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Discussion Starter · #1 ·
Hey everybody,

Just wanted to pass along some knowledge that I recently learned, though it may not be news to any of you. The DEF tank sensor apparently uses some sort of sonar to determine how full the tank is. Unfortunately, if you fill the tank right up to the top, it can't read the level at all: it needs some air to read properly (my only assumption is that it's measuring the speed of the sonar return through the two mediums of the water and DEF, since they would be relatively constant through differing environmental conditions, yet different from each other.).

I filled my tank all the way up, after the low DEF alert popped up in the cluster, and because it was too full, the gauge didn't move and the mileage countdown continued. After about 3 days, I topped it off again. Long story short, the counter went all the way down to zero and the truck wouldn't start. Luckily, I drove it to the dealership before I turned it off. The whole thing had the dealership scratching their heads too, but eventually they found some info about the sensor and were able to reset it and tell me how to avoid having it happen again. Hopefully this is helpful to somebody out there, and y'all don't make fun of me too much, haha!
 

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Discussion Starter · #4 ·
The DEF gauge sensor does not go all the way to the top of the DEF tank that is why the DEF gauge will read full for so long, there is a video or two that show the DEF gauge from the Ecodiesel (and other Ram Diesel trucks) DEF tanks and explains how it works, it is a long thin strip sonar board that sticks straight up, when the DEF covers the sensor the gauge reads full, but because the gauge does not go to the top of the tank it will read full for a long time before the DEF gets low enough to start exposing the sensor to air. Once the sensor is exposed to air in the tank it starts showing the level drop on the gauge.

Once the DEF fluid goes above the stand up sensor it does not know if you filled the tank to the top or just above the top of the sensor because it will read full either way.


It sounds more like you had an issue with your DEF gauge because once the DEF level went above the top of the sonar sensor it should have read full and again it does not know if you filled the tank to the top or just above the sensor it will read full either way and will not start to read the actual DEF level until the sonar board is exposed to air in the DEF tank and that can take a couple of thousand miles depending on how many gallons your DEF tank holds.

Not sure what to tell you: it didn't go back to full, and they didn't replace anything on the truck. I'm just passing on what the dealership told me, I'm in no way an expert on the subject.
 

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Discussion Starter · #7 ·
Is it possible that your 2nd gen EcoDiesel DEF tank uses different technology than the 3rd gen EcoDiesel DEF tank? The OP posted in the 3rd gen section of the forum, so I'm guessing he has a 3rd gen EcoDiesel.
That could be. I have a 2020 Limited, not the Classic, so it could be significantly different than his 2019. He seems really angry about it though....

Either way, I'm going to trust the dealer over some random guy getting mad about my story.
 
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