Hi,
I recently put a 90L sub tank (out of an AUS delivered V6 120 series petrol prado) in my FZJ105. The original owner put in a LPG system from new (& swingaway wheel carrier), but as LPG is barely available in the NT, I had this removed. It seems like the idea of putting a prado tank in a 105 is out there on the internet, but the practical steps involved are not, so here's how I did it, I'm sure if you're a mechanic/some other automotive professional, this amount of detail won't be of much use. Before I started, I got an FZJ105 sub tank and a prado sub tank from the wrecker, which definitely made it easier.
To summarise: works really well, practical additional fuel capacity, is nowhere near a "bolt on" solution; installation was fiddly and time consuming (for me).
Fuel pump, sender
The fuel pump assembly/sender assembly mounting holes and associated PCDs are the same for both tanks. I pulled the prado fuel pump assembly apart, modified it to be functionally the same as a 105 unit but longer (the prado unit does not have an in tank pump), hooked up the 105 wiring harness and that was that. I had to do a bit of electrical troubleshooting to get power to the sub tank pump, the culprit was the "sub fuel pump forcing driving relay". While it passed the test procedure outlined in the Haynes manual, it didn't actually work. I got a new one, problem solved.
I ended up reusing the sender out of the FZJ tank. I made the float arm longer (with an old coathanger) then bolted it in the tank and bent the arm so that it would travel freely through it's normal range from the floor to the roof of the tank. I'm pretty sure the prado sender is not compatible with the FZJ setup. The resistance values the prado sender was putting out were significantly different to the FZJ sender, and after an excessive amount of time trawling the internet, I discovered the prado sender is wired in series with the (prado main tank) sender and the instrument cluster sees the combined value. As distinct from the FZJ that reads the value of whichever (main or sub) sender is selected. I did some research and I think a FZJ75 main tank sender would probably be the elegant solution.
Mounting the tank
Initially I contemplated attaching the tank straps to the chassis. After some thought, I decided this might be a bad idea as it seems there is a small amount of potential movement between the body of the car and the chassis. Additionally, the original sub tank is hung from the underside of the floorpan and it seemed like replicating this was a good idea. I calculated the new setup would weigh ~30kg more than the original when full of fuel, so hopefully the floorpan is up to it. This part was pretty fiddly.
The floorpan members that allign with the wide shallow channels in the top the tanks are not the same - the prado tank channels are ~50mm narrower centre to centre. I made an H shaped frame so that when the tank is pushed up from below, the load is on the channels and floorpan members.
I needed to extend the straps ~60mm for them to be long enough. One of the original FZJ mounting straps had a longer L shaped bracket at one end that was the perfect length once pressed out and installed on a prado strap. For the other strap I made a bracket out of some 5mm flatbar. The recesed floorpan nuts that hold the straps to the floorpan don't allign perfectly with the prado tank (as they are 50mm wider apart) but it seems to work OK.
Hoses etc
The filler/breather locations and the hose dimensions are the same (or near enough). The prado tank has 3 hoses coming out the top, the FZJ tank 2. I bunged off the third (I think its a breather), pretty sure the FZJ tank 'breathes' via the fuel cap. The feed and return hoses are different sizes to the FZJ ones, so you need to buy/make adapters. The FZJ fuel feed hose is distinct as it is some kind of special fuel hose with special fittings, where as the return is a standard hose/clamp arrangement. I used a standard hose/clamp on the feed.
The tank guard
This was a pain in the arse. The guard has 2 'wings' on either side that need to be adapted to the FZJ chassis. While it fits between the frame rails unmodified, it's very tight. I trimmed 3-4mm off each side of the wings to aid positioning it correctly when bolting it up then made some brackets out of 40mm angle (with slots in them rather than round holes to further help with positioning). There was an ideally positioned hole in the chassis but it didn't go right through, so I drilled a corresponding hole on the inside. This was a monumental pain the arse, recommend doing before you put the tank in. For the front mount, I bent a piece of 75mm flatbar and drilled it to line up with some preexisting threaded holes on the backside of chassis crossmember. I had to remove it/modify brackets/reinstall a few times until the position was good and the tank had clearance from the tank guard.
