J

jimbo

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I checked my Vans stock float fuel senders mfg by Warner Stewart prior to closing in the tanks. Ohm readings were in line, float swung unobstructed and freely when installed. 3 years later after putting in fuel in tank both left and right senders don't read off empty. Plenty of fuel in the tanks and jiggled the wings. No change. Then checked the ohm readings between the sender terminal screw and the base with engine monitor gage wire off terminal screw. Reading between the sender screw and the sender base still reading empty (245 ohms).

Pulled the senders. Checked ohm readings-arm up-full and arm down-empty, float arm action, float buoyancy, bends in arm, ease of arm swivel. All checks out on both senders. Reinstalled. Put fuel in tank and still no reading off empty. The left sender has been off 2 times and checked.

Don't know what else there is to check. Don't think this is a ground issue because I'm checking between the sender base and the sender terminal with no wire connected to the terminal.

Brain stormers out there?

Jim
RV9A
Stewart Warner Model 385B-F and 385C-F lever fuel senders
 
If the empty and full positions read the correct resistance with the senders out of the tanks, but not with the senders in the tanks, I'd think it's either mechanical interference or a short. What's the resistance reading with the sender installed in the tank and fuel in it?
 
It sure sounds like whatever gauge you are using to read the resistance is bad. Connect the gauge to a variable resistor and check it.

Don
 
DaleB said:
If the empty and full positions read the correct resistance with the senders out of the tanks, but not with the senders in the tanks, I'd think it's either mechanical interference or a short. What's the resistance reading with the sender installed in the tank and fuel in it?
Click to expand...

Ohm reading out of the tank, float down, 245 ohms
Ohm reading in the tank with 5+ gals fuel, 245 ohms (had from 2 gals upward to 12 gals in the RV9A 18 gal tank-no ohm change)

Ohm reading out of the tank, float up, 30 ohms
Float arm wire secure in sender unit and float arm swivels up/down easy

Checked with 2 different ohm meters

Jim
 
If you're getting 245 Ohms with the sender in the tank -- not zero or infinity - then it's not sounding like a ground or wiring problem.

With the fuel level in the tank just below the opening for the sender, can you pull the sender out just far enough to get an inspection mirror in there and verify that the float is moving freely? It sure does sound like something is trapping the float at the bottom of the tank.

Or the float... doesn't. The symptoms indicate the float stuck at the bottom of the tank. Maybe it sinks. I haven't looked to see if those are hollow or not. If it's happening in both tanks, though... huh.
 
"float swivels easily"?

I had a similar issue - tested senders and all was OK, added fuel and one gauge read empty - due to a missing bend in float wire.

In my case, the final 90 degree bend called out by Stewart-Warner was not made by the original builder, thus the float arm swiveled (i.e. could rotate 360 degrees about the long axis.)

Next time you have the sending unit out, you might want to check that the preventive bend indeed keeps the float from swivelling (and allowing it to rotate to a "stuck" position.)

Good Luck!
 
DaleB said:
If you're getting 245 Ohms with the sender in the tank -- not zero or infinity - then it's not sounding like a ground or wiring problem.

With the fuel level in the tank just below the opening for the sender, can you pull the sender out just far enough to get an inspection mirror in there and verify that the float is moving freely? It sure does sound like something is trapping the float at the bottom of the tank.

Or the float... doesn't. The symptoms indicate the float stuck at the bottom of the tank. Maybe it sinks. I haven't looked to see if those are hollow or not. If it's happening in both tanks, though... huh.
Click to expand...

With the really tight space an inspection mirror is out of the question. Not sure if some sort of a scope could get in there thru the gas tank filler and the center rib holes to really see.

When I had the sender unit out, I put the float in a bucket of water and it raised the arm up nicely. No holes, waterlogging, etc.

Jim
 
Are your plate screws isolated? I ended up isolating the ground for my EFIS (here is my write-up with pictures). I used shoulder washers to isolate the screws from the sender plate. Otherwise, the common ground just seemed to 'swallow' the signal.
 
