Zenith Aircraft Builders and Flyers
Online Community of Zenith Builders and Flyers
Defective VDO Fuel Level Sender?
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
Replies to This Discussion
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Permalink Reply by John Austin on
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Got some not-so-good feedback from the author of the Sonex Belite thread:
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.
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Permalink Reply by Mark Maltais on
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I can agree with that. Went up yesterday... what is with the site only punching up the first sentence in my reply???
Anyways, I went up yesterday and had a buddy along so I only put 10 gallons on each side. Normally I fly pretty full. Tank readings went all over the place. 3 gallons on one side, 14 on the other, and other bizarre readings...
The fuel flow sensor is dead accurate by my guesses and I didn't know the xTreme could do both at the same time. Will have to look into that. Would be nice to be able to switch around.
Still boggles my mind on how the sender in my 20 year old truck is perfectly accurate and reliable but my AIRPLANE with $20k in avionics in it, can't read the fuel tank properly... $%^&.
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Permalink Reply by Don Morrisey on
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Interesting thread on VAF regarding a home grown solution to Belite:
http://www.vansairforce.com/community/showthread.php?t=128605
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Seems like you may have a poor ground connection from the tank/sender to airframe or back to your ground buss.
I checked the ground and it was OK. I'll have to wait until the sender malfunctions again to be sure, but seems like if the wiper contact was loose where it touched the resistor windings in the sender, this would increase the resistance and cause the same symptoms - low or "O" indication.
John
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Permalink Reply by Timothy D. McCall on
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If you can get to the connection at the sender you could bypass it with a 50 ohm resistor. Tank should read 1/2 or so and be absolutely stable.
Tim
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Just finished the condition inspection yesterday and did a test flight. During the flight, the fuel sender was back to its old tricks again, indicating "0" fuel although I knew the tank was half full. This time, it stayed at "0" throughout landing and shut-down on the ramp, so I whipped out the Fluke and checked the sender - 88 ohms, which is consistent with a full-scale empty indication. So, I definitely know it is the sender that is the problem!
It would be very easy to install a Red Cube fuel flow transducer on the right fuel line just before it enters the Andair selector valve and let the EFIS give a "virtual tank" or calculated indication of fuel quantity in the right tank. I know the MGL XTreme EFIS is capable of this, but I'm verifying with Matt at MGL that I can continue to use the left, conventional VDO sender for the other tank as it is working perfectly. If I can still display the two fuel levels with the two different configurations, I'll likely go with that vs either trying to remove/reinstall the sender through the access panel or drilling out the wing root top skin to gain access. I'm concerned in either case of installing a new sender that I might get leaks and since the defective sender is leak-free, I'd rather just disconnect it and leave it installed!
John
N750A
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I wouldn't touch the old sender, at least until it does start to leak. Using a fuel flow sender like you mentioned would be a great idea. I think it would be interesting to see a direct comparison since you would have a sender and flow meter at the same time. I'm thinking of doing the same since my senders are all over the place when I'm flying.
I've found my sender to be VERY accurate so far when I've dipped the tanks after a flight, and it would be nice to see a rock solid fuel level all the time flying instead of looking down once in a while and seeing a 2 gallon number in a tank, then 10 gallons which gives me a heart attack, until I tell myself it's not accurate! :-0
I wish there was some way to slow down the reaction time of the sender maybe, my vehicles don't have gas gauges jump all over the map...
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Mark,
The MGL XTreme EFIS has three levels of filtering the fuel level fluctuation, so I never had a problem with fluctuating fuel levels. However, with a float sender, the fuel level indication was slightly different sitting on the ramp vs the in-flight attitude. Seems to me that would be an advantage of the "virtual tank"/fuel flow setup: the fuel flow would indicate the absolute number of gallons that has flowed, so it should be accurate in any attitude and not be prone to fluctuation with turbulence, sloshing, etc.
Ironically, it looks like I've got enough room to install the red cube flow transducer in the center console just proximal to where the right fuel line enters the Andair fuel valve - if it had been the left line, it would be a much tighter fit!
