Bad Fuel Senders? Strange

edneff · #1 11-30-2007, 05:06 PM

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...

GrayHawk · #2 11-30-2007, 05:44 PM

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.

edneff · #3 11-30-2007, 07:46 PM

Quote:

Originally Posted by GrayHawk

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.

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.

GrayHawk · #4 11-30-2007, 08:01 PM

Quote:

Originally Posted by edneff

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.

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)

IowaRV9Dreamer · #5 11-30-2007, 08:07 PM

Quote:

Originally Posted by edneff

It seems equally strange that the unit would not be getting a good ground off the sender case installation.

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,

GrayHawk · #6 11-30-2007, 08:17 PM

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.

Mel · #7 11-30-2007, 08:24 PM

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.

mtclay · #8 11-30-2007, 10:45 PM

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.

edneff · #9 11-30-2007, 11:34 PM

Quote:

Originally Posted by GrayHawk

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?

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).

edneff · #10 11-30-2007, 11:36 PM

Quote:

Originally Posted by mtclay

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.

Something I will consider also, but I don't think so since the builder already replaced the left tank sender once.