I recently bought my first Superduty (since 1999). The DEF alarm on my new 450 goes off at 68% telling me to refill soon. I understand when I get down to 1/4 tank. Is this normal? Seems aggressive for an alarm and on-screen notification with so much left in the tank.

 

It is based off estimated range left in the DEF tank, I believe set to 500 miles. Usually guys note the issue when they switch to tow/haul, as it drastically under-estimates the mileage left. The thought process is that under heavy use, you use more DEF, so you'll trigger that alert at a higher DEF tank percentage.

I noticed this weekend that about 60% full on my F350 means I can put an entire 2.5 gallon tote into it, so that's likely where I'm going to fill it at going forward. I would rather keep it full.

 

I have had issues when towing and the notice says add DEF fluid.It would be nice if the gallons left were indicated. That would help in knowing how much fluid to by

 

IIRC it is approximately a 6 gallon tank. So take 6 and multiply by .xx where xx is your shown %. That's what you have left, subtract from 6 and you know what to buy.

 

I have been using the truck lanes and buying my DEF at the pump and not those stupid bottles. So much easier

 

It is a 7.5 gallon tank.

 

The DEF tank has a full, middle, and low level switch, much like an RV black holding tank, not like the continuous sensor in your fuel tank. The truck infers level based on these switches and mileage, so not very accurate. Like a lot of people I get DEF in 2.5 gallon jugs. In principle when a 7.5 gal DEF tank reaches 2/3rds (67.7%), there should be room for a 2.5 gal fill. First time I tried filling at 65%, overfilled the tank, evil stuff, it’s corrosive so immediately clean up spills on your truck. Second time at 62% tank didn’t take 1/2 oz or so. I now refill at less than 60%. Your tank calibration may be a little different, so probably best to try 60 and go from there. I’ve never had a DEF alarm at 60% level or higher, even when towing. Might be a question for your Ford dealer?

 

That hair trigger setting has been a source of annoyance for many of us.

 

For 23+ the DEF gauge does not work that way, from the Ford 23 6.7 Book "When DEF is in liquid form, the DEF gauge uses an ultrasonic level sensor to determine the DEF level."

 

Oh good grief, I stand corrected!! I hadn’t seen the 23 6.7 book, it’s not on the diesel teck website with the rest! Appreciate the heads up, I’m a little embarrassed to say the first search I made for “2023 ford 6.7 book” yielded The Book, and I’ve previously hunted for an update multiple times!! Sorry for any confusion. I am however surprised Ford selected an ultrasonic level gauge, in my experience they can be flakey in a confined space, mainly due to refections/echos; hence the comment about debris I suspect (see image from the 6.7 book on the DEF tank below).


Interesting comments about waiting for a low fluid warning to refill, and doing an SCR refill activation test!
My DEF gauge appears to have survived being overfilled, twice, as it’s reading about right. I was aware of the whole freezing expansion need for room in the DEF tank so don’t overfill thing, but the gauge calibration being affected by overfilling is news. Has anyone done an SCR refill activation test? Time for some additional reading…

 

that book is not how mine operates
it will stay on full for long time then drop to 97 or 95
then some times it hangs at that level for long time and others it plummets to below 80 or what ever it feels
at 80 i can get 2 gallons in,
i have also got 2 gallons in at 90 so its not IMO even close to being accurate

 

Explains a lot actually. In the mining industry we spend a lot of time tweaking ultrasonic level sensors mounted in confined spaces, and this is what they do. Flatline for a while, jump from 90 to zero and back to 80, all kinds of shenanigans, and they are expensive industrial versions at $5 to $10k each, not the presumably fairly simple low cost versions mounted in a DEF tank.

 

Ultrasonic sensors are...okay...for non-critical systems, or systems where a physical gauge or a laser sensor won't work. To date, I have never had to use one in any of my machine/system designs. Heck I have even used a laser to detect blocked sprues and metal level in an aluminum casting machine that SITS on top of a furnace full of molten aluminum - if you only leave it there a few seconds it's just fine.

Good to know, though, that that's how they're supposed to work. As for the comment about the 'gauge,' I think it's referring to the digital gauge, not the % readout (that I think is gone on '25 trucks, I haven't seen it, just the % readout, though my FIL's '23 also has the gauge). The % readout seems to hold steady on mine and go down gradually, so perhaps they have gotten some fine-tuning done?

 

Interesting! Agree ultrasonic sensors are excellent in the right application. Radar is replacing some ultrasonic sensors, as are laser devices, although I have no direct experience with laser based sensors. Each with their own set of plusses and minuses. At lest knowing the type of sensor used helps understand the behavior. No doubt Ford has heavily filtered the signal (hence the comment“The DEF gauge does not update instantly while refilling (unlike a fuel gauge).” The comment “The DEF gauge does not move off 'F' until the tank level falls below 90% capacity.” implies rules based filtering as well.

 

Oh, and on the 2024 KR the DEF gauge is available as one of the configurable gauges.