From time-to-time there have been reports of the VDO fuel senders leaking or even the central electrode falling out!  These senders were OEM for VW's and intended to be top-mounted on the fuel tank.  When side-mounted in the Zenith tanks, they are continuously immersed in fuel under slight pressure from the weight of the fuel. My VDO senders are at least 10 years old, installed for 8 years, and have never given a problem ... till now!

I recently flew to Mexico, MO for the Zenith Homecoming and when I returned, began my annual.  I opened the access covers on the bottom of my STOL 750's wings and the senders were bone-dry without evidence of leaking, nor was there even a hint of a whiff of gas fumes.  Since all was good, I buttoned that area up! However, I had left the tanks about 1/4 full on both sides when I started the annual.  Afterwards, I topped off both tanks.  The next day, I noticed a drop of fuel on the hangar floor near the aft edge of the wing near the root. Looking up, I saw a drop of fuel hanging from the inboard flaperon bracket.  I opened the access panel again and the sender was very wet with fuel!  It also appeared the white insulator was slightly shifted or cocked from its usual position.  I touched the insulator and a piece about the size of pea crumbled off!  Yikes!  It was obvious the insulator was deteriorated and I immediately set about draining the tank with visions of the insulator popping out and dumping a tank of fuel into the wing root! Fortunately, that didn't happen.

Here's a pic of the crumbled insulator: (Ignore the AN3 bolt - I had already removed one machine screw and just used the bolt to serve as a "handle" when I manipulated the sender out of the hole.)

Bad enough, right? No, it gets worse!  After draining the tank, I was walking around the plane and saw a drop of fuel on the floor in a similar location on the right!  I opened the right access panel and the right sender was leaking, too!  The insulator didn't seem as deteriorated as the left - it didn't crumble - but it did feel slightly loose and could be slightly rotated.  

Obviously, the fuel top-off triggered the leaks.  What are the odds that I could have just completed an 800 nm round trip with 3 refuelings and the senders wait to leak till I get home - and both senders at that!  I felt the angels were truly with me on that last trip!

So, I have drilled out the rivets enough to fold back the top wing root covers and remove the VDO senders.  I plan to install the Stewart Warner senders the Van's RV builders have used for years.  They appear to be of a much higher quality and use a "thick film" resistor that is superior to the VDO's wire-wound resistor.  It's a big plus that they mount in an absolutely identical manner as the VDO's (they even use the same screws and mounting ring).  One of our members, Mark Pensenstadler, has an excellent YouTube channel, Kitplane Enthusiast, and has a detailed video on these senders here.

After researching this somewhat, I am going to mount them the way Van's recommends - no gasket and use ProSeal instead.  I'll also ProSeal the screws to prevent leaks.  Obviously, one needs to dry fit and test the resistance, etc., before final installation as it would be very difficult to remove a sender later and no way am I going to drill out that wing root skin again! Ha!  Although these senders seem very reliable, If a sender ever failed, but was not leaking, I'd be very tempted to simply leave it and cut an access panel in the top skin over the tank and mount a sender there!  

I also plan to use my flexible "snake" video camera to verify the float action during a dry fit test.  You want the float to both touch the top and bottom of the tank to get the maximum measuring range.  I saw a post elsewhere where a builder dry-fit the sender and left a screw out and passed a piece of safety wire through and tied it to the float wire.  He could then verify the float had full range of motion by hearing it tap against the top and bottom skins as he pulled the safety wire. Obviously, I could also verify this by observing through my "snake" video camera.

Just thought I'd put this out to remind everyone to check those VDO senders very carefully!  If yours is leaking, I would recommend to definitely drain the tank below the level of the sender before probing around that white insulator!

John

N750A

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Ok. I’ll do a little research and find out if they make a sending unit that matches our stock fuel guages. Good to see you again too, &  enjoyed our flight in your plane Saturday morning.

Yep John - been there done that about 2 months ago.  If one goes to the search engine box upper right and types in fuel sender access covers, the discussion will come up and then click on the title at the top and the full discussion with photos will come up.  Boy, I can empathize with your frustration and all the rest who have had to go thru this exercise!

Does anyone have a link to the information on the proper bending of the angles for the float arms on the fuel senders? I built my plane so long ago, I have forgotten where I saw that information. Just went through my plans, it wasn't in there, but I believe I recall there was a builder's assembly guide on line somewhere. 

Builder guide 75-KA-1 in the STOL builder guide area. Let me know if you need the specific images.  Page 11

I believe the bends/angles will be totally different for the SW sender - it pivots from one side of the mechanism and the VDO pivots from the end of the mechanism.  Obviously, you'll want to end up with approximately the same arc.  I just finished making a mock-up of the side and bottom of the tank with the lower edge of the sender hole 62mm above the bottom of the tank as it is in my 750.  I mounted the original left VDO sender and the float hovers a few mm above the bottom of my mocked-up tank.  This is exactly the way it appeared when I peeked in with the "snake" video camera before removing the sender!  So, I plan to mount the SW sender and use coat hangar wire to make bends (same method as Mark in the video) to make a pattern for the SW float arm.  I'll attempt to put the new float in the same position and have the same amount of travel.  I'll likely lower the float slightly so it just touches the bottom of the tank so as to reduce even further the amount of unmeasured fuel when the float bottoms-out.

Once I get the new senders, I'll take some pics of the old and new senders in the mock-up rig and post my results.

John

That would be a great help, thanks!

