I've had a recent problem with one of my fuel level indications and wanted to see if anyone else has seen this. I have the STOL 750 with the Zenith-supplied VDO/Volkswagen resistive fuel level senders mounted per plans. The senders are wired to my RDAC which then displays the fuel levels on my MGL XTreme EMS. This has worked great for a number of hours. 

Recently, however, my "Fuel level #2" (right tank) began to indicate a few gallons lower than what I knew was in the tank. As the tank got low, it then would indicate "0" or empty even though it was not.

After re-checking all my wiring connections, I contacted Matt at MGL and he suggested re-calibrating the EMS with the sender.  I did this and it seemed to be working perfectly again. I flew about 2 hours today and the first 3 legs of the flight it worked normally. However, when I fired up for the 4th leg and final flight home, it indicated several gallons low again and eventually fell on off to "0" or empty when I knew there was still several gallons of fuel in the tank. (I have a fuel totalizer and could subtract the fuel in the left tank from the total and knew how much I had burned out of the right tank.)

I'm now wondering if this might be a defective sender? In all of my time on this forum, I've never heard of one going bad that was working properly initially.

Just wondered if anyone else has ever experienced it? If the sender does turn out to be defective, is the VDO/Volkswagen sender that Zenith uses the best quality sender available?

Thanks,

John

N750A

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John,
Isn't the VDO senders range 10-70 ohms? I know you checked your ground but since I'm not certain of your setup, sender grounded to tank, tank grounded to airframe; or ground from sender to airframe. If it's grounded the first way it could be a bad ground from sender to tank. If it was a broken winding at the sender or a bad contact i would expect infinite ohms, also a bad contact usually reads good in the ranges above an below the bad spot and a broken winding will read good when fuel level drops below the spot where the break is.
Lol to keep it brief, a simple check would be to jumper off one of the sender screws to airframe ground an see if it does any better. Worth a try before going through the hassle of a sender replacement
Dave
P.S. The fuel senders on a grand caravan, Cessna 207, and piper Navajo are the same in design shape an ohm ranges are different but other than that they are basically the same thing
One addition, this type of sender will last well beyond a thousand hrs. (In my experience) before attention is usually needed. The one thing that kills them fast is if the nozzle is pointed at them when refueling. Pilots keep telling me they never do that (I don't believe em). But that kind of abuse is usually related to float/arm damage. When the resistive winding fails it's usually a break in the winding and it's almost always open in the spot where the fuel level is kept at, example, 700lbs a side is the normal fuel load in the 208b so that is where the resistive winding fails... Hope some of my rambling on helps
Dave

The senders are supposed to be approximately 10 - 80 ohms. Mine was failing intermittently, first tending to read a few gallons low and when a few gallons have been burned, then falling off to zero. Sometimes it worked OK. But the last few flights it has been in a completely failed state,reading zero.

While the sender was in the failed state indicating "0", I checked sender with my meter. With a half tank of fuel, the sender was indicating 88 ohms - consistent with the "0" fuel indication I was getting on the EFIS.

My senders and tank are grounded per plans - A tab on the tank is grounded to the airframe and the sender is grounded to the airframe. The sender is also grounded to the tank by the mounting method. While the sender was in the failed state, the tank-to-airframe ground was 0.3 ohms, the sender-to-airframe was 0.3 ohms, and the airframe where the grounds terminate to a remote airframe ground was 0.3 ohms. So, seems to me the grounds are fine!

My theory is rather than a broken winding, the wiper is not making good contact - about the only way one can explain it sometimes working, sometimes not.  A loose wiper would cause increased resistance, giving erroneous low fuel indications.

I guess another possibility would be a leaky float - obviously a heavy float would sink and indicate lower fuel than actual - don't know why it would "get better" sometimes, though?

In either scenario, I see no option but to replace the sender.

I fuel my aircraft myself 99% of the time and have a "gentle" 12v pump - the stream is much less forceful than the typical big pumps at the FBO's - and aim it straight down the inlet. However, I guess it would only take once by a careless lineman to point a nozzle in the float and arm's direction and perhaps do some damage.

John

Dave,

I'm cooking up a big helping of crow for dinner tonight! After reading your comments and assuring you "the grounds were good,"  you left me with enough nagging doubt that I thought "I'll check the grounds again one more time before replacing the sender," ... and THEN I discovered a ground problem!

In my defense, this has been an intermittent problem, but lately the "sender" seemed totally failed. For some reason, I left the EMS on while checking the grounds. As you suggested, rather than just checking with an ohmmeter, I jumpered a ground and when I touched it to the body of the sender, the indicated fuel level marched right up from zero to the correct level!

Here's what I found - I had put the sender ground UNDER a rubber washer on the sender screw, so it was in contact with the sender flange. However, the ground ring terminal was a loose fit on the screw, So, IF the sender flange is not in contact with the surrounding aluminum tank externally, it is insulated by the gasket underneath. Also, IF the ground terminal ring is centered on the screw and is a loose fit, it might/might not actually touch the screw since it was insulated by the rubber washer above it.  Although it worked fine for 300 hrs, I think with time and perhaps shrinkage of the rubber washer or gasket, it somehow slightly shifted and was making poor or intermittent contact.

Solution, I removed the rubber washer so the ring terminal was in solid contact with the screw head AND the sender flange. I used Loctite 567 on the threads and so far (keeping my fingers crossed!) no leaks. Moreover, the fuel sender is indicating perfectly and reliably!

I hope this is the end of the saga!  Dave, to paraphrase your later comment, "Your rambling DID help!"  ;>)  I'm NOW "well-grounded" in sender diagnosis! (pun intended!)

Thanks,

John

I'm glad I helped John.

Dave
I have to get senders for my new wings.
Does anyone have experience with the Belite ones. Seem real simple they measure the weight of the fuel above them.

Ray Toews

Ray, 

The Belite senders are discussed earlier in this same thread. Here's what a Sonex builder said about his experience with Belite senders:

The fuel sender works, but I don’t trust it in flight.

Its susceptible to electrical noise (lots with the Jabiru).  In turbulence, the readings are all over the place.  On the ground, they are stable.

So I use it only to check level before flight.  I use my MGL with a RedCube flow to calculate fuel level in flight.  Its highly accurate.  The xTreme allows you to have two fuel levels, one based on a sender, one based on flow.  I use the latter.

It is too good to be true.  I won’t remove it, it provides some benefit.

Yes, I tried resistors, but that is not my strong suit and gave up quickly.

I suppose I could spend more time troubleshooting it, but I’ve found a good solution.  It is what it is.

Not exactly an enthusiastic endorsement! In fairness, it could be installation-related, but doesn't sound like it is a simple "plug 'n play" solution.

John

Hi Guys

 

I am having the same problems as far as accuracy.   I  want to order a fuel flow for my tanks,  but I was wonderiing if you guys have any models in particalur that you would recommend. 

 

I have an Mgl ff1 that can take a 2 unit.

 

Thanks

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