edneff

edneff

Well Known Member
My RV7a has two fuel gauges indicating zero. The senders are the Van's float type.

The builder replaced the left sender at about 30 hours-- I think it was because he was getting zero indication on the gauge. The aircraft has about 75 hours now, and for the last 40 hours or so I have been getting inconsistent readings on both gauges. For the last 10 hours or so the gauges have been mostly reading zero.

I went through the troubleshooting procedure in the Van's Fuel Gauge instruction sheet. Good power to the gauges, and good ground. When I grounded the sender wire at the unit, the gauges went to full scale, i.e. Full Fuel, 15+ gals.

I closed it up and decided I'll keep flying it for awhile until I decide what to do. I know it's going to be an awkward and messy job with the wings installed and the fuel tank on the wing, but the senders are not expensive and I think I can handle it.

Two questions for my fellow VAF readers:

Is there anything else I can troubleshoot before I settle on replacing the senders? I plan to put an ohmmeter on the sender unit and see if I get an indication before I remove them.

Do you have any suggestions as to how to seal the new senders? I have seen lots of opinions here, like gasket, no gasket, proseal, etc. I believe the builder said he used Permatex type II to seal the left hand sender when he installed it. And I saw a post on VAF about a "less agressive" pro seal...
 
Assuming that the sender units and the gauges are the 'standard' ones used by many RVs; the fact that you can get the gauges(s) to go from empty to full by grounding the lead at the sending unit (out at the wing, right?) means that the gauge(s), wiring, supply voltage, etc. are OK. And I feel the sending units are probably OK also, unless somehow jammed or damaged during installation.

Measuring the resistance at the sending unit terminal (resistance to ground) is a good step. A full tank should read low (30-40 ohms?) and an empty tank should read high (240 ohms?). Measure to both the wing or fuselage and then to the metal of the sending unit itself (should be the same). If you do not get reasonable readings, then suspect grounding of the sending unit itself. Take a short jumper cable and jumper the metal of the sending unit on the access plate over to the fuselage. Anyways I can't believe 3 sending units bad in one low hours airplane.
 
GrayHawk said:
Assuming that the sender units and the gauges are the 'standard' ones used by many RVs; the fact that you can get the gauges(s) to go from empty to full by grounding the lead at the sending unit (out at the wing, right?) means that the gauge(s), wiring, supply voltage, etc. are OK. And I feel the sending units are probably OK also, unless somehow jammed or damaged during installation.

Measuring the resistance at the sending unit terminal (resistance to ground) is a good step. A full tank should read low (30-40 ohms?) and an empty tank should read high (240 ohms?). Measure to both the wing or fuselage and then to the metal of the sending unit itself (should be the same). If you do not get reasonable readings, then suspect grounding of the sending unit itself. Take a short jumper cable and jumper the metal of the sending unit on the access plate over to the fuselage. Anyways I can't believe 3 sending units bad in one low hours airplane.
Click to expand...

That's correct, I checked the gauges by grounding the lead at the sending unit, and the gauge read full.

And yes, I'm having a hard time believing that I got three bad senders on one airplane in 75 hours... Hence the thread title, but STRANGER things have happened I suppose. It seems equally strange that the unit would not be getting a good ground off the sender case installation.
 
edneff said:
That's correct, I checked the gauges by grounding the lead at the sending unit, and the gauge read full.

And yes, I'm having a hard time believing that I got three bad senders on one airplane in 75 hours... Hence the thread title, but STRANGER things have happened I suppose. It seems equally strange that the unit would not be getting a good ground off the sender case installation.
Click to expand...
When you say
"I checked the gauges by grounding the lead at the sending unit, and the gauge read full"
exactly what points did you connect to ground it? In other words, I assumed you used a test wire ro ground it. Where was the ground end touching or connected?

(From a person who has an marginal & intermittent ground at his right tank sending unit & sometimes sees that tank go to empty (on the gauge) in a second or two, then comes back later)
 
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edneff said:
It seems equally strange that the unit would not be getting a good ground off the sender case installation.
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My plane isn't flying yet, so feel free to ignore my post, but I think the grounding may be the issue.

The sender ground is it's metal body. The body is attached to the access panel with lots of proseal (an insulator) and maybe a rubber gasket (another insulator). Yes, there are screws but I bet they may have (probably have?) proseal in the threads and under the heads. At least mine do because I didn't want it to leak.

After that, the access panel is attached to the tank body with more proseal (insulator) and maybe a cork gasket (you guessed it - an insulator). Again, the screws probably have proseal in / on / under them.