You seem to have thoroughly checked the "in tank" part of the circuit

Deleted - after reading you test method later my statement doesn't apply.

Bob Axsom
 
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Bob, I think that's 3 years of sitting while the rest of the plane is built. I could be wrong.
 
You have received a wide variety of advice.
The thing is most of it is looking to make this much more complicated than it probably is.

90+% of the time this problem is caused by no continuity between the mounting flange of the sender and airframe ground. That flange is part of the circuit path to the instrument. It uses airframe ground to connect the flange to the ground side of the instrument.

If you touch one meter probe to the sender flange, and the other probe to a ground point on the airframe, the resistance should be zero (or nearly so).
If it is not (and I bet that is the case), then you have poor electrical coupling between the sender flange and the tank access plate, or the tank access plate to the fuel tank (or both).
 
OOPS!

DaleB said:
Bob, I think that's 3 years of sitting while the rest of the plane is built. I could be wrong.
Click to expand...

-deleted non applicable information - When we calibrated our gages the first couple of 2 gallon increments were required to get to the point where the next 2 gallon increment registered on the Electronics International digital instruments. The system is not linear down at that part of the fill. Is that where you are at? If you have not filled your tanks in 2 gallon increments for calibration you do not know if you system works on not. Given your test method this is still valid in general but if you put in 12 gallons that would overcome this.

Bob Axsom
 
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Both my fuel gauges showed empty on the initial fueling. I went to the wing tip and did a vigorous up and down jiggle. Problem solved never to return.

Edit:

Now that I think about it, I think this happened during calibration. It was probably only 2 gallons in the tank and the sloshing broke something loose. It did happen on both tanks.
 
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Wires crossed

23 years ago I had a similar problem with my Vans gauges. It turned out that I had the wires on the wrong post on the back of the gauges.
 
reread my first post

Everyone is saying that its a ground problem or faulty wiring, etc.

When I took the sender out I checked the ohm resistance from the sender screw terminal and the sender mounting plate. No instrument wires connected, just my ohm meter. Moved the arm up and down. Got 30 ohms up and 245 ohms down. Found float arm connected good and the heavy wire float arm doesn't twist. Arm freely moves up and down.

Put the sender in the tank. Still no instrument wires attached. Still read 245 ohms empty tank-good. Put gas in the tank (have had from 2-12 gals in the tank). Repeat no instrument wires connected. Connected my ohm meter to the center terminal and the other one to the center mounting plate of the sender as before. With gas in the tank it still reads 245 ohms where it should read something less than empty tank reading.

I have taken the instrument wire and sender ground out of the equation by checking direct with my ohm meter.

I am going back and shake that wing like theres no tomorrow as my analysis is that the float arm is not coming up freely with fuel in the tank.

Jim
 
Jim,

I might have missed it but what are you trying to read the fuel levels on? On much of it, like GRT equipment, you need to excite the circuit, a small resister between a power source and the sender. I had the same issue until I added this, now they read well!
 
I'd agree with the shaking approach. Having read what you posted, it's obviously not electrical. It's mechanical. Something is keeping that float rm from moving.

When you first built the tank, did you install the fuel sender and flip the tank over to listen for the "clunk"? I was more than a little worried about mechanical interference -- rib, vent line, whatever -- so I checked it all before installing the rear baffle, then flipped the tank right side up - upside down a few times to verify that I was hearing the float "clunk" both ways (and verified full travel with an Ohm meter). If it's on the wing now, it would be a hassle but you may have to bit the bullet and unbolt that sucker.
 
Paul K said:
Jim,

I might have missed it but what are you trying to read the fuel levels on? On much of it, like GRT equipment, you need to excite the circuit, a small resister between a power source and the sender. I had the same issue until I added this, now they read well!
Click to expand...