I'll update when I hear from Matt at MGL that I can display the two tanks' fuel levels with the two different configurations - I'm pretty sure I can, but I want to hear it from him, too!
John
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Update -
I contacted Matt at MGL and he said a "virtual tank" can only be set up on the primary fuel flow sender. (Don't know if that's just unique to the XTreme EFIS or all MGL EFIS). Of course, my primary flow sender is on the main line to the engine, so the virtual tank would have to be for 30 gallons and that doesn't help me get individual left and right tank fuel level indications. I really want to keep that sender where it is so as to have total fuel flow information.
So, I don't see any good option except to replace the VDO sender. I notice that replacement senders come with a fresh gasket. Since Jimmy Young says replacement is do-able through the oval access hole, I'll give it a try before resorting to drilling out the top wing root skin. After 30+ years of performing gynecologic surgery, I'm quite experienced at operating in tight spaces! ;>) Additionally, I've got one of those "snake" video cameras that I can thread into the space and be able to watch what I'm doing. If all else fails, I still can drill-out the rivets and lift the top skin!
John
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Permalink Reply by John Burns on
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I found this interesting article in the Rock Auto news letter and wonder if this could be the issue? However I would not be rally happy about running an untested fuel additive in an aircraft.
The fuel gauge in my 1992 Dodge B250 van should have been reading about ¾ full, but instead the needle was swinging wildly to different spots between the ¾ and empty marks. Every few seconds, the needle would start its crazy dance. The needle was moving even when the van was sitting still at stoplights. It was hard to concentrate on driving while the big gauge in the center of the dash needed an exorcist.
It is an older vehicle and the problem came on suddenly. My first assumption was that the fuel gauge sending unit in the fuel tank had worn out. The sending unit is an arm with a float on one end and a variable resistor on the other. When the fuel tank is full, the float rises up and moves the variable resistor to its low resistance (impedance) end. Low resistance means more electrical current gets back to the gauge in the dash and the needle moves up to the full mark. As the fuel level falls in the tank, the float moves the variable resistor towards the high resistance end, less current makes it to the gauge and the needle moves towards the empty mark. An electromechanical system that is bound to wear out eventually.
The sending unit is an arm with a float on one end and a variable resistor(circled)on the otherLuckily, I remembered an old Goss’ Garage segment from an episode of MotorWeek (RockAuto is a sponsor of MotorWeek). Pat Goss said sulfur could become too concentrated in old gasoline as more volatile chemicals evaporate away. Sulfur tarnishes the silver-based contacts on the variable resistor, creating random spots of high resistance that lead to random movements of the fuel gauge needle. The recommended solution was to pour a bottle of fuel system cleaner in the gas tank. The right cleaner in the tank would remove the corrosion and the fuel gauge would start working again.
I am skeptical of potions, but nothing is easier than pouring a little bottle into the fuel tank, so I gave it a try. A typical grocery/variety store has a dizzying selection of bottles labeled “Fuel System Cleaner.” I chose the first bottle I found that had printed on the label, “Removes deposits from the fuel sending unit, which can cause erratic fuel gauge readings.” The top ingredient listed on the label was kerosene.
The bottle of fuel system cleaner worked! There was an almost immediate reduction in the crazy swinging, but it took a couple of weeks before movement of the fuel gauge needle was back to normal. I avoided replacing the fuel pump/sending unit module (found under Fuel/Air in the RockAuto catalog) unnecessarily and perhaps inadvertently installing new parts in a fuel tank still full of bad gasoline.
Tom Taylor,
RockAuto.com
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That is interesting! But, like you said, I'd be reluctant to start putting cleaners in the fuel. Guess I'm a hypocrite, though, ... I run 94 UL/no ethanol in my Jab 3300, and since auto gas has a reputation for "going bad" quickly, I treat it with "PRI-G." It's not a system cleaner, however, but a fuel stabilizer.
John
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I suppose you could put cleaner in the tank, let is set a few days and then drain it out for use in the tractor or mower? Still makes you think though ?? Keep us posted. It will be interesting to see what the sender looks like when you take it out.
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