The ground that matters is the ground for the fuel gages. It may be that the small sensing current is having trouble getting through all that insulation to the airframe to get back to the gage.

To fight this, I attached a small ground wire to the sender body. I plan to run this either all the way back to the cockpit (fuel gage) directly, or to the airframe.

Good Luck,
 
What Iowa said!
And it's easy to test without messing with anything. Just ground the sending unit metal to the fuselage with a good ground. If that's it, then go ahead and create good grounds with the wire mentioned.
 
What everyone has already said.

Almost ALL electrical problems like you are getting turn out to be grounding problems. My float gauges are almost 15 years old and work great. I did have to replace one of them when I removed the slosh compound from my tanks, but that was because I damaged it myself.
 
OFF THE WALL POSSIBILITY

I too purchased a flying RV. The timing of my purchase was right after Van's issued the fuel tank SB, and the previous owner agreed to complete the SB prior to me taking delivery.

The aircraft was delivered to my location. On my first flight a couple of days later after filling both tanks I noticed that both fuel gauges read zero. I initially thought that either the senders or the gauges were bad, but I couldn't understand or believe why both sides would fail at exactly the same time.

A lot of troubleshooting later (getting the same electrical indications you reported) an A&P buddy (also an RV builder) and I theorized that maybe the previous builder had accidently swapped the senders to the opposite tanks when reinstalling them after complying with the fuel tank SB (each sender is side specific).

We drained the tanks and removed the senders. It immediately became clear that we had solved the problem just by comparing each sender to the drawings. The left one was in the right tank and right one in the left tank.

The senders were reinstalled in the proper tanks -- PROBLEM SOLVED.

The solution to the problem would have become apparent much sooner if I wasn't so anal about keeping the tanks full. I stayed in the local area never exceeding 1+30 or so, and refueled after each flight. For the gauges to even begin to budge upward off zero the fuel in either 21 gallon tank must get below approximately 15 gallons. I either didn't let that happen or didn't notice the gauge moving up prior to solving the problem.
 
GrayHawk said:
When you say
"I checked the gauges by grounding the lead at the sending unit, and the gauge read full"
exactly what points did you connect to ground it? In other words, I assumed you used a test wire ro ground it. Where was the ground end touching or connected?
Click to expand...

I disconnected the wire from the sender, and connected that wire to one of the bolts on the spar with a test wire.

I tend to think that I have a ground issue also. I am going to explore this more before I remove the sender(s).
 
mtclay said:
It immediately became clear that we had solved the problem just by comparing each sender to the drawings. The left one was in the right tank and right one in the left tank.
Click to expand...

Something I will consider also, but I don't think so since the builder already replaced the left tank sender once.
 
On my 9A, the senders are electrically the same left & right - the differences are mechanical. I bet that isn't your problem.

I'd turn the plane on, and touch a ground wire between the body of the sender and the airframe and see if that helps.
 
The very first thing that should be checked...

has already been mentioned a number times. Please verify it before removing the senders.

The body of the sender is part of the circuit and it must be connected to airframe ground.

Use an Ohm meter to check the resistance between the sender body (the gold cad plated disc that the screws are in) and a good ground point on the airframe. It should be zero resistance. If it is not, that is your problem.

If you are unable to check the resistance, connecting a wire between a good ground and the gold cad plated surface (don't just clip a wire to the screw head) will give you a good ground connection, then recheck the gage operation.

If you find that this corrects your problem, it is being caused by the sender mounting screws not making a good electrical connection to the surface of the sender body.
You can fix it by removing one screw, put a small amount of tank sealant in the screw hole, and then reinstall the screw with a toothed lock washer. The lock washer will help make a good (uninsulated) electrical connection between the screw and the sender body.
 
<<The left one was in the right tank and right one in the left tank.>>

Which means they were upside down. Sheesh...can't blame the previous owner for that one, but clearly the mechanic who installed them late one night was an idiot.

Sorry Mike.
 
Which means they were upside down. Sheesh...can't blame the previous owner for that one, but clearly the mechanic who installed them late one night was an idiot.
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Dan,

I would never mention any names.
 
<<I would never mention any names>>

An officer and a gentleman forever <g>.
 
If all else fails

I have been through all the above scenes regarding my fuel senders plus some more. The first issue was resolved when I finally realized that I had received two left hand senders in my kit. Then I could not get any more than a 10 gallon indication and I realized the fuel return lines I had installed were interfering with the travel arc of the floats. I finally removed the tanks form the wings so I could truly see what was actually happening. I was able to hang the access panel from the fuel lines and see how everything fits up next to the spar etc. Rerouting new return lines solved the problem. The point I am making here is that it was a big job, looking back on it I would say it is not all that difficult to do. 21 AN3s on the back side of the spar on each tank can try your patience but once you get going on it not that bad. If I had built the tanks myself I probably would not have had these problems, (quickbuild wings).

