FZJ80 Sub Tank Installation Notes in 2020
I’ll preface this by saying I wouldn’t have been able to do this without some of the pioneering members posting their info over the years. The threads I referenced are below. That said, a lot has changed since people have done this and its less feasible than it once was. That said, it is still possible with patience (this took me almost a year to complete). Here are my notes for attempting this project in 2020.
Sub Tank
These are NLA and you’ll have to find a used one. There are two versions; a diesel and a petrol. Both will work, but the diesel unit has an extra pickup tube that needs to be capped. Both pickup assemblies are NLA, so make sure the unit comes with a functional one. Mine was damaged in shipping and I had to fabricate a replacement. You’ll also want to replace the fuel sock on the end. These are still available.
The tank should also come with a fuel sender, if not or if its broken, you can replace it as they are also available. If you’re lucky, the tank will come with the harness to connect the sender to the main wire frame harness under the back center of the truck. It’s a 6-pin connector listed as Bb1 in ROW wiring diagrams. If your tank doesn’t come with this harness, you’ll have to make one.
To secure the tank, two straps are used (also NLA), but they can be fabricated if necessary. They are secured to the floor of the truck with two rivnuts and bolts per side. US trucks have holes pre-drilled for two of the rivnuts, but the rear two will have to be drilled out. It’s very tricky to get a rivnut installation tool in the two closes to the front of the truck and will require some creative thinking.
The final piece of the rear tank setup is the service cover for the floor. US trucks have an embossed area where the hole should be cut. Take a hole saw, cut out this raised piece of the floor, and cover it with the service cover. This will allow you to access the sender without dropping the tank.
Dual Filler Assembly
There are two versions of the dual filler assembly; petrol and diesel. There are also two iterations as they were revised for the newer trucks. The ideal setup is to find a petrol assembly from an early truck. These are very hard to find so you’ll probably end up with a late model diesel version. These do not come as a complete assembly. There are many different parts (diverter valve, filler neck, spring, activation rod, knob, etc.) and you’ll have to assemble them to create the unit. This version will not have a gas restrictor in the neck, will require a separate mounting bracket to secure to the truck (which is NLA and will need to be fabricated), will have less than ideal fuel pipe angles, and you may have to modify the diverter rod as it was too long for my truck and interfered with my gas cap door.
One you have a functional unit; you’ll have to cut the OEM filler tube on your truck right by the inside frame rail. You will not be able to remove the hard pipe without lifting the body off the frame and even if you could, the replacement pipe is NLA. I used an OSHA approved non spark manual hacksaw $$$ to cut this because I didn’t want to drain and remove the main tank. It was tedious and nerve wracking and I assume no responsibility if you blow yourself, truck, house, etc. up
It’s easiest to cut and install everything by jacking the rear of the truck up, removing the rear wheel, liner, and interior cargo trim panel. This will have to be removed anyway to fit the ECU (NLA – are you sensing a theme here?) and modify the existing wiring harness. Once fitted, everything is connected via OEM fuel hoses/vent hoses/clamps. You can also add the special dual filler instruction sticker to your gas door for that OEM look.
Sub tank Transfer Assembly
Also NLA – you’ll have to find one used. It should come with the pump, bracket, hoses, and solenoid. This is attached to the underside of the truck using a few existing captive nuts. Technically there is a larger bracket that everything bolts to, but I found that it wasn’t necessary, and everything was secure with the existing mounting points. YMMV. You will need to find the harness for this, and it ties into the existing wire frame harness in the truck. Last I checked there were still a few available new.
ECU and Wiring
ECU is NLA and will need to be sourced used. The other option is to use Georges ECU (he is still making them). I won’t cover that route as I found an OEM ECU, but it’s a viable option if you cannot find one. I purchased one just in case my ECU dies someday. Both the momentary switch (OEM ECU) and latching switch (Georges ECU) are still available. I was able to source the harness that ties the solenoid, pump, senders, and switch a few months ago, but I was unable to find the main tank switch assembly. This tells the ECU that the main tank is full and to shut off. I ended up fooling the ECU into thinking the tank was always ready for transfer.
The wiring portion of everything is described well in the reference threads so I wont be going over it here.
Overhead Console
This is optional, NLA, and hard to find. I was unable to source an oak colored one, but successfully dyed a gray one. I also had to fabricate my own harness.
Final Thoughts
While still technically doable, this isn’t for the faint of heart. It’s expensive, complicated, time consuming, and quite a few parts are no longer available. If you come up short finding some of the parts your project could be dead in the water with a potentially significant loss of investment. That said, the convenience is of being able to transfer extra fuel on the go is amazing and it has a 10/10 cool factor, especially considering how rare and desirable this setup is now. Hope that helps anyone looking to add one of these in 2020! I also have some parts left over if anyone wants a head start on the project.
