Installed a larger fuel tank? Need to have the J51 fuel gauge program installed so your gauge works correctly? Easily....do it yourself !! No dealer needed!
A while back I installed a new S&B 47 gallon fuel tank in our 2017 4x4 Sprinter (NCV3) and afterwards my gauge wasn't quite working correctly. I was told the fix is to have Mercedes flash the J51 fuel program (Description below) into my van to fix the gauge issue, but when I stopped by my dealer I was told that Mercedes will no longer allow dealers to flash the J51 program if you've modified your fuel system...including installing a larger tank. Dang! Time for a different solution.
Note on the J51 Fuel Program: Mercedes has two programs they use to determine how your fuel gauge functions. With their Fuel Tank Algorithm Activated the level sender in the fuel tank tells the computer when the tank is full and then the computer stops using the float and the gauge is an estimation of fuel left from that point on, based on fuel consumption etc) But if you have a diesel heater installed that's pulling fuel from the tank each night then the van won't know this extra fuel is being used and the gauge won't reflect this, you could end up with an empty tank and the fuel gauge still showing fuel. No good!.....Enter the second fuel program.
When the Fuel Tank Algorithm is Deactivated (J51 program) then the gauge actually uses the fuel sender in the fuel tank at all times and shows you a legitimate reading through the range, even if something like a diesel heater is using extra fuel. (Note that your range to empty will still be inaccurate, but the gauge will function correctly). Perfect! Just remember that when the gauge is empty it will be Empty with very little reserve.)
I read that the Thinkdiag brand of scan tools have the ability to not only scan and reset codes but will allow you to perform variant coding...which is where you change the fuel/gauge programs. Also I read where people have successfully used these tools to Install the J51 program in Sprinters...yes! (J51 = Deactivate the Fuel Tank Algorithm). With that i ordered the Thinkdiag 2 programmer ($119 on Amazon "link Below") and anxiously waited for it to arrive. The Thinkdiag2 plugs into your OBD2 port and uses your phone or a tablet (need internet connection) to control the system via Bluetooth.
Fast forward to today and I successfully and easily installed the J51 program in minutes! This tool will be awesome to carry in the van as it is a powerful troubleshooting tool as well allowing you to manually force systems, plus does normal stuff like clear SRS codes, etc.
Note on Android vs IPhone: The Thinkdiag app is available on both platforms but has limited functionality with the Iphone. On the Iphone changing Variant coding isn't available. If you want to change variant codes (like the fuel programs) then that can only be done on the Android platform/App.
After downloading the Thinkdiag+ app and entering the serial number and activation code included with the unit I was ready to get started. I took screen shots to step you through the entire process if you are looking to do the same setup
I tested this on my fathers VS30 (2021 Revel) and although there is a TON of Variant coding available I couldn't locate the ability to change the Fuel programing for the gauge cluster like I could on the NCV3 models. It's still an awesome powerful tool to carry with you in the van but appears it can't flash the J51 program on the newer vans if that's what your after.
Supplies used:
Note: The links below are my Amazon Affiliate links, I make a small portion each time someone purchases from Amazon using one of my links, at no extra cost to the buyer. I only provide links to items that I personally like and use myself. i use the money to put back into my builds and support making videos/posts. If you prefer not to use my links you can search for the same items directly off Amazon. Thank you for the support!
Thinkdiag2 Scan Tool:
CLICK HERE
The tool comes nicely packaged and a zip up carry case is included.
After you register the unit (Comes with a 1 year activation code) and update the firmware on the device through the phone app your ready to go. Let's get into installing the J51 Code. 1st Turn your ignition key to the "On" position and click on the "All Systems Diagnostic button".
Next click on "Vin Decoding". The unit will locate your vin and verify the vehicle model and year your working with.
Next press "Automatically Search".
Next select your year range, for me it is 2013 and up.
Diesel engine for me
Left hand steering.
Automatic Transmission
Next click "System Selection".
Now click "Information and Communication"
Next is "Variant Coding". This is where we we will find/change the Fuel Programs for the Level Gauge.
Now we select "Instrument Cluster"
Continued Below....
. Squirrel mode has once again kicked in and our van is now up for sale. I sold the last vehicle to fund an airplane and ended up building the van instead, now I'm selling the van to make the airplane dream a reality. 