Here's some pics of my mock-up/test stand. The lower edge of the sender holes in my tanks are exactly 62mm above the inside bottom of each tank, so I replicated that on the stand:

When I mounted the VDO senders, their floats were suspended above the bottom of the test stand the same distance as in the actual tank - I had estimated the distances relative to the thickness of the wire holder and the float when I peeked in the tanks with my snake camera - the gap below the floats appears absolutely identical on the stand!  Here's a comparison of the left float in the tank and on the stand.  I had estimated the gap at about 2 diameters of the wire holding the float (since I couldn't directly measure the gap in the tank) and it came out about the same on the stand. There's a slight difference in magnification in the two pictures:

Since the stand seems to replicate the float's position in the tank accurately, I think I can work out the bend(s) of the new SW's floats and get it pretty close to correct, then dry-fit them, look in again with the camera to check the position and tweak the bend(s) if necessary.  I'm going to try to get the float just touching the bottom of the tank so the amount of unmeasured fuel will be minimized.

The new senders are scheduled to arrive next Monday.  I'll then bend some mock-up wire arms for the new floats and then bend the actual arms to match.

John

Following with interest. Nice work!

Not just Zenith (vw) senders that leak like this, just saw a video of an A&P doing an inspection on a Cessna 182, and both fuel senders are leaking. 

Yesterday I installed the SW senders.  I did several dry-fit practice trials because when you do the final installation with the ChemSeal (same as ProSeal), it would be a huge mess to have to remove the sender again with the wet sealant on it.  I cleaned the sender's flange and the tank hole with acetone, scuffed the areas where the ChemSeal would go, and wiped down with acetone again.  I had further deepened the notch in the retaining ring the way Mark did in his video I referenced in the first post.  This made it much easier to insert the retaining ring when installing the sender. Thanks, Mark!

I used ChemSeal B2, which has a 2 hour application time before it starts to set up.  I had a caulk gun that fit the ChemSeal cartridge perfectly and used it to apply the sealant - first a bead around the fuel tank opening and then a dab under each screw to seal them as they are seated.  One screw has the ground lug under it, so I tightened it without sealant and then applied the sealant around it afterwards.  I used my ohm meter to assure I had a good ground before sealing it!

I also used hex socket cap screws (stainless) instead of Phillips.  These work well with the sealant since the heads are taller and you can still access the hex socket with the sealant around the head.  

I guess in the rush to get the senders installed before the sealer set up, I forgot to take any photos.  Turns out I had plenty of time - it took about 1 hour to completely install both senders.  I did, however, take a post-installation photo and you can see the float almost touches the bottom of the tank and is perfectly aligned parallel to the bottom:

(Sorry for the skewed photo - I usually just rotate my phone's screen to make the image "upright" and not try to rotate the "snake" video camera.)

I did measure the clearance above the sender to ensure the float arm would clear the top of the tank when the float is fully up.  The picture also gives you an idea of the arm's acute angle (I didn't measure it, I just bent it identical to my coat hangar wire template) necessary to position the float.  The SW sender pivots from the side and the original VDO pivots from the end of the unit, so the arm bends will be completely different.

Now to sit around and twiddle my thumbs for 72 hrs (B2 needs 72 hrs to fully cure), then fill the tanks and observe for leaks before re-riveting the top wing root skins.

John

John, on your coat hanger wire template, is it pretty much an exact replication of the float arm wire? I’m curious if I could use those dimensions, (lengths and angles) should I take this change out on myself. 

The wire template is very close in dimensions to the actual float arm wire.  Once I made my bends on the actual float arm, there was some minor tweaking or bending of the wire but I did not have to re-do or cut anything shorter, etc.

The proximal portion of the wire from the edge of the sender's pivot hole to the apex of the bend is 102mm.  The bend is an acute, 65 degree angle, and the distal arm is 117mm long from the apex of the bend to the bottom of the semi-circular portion of wire that goes around the float:

Remember you'll have to allow some additional length on the proximal portion of the arm for a sharp 90 degree bend and 3/4" overall length of wire to insert into the sender's pivot hole to secure the arm in the nylon clip of the pivot - that dimension is illustrated in the instructions included with the sender.

My tanks are the 15 gallon "long range" tanks.  Since they are essentially hand-made at the factory, there could easily be some variations to have to account for.  Also, accurate arm dimensions and location would be dependent on the sender hole location being exactly per plans.  One could/should either make a mock-up test stand and verify the template or mount the template wire in the sender and temporarily install it in the tank and verify with a "snake" video camera.

One negative point about this configuration: The new sender's pivot point and arc of swing seem to not allow the float to rise as high as the original VDO.  What this means is that the sender will indicate "full" before the tank is actually completely full.  What will happen then is that when the tank is completely filled, the fuel level indicator will stay on "full" longer before it begins to drop.  However, with proper calibration, once the indicator starts dropping, the indicated fuel quantity remaining should be accurate down to empty.  I believe that FAA regs only require that a fuel quantity indicator be accurate at empty, anyway.  I have a fuel flow totalizer on my EMS, so early in a flight I can see precisely how much fuel I've burned even if the gauges are indicating "full."  (Hope this makes sense!)

One could get the float closer to the top of the tank by lengthening the arm, but then, due to the dihedral angle the tank is at,  you'll have more unmeasured fuel below the float when it touches the bottom of the tank and indicates "empty."

Today I hope to fill the tanks, check for leaks, and calibrate the senders to my MGL XTreme EMS's fuel level indicators.

John

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