On my 750 I installed the stock VDO fuel sending units in the stck positions and bought a set of VDO fuel gauges from Zenith with the matching ohm resistance. I have my wings on now & I put a total of 20 gal of fuel in the plane and I have the 15 gal tanks. I added fuel in 5 gal. increments to see what the gauges would register.

At 10 gal, (5 in each tank) the gauges just started to register. At 20 gal, they are both showing just over a 1/4 tank. I've got 10 gal in each tank so they should be reading 2/3 full.

I did the sender installation almost 2 years ago, and I am almost 100% certain I bent the float rod exactly like the plans called for because I remembered the 125 deg. bend before I went back to look it up. I've also got the "up" arrow drawn on my sender unit just like the manual shows, & I'm including an old photo to show one of the units when I built my wings.

 

After thinking about what could be going on for a couple of days, the only thing I can think of causing these incorrect readings is that I either have the wrong angle bent on the float rod, or I have the rod in a position other than vertical. I've tried to access the sending unit thru the access panel and I can get a screwdriver on it, but it's looking pretty difficult to back the phillips head screws out without stripping them due to lack of space to work. I'll pull the wings off and drill out the inboard top skin to get to them should I have to, but I'd obviously rather get the senders out without having to do that if I can.

 

Has anyone else had a similar situation occur with the erroneous fuel level readings, and has anyone else successfully removed a fuel sending unit via the stock access panel? Any and all thoughts, comments and suggestions are appreciated.

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What intrument do you have it hookup to? On a dynon or any efis you just calibrate the tanks it doesnt matter the ohms

Tracy - it's hooked up to a VDO analog gauge. 

Steve - I'll check that out, hope that's the case. I learned last night reading some troubleshooting articles that if the ohms are not the same on the sender and gauge that I could be getting similar readings. Either way, it will be an easy fix if the problem is with the gauge vs. pulling the sending unit out of the wing tank. Thanks.

Jimmy - I don't know if this applies to your problem, but it seems to me that the gauges should be inaccurate between "full" and "empty"! Why? I "assume" a steady change in ohms as the sender swings down, thus a steadily decreasing indication of fuel. BUT, our tanks, and most wing tanks, for that matter, don't have a constant surface area as the fuel volume goes down, or in other words, the tanks are not constant in their capacity from bottom to top since the tank has an irregular, wedge shape. It'll take a lot of fuel volume to float the sender off the bottom where the tank is widest, and then as it narrows near the top, small volumes of additional fuel will move the sender relatively greater distances.

Even my GRT EIS doesn't correct for this - you calibrate it at empty and full and it evenly divides up the gallons of fuel in between - but I'm willing to bet the only 2 points that are accurate are "empty" and "full"! To have a truly accurate gauge, you'd have to be able to calibrate it at each additional gallon or whatever unit you're using.

 

John,

I understand what you are saying and it makes sense. I could put 5 more gal in each wing and they would be full, then check the gauges & see if they read "full". Still would not be happy with a gauge showing 1/4 when I've got 2/3 of a tank of fuel though.

I found some info on VDO fuel gauges that show an adjustment screw in a hole on the outside of the barrel of the unit. However it was for a tube type VDO fuel sender, but it's still a resistance-type sender. The instructions said to set the dial to empty when the tanks are empty and you are done. I've found other info that says if the sender & gauge are matched with the same ohms, no calibration is necessary. What I find interesting is that of all the Zenith builders who have used the VDO gauges over the years, I would think this would be a well discussed topic if adjusting the gauges was commonplace. At this point I think my problem is that I've got a 70-10 ohm sender tied into a 180-10 ohm gauge. The math works out to 26%, and I'm showing 1/4 full readings on each side.

Hi Jimmy,

From that last comment about the 70-10Ohm sender on a 180 - 10 gauge, I think if that was the case your gauge would read high. When the sender was at the empty position the gauge would expect 180 Ohm resistance. If your sender empty is 70 Ohm then your gauge would be well off empty as that would represent a semi-full tank.

I'd first measure the sender empty resistance to see what's going on. If the sender is really a 70 - 10 unit, I'd be inclined to change the gauge for a matching one for the sender. It's easier than changing the sender.

Hope you get it sorted. I'm about to calibrate mine but with a fuel computer.

Paul

A follow up to this thread: It turns out the cause of the problem is I did not bend the float rod the required 125 degrees as is clearly shown on the assembly manual. After trying a couple of other gauges and everything else I could think of, it got down to the dreaded fact I had to pull the senders out of the tanks. They come out fairly easily, but the re-install was what I was concerned with as I've got my wings completely buttoned up & on the plane. When I got the first one out, I found the rod was bent (by me) at 105 degrees instead of 125. Fixed that, then took about 3.5 hrs to get the first sender back in the tank. There is a trick to it once you go through the learning curve as the second one only took about 45 minutes to remove, fix and re-install.  

The trick to reinstalling in a mounted wing is, after you put the long screw in to hold the inside retaining ring and while you still have the sender in your hands, go ahead and pre-set the rest of the screws so they just barely thread thru the gasket. This helps the alignment in the hole once you work the sender back into the tank as the outside diameter of the screws are right at the 59 mm diameter of the tank hole. Then you just slip it in and the screws are able to mate & thread up.

After I filled the tanks 1/2 full I've got good readings on both gauges. Hard for me to call a product junk when I installed it wrong. The VDO gauges are only adjustable for the purpose of setting the needle on "0" or empty before you install fuel. There is a hole for a slotted screw on the outside of the barrel for this, and you must have the power on to do the adjustment.

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just curious but could you get to the rubber fuel line we are suppose to change out in 5 years or will i have to train squirrels to do this for me. looks like great work you are doing.

best chris

I think it would be tough to get to thru the stock access panel. I added another football-shaped access panel under the wings at the hose/tank connection to get to mine. I used an Aeroquip hose (part # FC598-06) that is much better quality than the factory supplied hose. I used Aeroquip "socketless" AN fittings with a bulkhead fitting thru the fuse/wing connection. I left all of my fuel hose exposed, run down the back side of the upright so I'll always be able to inspect them and get to them if needed. Could send you a pic if you need one.

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