I have had good luck with straight pro seal.

Now I wish I could electronically calibrate Van's gauges. They are not all that accurate and going back in there to bend the wires over and over does not seem to be a good way to get the accuracy. I am leaning towards the Electronics International units that allow full calibration with float senders.

Randy C
 
Fuel sender issues

I, too, was having trouble getting my fuel gauge to register on my left (port) side fuel gauge. I checked the wiring, fuses, fuel gauge and sending unit. Only after calling Van's who knows how many times did I realize I was supposed to put a little bend at the tip which locks the float in place and keeps it from pivoting on the shaft of the float in the sending unit. It turns out this is necessary to keep the float from rotating. The float would rotate when fuel was in the tank indicating zero fuel even though I had about 6 gallons in there.
I reviewed my plans and couldn't find where it said I was supposed to put the little bend in the float. It's probably in the plans somewhere but I couldn't find it.
 
Too much proseal?

Ed, Iowa RV9er is likely right on the money. I had what you describe in my last RV6 and backing all the sender assy to tank screws out and then re-tightening solved the problem. In your case, you might back one off, try to ground it to the airframe, etc., for a more permanent fix. The problem eventually came back using method A.
 
wing grounds

Don't forget, the anodized spar is non conductive on it's surface, so grounding the wing to the frame through the harness is a good idea IMHO. I ran a 14 ga. wing ground through the harness from a bond on the wing rib to the outboard seat rib, and an auxilliary from the fuel sender case to the same bond on the seat rib at each wing. This will guarantee equal potential between the wing and the fuse. Don't rely on the spar/center section attach bolts to ground the wing, as the anodizing can insulate the bond. As others have said in this thread, I can't believe it's not a ground potential issue causing your fuel gauge woes.

HTH...Chris
 
carguy614 said:
Don't forget, the anodized spar is non conductive on it's surface, so grounding the wing to the frame through the harness is a good idea IMHO. I ran a 14 ga. wing ground through the harness from a bond on the wing rib to the outboard seat rib, and an auxilliary from the fuel sender case to the same bond on the seat rib at each wing. This will guarantee equal potential between the wing and the fuse. Don't rely on the spar/center section attach bolts to ground the wing, as the anodizing can insulate the bond. As others have said in this thread, I can't believe it's not a ground potential issue causing your fuel gauge woes.

HTH...Chris
Click to expand...
The resistance only has to be greater than ~250 ohms for the gauge to peg at empty.
 
What is normal??

I just completed calibrating my Van's float senders. I have the GRT EIS, and followed the steps for calibration. In the end, with a full tank, my gauges read 16 gals each side until the sender starts moving (to calibrate I bought 21 gals 100LL and filled each tank, one at a time.)

However, when I split the fuel between the tanks the Right reads 16, the Left reads 9?! I know I added 10.5 to the Left tank last!? What is "normal" accuracy for the float senders?

I have quickbuild wings, so I've never taken the fuel tank off, but is it possible to replace the float senders with capacitance senders?
 
Gsuit said:
However, when I split the fuel between the tanks the Right reads 16, the Left reads 9?! I know I added 10.5 to the Left tank last!? What is "normal" accuracy for the float senders?

I have quickbuild wings, so I've never taken the fuel tank off, but is it possible to replace the float senders with capacitance senders?
Click to expand...

My impression is that if you are getting accuracy within 1.5 gals at over half full, that you are well within the typical range for these types of senders-- I'll let others chime in here on this topic, since I'm no expert.

There are a few threads which discuss capacitance vs floats, and you can retrofit but it would be a big project.
 
edneff said:
I am not sure I understand the significance of this statement. Could you expand on it? Tks.
Click to expand...
What the gauge is really indicating is the resistance of the wire out to the sending unit plus the resistance of the sending unit to its metal 'flange' plus the resistance from this flange back to wherever the meter is grounded (instrument panel). Usually the first and last items are roughly zero or close enough to zero to be ignorred. If this sum of resistances is roughly 30 ohms or less, the gauge will read full (regardless of the amount of fuel in the tank). If the sum of resistances is roughly 250 ohms or greater(don't remember the exact values) then the gauge will read empty (regardless of the amount of fuel in the tank). If the sum is zero (ground the wire at the gauge) it will peg (hard) FULL, disconnect the wire from the gauge with the gauge still powered up, the gauge will peg EMPTY.

