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