Online Community of Zenith Builders and Flyers
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!
Hi John, very detailed write up, thanks. I realize it’s after the fact but I just found this post so I will ask, why did you drill out skin rivets to remove and replace the sender? Why not just remove it from the mounting hole it sits in?
If I had used an identical VDO sender and installed it "dry" with no sealant and only used the gaskets as a seal, I "might" have given it a shot at doing the job through the access hole, but from others' experiences with sender replacement on the STOL 750, it's no fun as you have to be a contortionist, can't use both hands, and have limited visibility. Some have even cut a second access hole/inspection plate so they could get another hand in there!
However, I used the Stewart Warner senders which are very popular in Van's RV aircraft. Van's recommends not using the gaskets and use Proseal or Chemseal instead. This material is very messy and sticks like nothing else! Without better access and visibility, I didn't see any way I could cleanly and accurately install the senders with one of these adhesive sealants - you really need to be able to hold the sender in position while the screws are tightened and sealed. Having good access and visualization also lowers the chance you knock a clump of sealant into the fuel tank or gum-up the sender pivot, etc.
In my particular case, I had left the top wing root skins off the plane at final assembly and had them painted separately. I did this to be able to observe the senders for leaks once the plane had it's initial fueling and easily fix any leaks if they did occur (none did!). I later installed the skins and touched up the rivets. So, this made it even easier to take the approach I did in that the skins weren't adhered by paint to the surrounding skins - it just took a few minutes to drill out the rivets. This gives excellent visibility and access!
BTW, it's not necessary to completely remove the top skins - just take out the aft rivets and work forward up to and slightly beyond the main spar and gently arc the skin forward. It will naturally arc towards the mid-line of the fuselage and you can cleco it to the opposite top skin if you're opening both and that will keep them from flopping back into their normal position. If just one skin is opened up, you can use a tape strap to keep it arced forward out of the way.
Well the same issue of side mounted leaking VDO senders has happened in my Cruzer which I caught on last week's condition inspection. Here I have what appears to be slow seepage on both wing tanks as evidenced by dark blue 100LL color around the rim and maybe through the mounting screws.. That said and having read your detailed change to the Van's units. My only question is whether or not you had to clean up the inside tank area where the inner flange plate mounts. I am sure that the material I used will to a limited extent be inside the tank (I believe I used a fuel resistant gasket maker material). I don't want to fuss with the inside of the tank too much but obviously want a good seal. Finally it appears that you did not use a VDO kit rubber seal in your installation of the VAN's units but Mark seems to have included it in his top mounted senders so a little confusion there for me. Can you confirm if you used the rubber seal or not for me John? I really enjoy your detailed input as well as your monitoring activities John. The whole zenith community is indebted to you!
I had the original VDO senders mounted with the rubber gaskets and no other sealant around the periphery, so there was no cleanup of the inside of the tank necessary. I did lightly scuff the inner rim of the hole with Scotchbrite and cleaned with acetone as a precaution that if any Chemseal (same as Proseal) oozed inside the rim of the hole, it would have a good surface to bond to. (After my installation, I looked at the area with my snake camera and very little, if any sealant had squeezed-out inside the tank.)
I did not use the rubber seal when installing the new senders - Van's recommends using Proseal/Chemseal only and not to use the rubber seal. The RV series mounts the SW senders on the side of the tank, same as Zenith. I had a Van's RV builder tell me he used this method 25 years ago and still no leaks, so that's good enough for me! Haha!
In Mark Pensenstadler's excellent videos, I believe he used the Proseal and no rubber gasket on one of his builds (?the Cruzer?) and he used the rubber gasket on the other (?Super Duty?) I don't know his reason to switch back to the rubber gasket but I decided to stick with the tried-and-true method that Van's recommends. If one top-mounted the senders with an access plate directly above, it wouldn't be too hard to change them out if they leaked, so that would be one argument to use the rubber gaskets and avoid the very messy, very "fiddly" Proseal method! However, I think of the Proseal/Chemseal method as a "permanent" method. I'm sure the rubber gaskets are OK, too, but remember, the actual source of most builder's leaks is the central insulator terminal in the VDO senders - the SW senders have a much-improved design.
I just got back from the Homecoming Fly-In at Zenith, so it's been one year since I installed the new senders and no leaks so far! Fingers crossed!
Thank you for the quick reply and clarification. I ordered the Stewart Warner senders and matching VDO analog gauges and the Chem Seal too. I’ll let you know how my install goes but I think your mock up idea is a great one as well as no sharp first bend angle. It’s been so hot down here in Florida that I decided not to fly all of August and most of September too and so I had the plane sitting for the first time ever over an extended period of time (at least 45 days) with full tanks which I believe may have contributed to the weeping seal along with the oppressive heat. While I wait for the new parts to arrive with cooler days now in sight I’ll see how bad the seeping will be or if it gets worse by checking after each flight. I was able to carefully clean up the area with a solvent wet rag so it looks like new. My mind will be at easy once I have installed the new senders with the Chem Seal. I think my real issue here is that I did not use the correct bonding agent in the first place but had been lucky for the first 3 years of flight in the Cruzer despite this apparent building error.
On the SW senders you used - are they designed from the get-go to be side mounted, thus no bending of the float arm is required?
I doubt they were intentionally designed to be side-mounted (I suspect they are intended to be a replacement for the VDO's, which are actually mounted on top of the tank in OEM applications like the old VW bugs), but as I've said elsewhere, apparently they're much more resistant to gas leaks as they have a totally different central terminal and resistance wiper design while maintaining the exact same diameter and bolt-hole pattern of the VDO.
If replacing the VDO sender with the SW, you'll definitely have to bend the float arm to get a complete range of motion that mimics what the VDO sender had. However, the arm pivots on the side of the mechanism on the SW and pivots on the end of the VDO mechanism, so the pivot points are different and thus the bends will not be exactly the same.
there are two different types of sender installs
side mounted and top mounted
you are correct about a top mount, there is an access panel
on the side mounts it’s a lot more difficult to get at, requiring
panel or tank removal for access
Yup. I have a small access panel under the wing that allows me to inspect and reach the sender, I assumed John did too. I can remove that panel cover and remove and reinstall my side-mounted sender from its mounting location on the tank. Just did so last weekend and (argh) gotta do it again next weekend, those senders are a joy. Anyway wondered why John had to drill out skin rivets to do remove and replace tasks.