Summary: If the resistance from the flange of a sending unit back to the ground of the gauge is marginally high (above say 270 ohms), because of too much corrosion, permatex, tank sealant, anodized spar,... or whatever; the gauge will read empty, even though the tank is full of fuel & and the float unit is working.
 
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Well today I did a little more troubleshooting. For whatever reason, maybe due to taking the sending wire off the sender numerous times, or shaking something else loose, I started to get a tiny indication on the gauges-- about 1/8 inch off the pegs. This is with about 7 gals in each tank.

I also measured the resistance of the sender and they indicated about 230-- which corresponds to empty according to Van's tech support.

Tomorrow I plan to attempt to view the floats from the filler neck using an inspection mirror and a flexible flashlight. I want to see if the float is oriented properly in the tank. If that checks out, I plan to add fuel slowly and see if I get appropriate indications on the gauges, and appropriate resistance values on the multimeter. If that doesn't work out, I guess I'll be pulling the senders after all.
 
Ed,
Have you done as I and many other posters already recommended???... Measured the resistance between the flange (gold colored disk) of the sender and airframe ground (any bare aluminum spot)? It should be zero ohms.
If you have no way to measure the resistance, or don't know how to do that, then just temporarily touch a scrap piece of wire between gold colored disk of the sender and a good ground on the airframe. Then see if the fuel level reads correctly.

The round disk (body) of the sender must make a good connection to airframe ground to complete the sensor circuit. This is a very common cause of the problem you are having.
 
rvbuilder2002 said:
Ed,
Have you done as I and many other posters already recommended???... Measured the resistance between the flange (gold colored disk) of the sender and airframe ground (any bare aluminum spot)? It should be zero ohms.
If you have no way to measure the resistance, or don't know how to do that, then just temporarily touch a scrap piece of wire between gold colored disk of the sender and a good ground on the airframe. Then see if the fuel level reads correctly.
Click to expand...

I did use a test wire with clips on either end to go between the sender disk and the airframe and I still got a zero reading on the gauge. I got a positive continuity test between the disk and the airframe. I got zero ohms. I also disconnected the sending wire and checked resistance on the sender and it read about 240, drained the fuel, checked it again, then put the fuel back in. Didn't change the resistance.

My next step is going to be to order some new senders, then remove and test the old ones. If I can't get them to work I'll install the new ones. Can't think of anything else to do.
 
STICKING GAUGES

Be suspicious of your gauges when you first start flying. My gauges worked fine during calibration with my VM1000C but for about the first 40 hours they would both stick in the FULL position. I needed to pull a couple of Gs to get them to unstick. Even then they would sometimes stick in intermediate positions. They must have loosened up as they always read accurately now.

FIN 9A
 
Well, I think I have figured out the problem-- it's one other members of the forum have described.

I removed the left sender today and found that the sender wire was not bent 90 degrees where the wire terminates on the plastic bracket that holds it to the sender. So, the float has most likely been twisting around in the tank, and not pulling the sender arm up, thus reading zero.

I suspect the right sender has the same problem, although coincidentally it was working normally today. I'll remove it tomorrow.

Now, what about installing the bend and getting the floats reading correctly? Gus at Van's suggested just adding the 90 degree 1/4 inch bend to the existing wire. The left side wire measures 3.5" long from the sender bracket to the elbow. It measures only 2.25" from the elbow to the sheperds crook for the float. The plans indicate 3" for both. I saw a different measurement on this forum for an RV8. The RV7a probably requires a different measurement than the 8, right? Does anyone have a recommendation?

Thank you.
 
We are finding as well the Gauge readings are sensitive to slip... pressure on the left rudder can increase the L Tank level by quite a few litres. Aircraft is well balanced overall, but at high speeds a little left rudder is needed for the ball.... and if we are lazy :rolleyes: I suspect the fuel drifts outboard away from the sender :eek:

Andy & Ellie Hill
RB-8 QB nearly 20 hours now
www.g-hilz.co.uk (under development ;) )
 
Just to wrap up this thread...

I removed the senders and replaced them. I had to fabricate a new wire for one, the other one I just rebent it correctly.

I tried to bend them both so that they just barely touched the bottom of the tank. I added fuel a gallon or so at a time and checked the gauges. They both read about 2 gallons higher than actual. Disappointing, but I don't want to take the senders out and start over just now. I should have bent the wires so that they were not touching the bottom of the tank, perhaps a half inch or so above the bottom.

I'll live with the discrepancy for now. At least I am getting consistent readings now on both gauges if not entirely accurate ones.

Thanks for all of the advice and suggestions. VAF is a great resource. I would be lost without it.
 
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