuel sender base mounting screw. This has worked well for me -- YMMV.

Agree with DHeal. I found the same problem: intermittent ground via the tank itself. I found a silver impregnated epoxy (Permatex 21351 Electrically Conductive Epoxy)and used that to attach a ground wire to the sender plate and have had perfectly steady fuel indications since.