Paul....No, not trying to read with any gages on or connected, just the ohm resistance at the sender using a hand ohm meter per my above post.

Jim
 
Jim,

From every thing you say you've done, I'm siding with the mechanical interference or floats-not-floating theories. I didn't use float senders so this might be a silly question, but I'm wondering if it's possible they might be mis-installed left to right, and what effect that might have?
 
Is your installation something like this?

(With the pick up open of course)

IMG_0001_zps0d314acf.jpg


IMG_zps69135795.jpg


Bob Axsom
 
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Isn,t there a way to stick a bent welding rod or something into the filler opening and lift the float with it. Usually works for me.
 
aerhed said:
Isn,t there a way to stick a bent welding rod or something into the filler opening and lift the float with it. Usually works for me.
Click to expand...

With the filler neck at the outboard end of the tank and the float at the inboard - and all those ribs in the way this sounds pretty tricky....

After reading all of the posts in this thread Jimbo, I have to vote with a mechanical issue, and the one I can associate with most closely is to make sure that you have that last 90 degree bend in the float wire to keep it from rotating relative to the resistor.

As a note - We had to recalibrate the sensors on the RV-6 last month with the avionics upgrade and it took a lot of tapping/banging on the tank root after adding each gallon to get the float to move and settle to it's new position. In this case, vibration is your friend!
 
Ironflight said:
make sure that you have that last 90 degree bend in the float wire to keep it from rotating relative to the resistor.
Click to expand...

Another mechanical assembly error I am aware of is builders having teh small bend at the end but failing to snap the wire into the tangs on the wiper arm of the sender. Float stays in place and pivots just fine, but it is not always moving the wiper arm.
 
You should be able to tie a thread, string or small wire to the first bend in the sender arm. Run the thread out the top mounting hole of the sender plate. Screw the sender and plate back into place leaving out the top screw and see if you can pull the sender arm up via the thread and notice any change in resistance.

If I recall, there is a stiffener that could cause the float to jamb at the bottom or maybe it was the top of the tank. I remember reversing the float on one side so the float was in front of the wire on one side but behind the wire on the other.

Also, there may be some dead range on the sender. That is, if you are not adjusted properly, you may have to lift the float up quite a bit before you will see a change in resistance. Ideally you should adjust the bend for max resistance when the float is at the bottom of the tank and the resistance should start to decrease with a very small upward motion.

I adjusted my senders using two threads when the wing was in a vertical wing rack. I ran one thread out the top sender mounting hole and the other out the fuel drain hole. I was easily able to pull the sender from empty to full positions with the threads and with a little patience I was able to gets the bends in the sender arms just where I wanted them.
 
Found the problem

The pictures that Bob posted is the sender alright and the last post from Joe hinted at the culprit.

I can't say for 100% certainty because its all closed up and you can't see in there but I think what is happening is as follows: in order to get the sender, with bent arm and float thru the hole in the rib plate you have to turn the assemble every which way due to the small space (3" +/-) between the fuselage and the rib when the wing is installed. While doing so the float arm will want to tip down due to its weight. Well, on the bottom of the tank there is a stiffener that runs parallel with the wing. I think the float got on the wrong side of the stiffener and it put a tension on the wire float arm. When fuel was added to the tank the tension prevented upward buoyancy movement.

I shook the wing and the float arm seems to be freed up and moving as it should (correct ohm readings). I shook the wing a couple days ago with no response. Why it worked today I can only attribute to the right amount of fuel sloshing around in there having the most impact on freeing the float.

Whew. I hope this is helpful to anyone in the future having this problem. Of course everything looked perfect when I closed in the tank years ago.

Jim
RV9A
 
jimbo said:
I think the float got on the wrong side of the stiffener and it put a tension on the wire float arm.
Click to expand...

If the float was interfering with the stiffener angle enough to restrict it's movement, there is probably a good chance that once broken loose, it could be slightly resting on the angle and not going all the way to the bottom of the tank.
 
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