At first glance looks relatively factory.
I recently put a 90L sub tank (out of an AUS delivered V6 120 series petrol prado) in my FZJ105. The original owner put in a LPG system from new (& swingaway wheel carrier), but as LPG is barely available in the NT, I had this removed. It seems like the idea of putting a prado tank in a 105 is out there on the internet, but the practical steps involved are not, so here's how I did it, I'm sure if you're a mechanic/some other automotive professional, this amount of detail won't be of much use. Before I started, I got an FZJ105 sub tank and a prado sub tank from the wrecker, which definitely made it easier.
To summarise: works really well, practical additional fuel capacity, is nowhere near a "bolt on" solution; installation was fiddly and time consuming (for me).
Fuel pump, sender
The fuel pump assembly/sender assembly mounting holes and associated PCDs are the same for both tanks. I pulled the prado fuel pump assembly apart, modified it to be functionally the same as a 105 unit but longer (the prado unit does not have an in tank pump), hooked up the 105 wiring harness and that was that. I had to do a bit of electrical troubleshooting to get power to the sub tank pump, the culprit was the "sub fuel pump forcing driving relay". While it passed the test procedure outlined in the Haynes manual, it didn't actually work. I got a new one, problem solved.
I ended up reusing the sender out of the FZJ tank. I made the float arm longer (with an old coathanger) then bolted it in the tank and bent the arm so that it would travel freely through it's normal range from the floor to the roof of the tank. I'm pretty sure the prado sender is not compatible with the FZJ setup. The resistance values the prado sender was putting out were significantly different to the FZJ sender, and after an excessive amount of time trawling the internet, I discovered the prado sender is wired in series with the (prado main tank) sender and the instrument cluster sees the combined value. As distinct from the FZJ that reads the value of whichever (main or sub) sender is selected. I did some research and I think a FZJ75 main tank sender would probably be the elegant solution.
Mounting the tank
Initially I contemplated attaching the tank straps to the chassis. After some thought, I decided this might be a bad idea as it seems there is a small amount of potential movement between the body of the car and the chassis. Additionally, the original sub tank is hung from the underside of the floorpan and it seemed like replicating this was a good idea. I calculated the new setup would weigh ~30kg more than the original when full of fuel, so hopefully the floorpan is up to it. This part was pretty fiddly.
The floorpan members that allign with the wide shallow channels in the top the tanks are not the same - the prado tank channels are ~50mm narrower centre to centre. I made an H shaped frame so that when the tank is pushed up from below, the load is on the channels and floorpan members.
I needed to extend the straps ~60mm for them to be long enough. One of the original FZJ mounting straps had a longer L shaped bracket at one end that was the perfect length once pressed out and installed on a prado strap. For the other strap I made a bracket out of some 5mm flatbar. The recesed floorpan nuts that hold the straps to the floorpan don't allign perfectly with the prado tank (as they are 50mm wider apart) but it seems to work OK.
Hoses etc
The filler/breather locations and the hose dimensions are the same (or near enough). The prado tank has 3 hoses coming out the top, the FZJ tank 2. I bunged off the third (I think its a breather), pretty sure the FZJ tank 'breathes' via the fuel cap. The feed and return hoses are different sizes to the FZJ ones, so you need to buy/make adapters. The FZJ fuel feed hose is distinct as it is some kind of special fuel hose with special fittings, where as the return is a standard hose/clamp arrangement. I used a standard hose/clamp on the feed.
The tank guard
This was a pain in the arse. The guard has 2 'wings' on either side that need to be adapted to the FZJ chassis. While it fits between the frame rails unmodified, it's very tight. I trimmed 3-4mm off each side of the wings to aid positioning it correctly when bolting it up then made some brackets out of 40mm angle (with slots in them rather than round holes to further help with positioning). There was an ideally positioned hole in the chassis but it didn't go right through, so I drilled a corresponding hole on the inside. This was a monumental pain the arse, recommend doing before you put the tank in. For the front mount, I bent a piece of 75mm flatbar and drilled it to line up with some preexisting threaded holes on the backside of chassis crossmember. I had to remove it/modify brackets/reinstall a few times until the position was good and the tank had clearance from the tank guard.