DEF Capacity - Gauge

[JHC1]

Being new to the RAM 3500 / Cummins engine, I am trying to figure out how to gauge the volume in the dash DEF Gauge. I understand the total volume is 5.5 gallons (I think...) but how does that equate to the gauge? I am showing DEF at the half way mark so can I fill it up with at least 2 gallons? I don't want to open a gallon and not able to use the whole gallon. Thank you!

 

[DougB]

You can add a 2.5 gallon jug at half way. It’ll show full after that. That’s usually when I add to mine.

 

[steve49]

If you live in an area with freezing winter temps, I wouldn't fill it full since DEF can freeze!

 

[mountainears]

I wait until it gets to 1/4 then add a jug. I don’t like over filling and want to be sure a full jug will fit.

 

[CdnHO]

I wait until it gets to 1/4 then add a jug. I don’t like over filling and want to be sure a full jug will fit.

What he said. Put a jug in at a 1/4. Beats spilling the stuff all over and people looking at you like you peed yourself. LOL. Besides overfilling can sometimes cause issues. If I fill up at a truck stop, I just let it go until the DEF pump clicks off.

 

[wrvond]

Being new to the RAM 3500 / Cummins engine, I am trying to figure out how to gauge the volume in the dash DEF Gauge. I understand the total volume is 5.5 gallons (I think...) but how does that equate to the gauge? I am showing DEF at the half way mark so can I fill it up with at least 2 gallons? I don't want to open a gallon and not able to use the whole gallon. Thank you!

Not based in anything scientific, but my personal belief and practice is that I add a jug from WalMart when the indicator is between 1/4 and 1/2 marks. The DEF freezes easily and expands. It's my understanding the tank is designed with this expansion in mind, but in the winter I don't fill it all the way up "just in case". There is also a heater inside the tank to thaw that frozen DEF or prevent freezing as you're driving. As with most submersible pumps, motors and heaters, the device is cooled by the liquid surrounding it. I feel that if too much of the heater is exposed to air it can overheat and break, similar to an aquarium heater.
Again, this is just me, how I do it, and why.

 

[Brutal_HO]

Full is 5 gallons.

That makes each 1/4 = 1.25 gallons.

Don't overfill regardless of ambient temps, the sensor is sonic and can register false/"stick" if overfilled. Though I think there was a TSB that fixed that stuck reading issue.

There's two schools of thought on DEF tank level during below 12F weather when it can begin to freeze.

1. Keep it topped off to reduce the likelihood of freezing (more mass=less chance)
2. Keep it low to prevent a solid frozen 5 gallon chunk of DEF that will never unthaw unless parked in a heated garage 24-48 hours

I'm in the #2 camp.

First, the software will not put the truck in limp mode if the DEF is frozen and can't initially pump (to a point).
Second, I can add fresh DEF to assist with the unfreeze.

This is all, of course, going to depend on weather forecast and use/non-use patterns that are unique to every owner. I'm not going to add more DEF if the truck's just going to sit parked below freezing.

 

[AH64ID]

Here is what I’ve found only filling with pumps for the last 20K miles.

Gauge at 1/4, it will take just under 4 gallons
Gauge at 3/8 it will take about 3 gallons
Just under 1/2 will take about 2.75 gallons

The gauge isn’t a float, so it’s not quite the same as a fuel tank.

 

[JHC1]

Thanks everyone. Good info.