100 series subtank wiring/plumbing oem style

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semlin

curmudgeon
Joined
Mar 27, 2003
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336
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north of 49
looking for comments and feedback

i have been researching how to wire and plumb a subtank in the factory manner fror a 100. that means the dash subtank switches between tanks. there will be no transfer between tanks.

going full oem is impossible or at least impractical unless your harness is subtank ready. oem includes separate relays in the rear quarter panel for the sub fuel pumps and the fuel pump selector that are controlled by the ecu. in fact, there may be a separate fuel control computer for some models although it shows up in the 1999 uzj oz epc as "not applicable".

anyway here is my tentative plan for "near oem".

1. disconnect the main fuel line and return line from the main tank behind the tank. these parts are fully compatible with the subtank harness. you join the subtank line assembly in series into these parts.

2. install the oem fuel line-solenoid 77720-60070 for subtank vehicles. this is a "t" solenoid that switches line flow between the two tanks, apparently on the return line.

3. install the oem y pipe for subtank vehicles on other line (appears to be main fuel line) 77205-6A060

4. connect everything either with proper factory lines or flexible fuel line.

4. install the oem subtank stub wiring harness 82414-60010. this will reach from the subtank fuelpump/sender and the solenoid up into the rear quarter panel. in the oem configuration it would connect there to a relay for the solenoid and a relay for the sub tank fuel pump. for reasons explained below i think neither is needed.

5. make up a harness that interrupts the main fuel tank stub harness at the connector under the driver's side passenger door sill and also connects to the sub tank sub harness and the subtank dash switch.

to do this you need

-harness wire
-male and female connectors to match the main tank harness stub plug
-connector to match the subtank harness stub plug
-connector for subtank switch
-a single pole double throw (spdt) relay (oem part suggestion welcomed)

the harness is then rigged as follows

-12v power for both pumps comes from main tank fuel pump relay at all times so the ecu does not know of the existence of the second tank. this power goes to the spdt relay which is controlled by the subtank switch.

the subtank switch operates the relay which switches between
a) main tank and subtank pump
b) fuel line solenoid position "main" or "sub"
c) fuel line sender wires from main and sub

This leaves a few unknowns

-do i need a momentary overlap of power from both fuel pumps when switching? it looks as if the factory system has an open y where the fuel lines join. as long as the power switch is instantaneous i think that the fuel pressure regulator and the volume of fuel in the fuel filter will cover for me.

-do i need a separate power source and relay for the fuel line solenoid or can i piggy back it off the fuel pump relay using just one spdt relay? the factory setup includes a small efi style relay for the subtank fuel pump (oem parts are 90987-02004/85999-12050) and a larger relay for the solenoid 28380-50020. i do not need these to control these devices. i also assume i do not need these for voltage drop since the main tank fuel pump is powered from engine bay to middle of the rear by a 14 gauge wire (i will use 12 gauge wire to run power down to the subtank stub harness). however, perhaps i should find a separate power source for the solenoid and rig two spdt relays from the subtank switch.

-in the oem setup the oz websites indicate the ecu will redirect the return line fuel on shut off to the subtank so it gets the dregs and does not become completely empty. i will lose this function. not sure how important it is to tank function. i cannot figure out how to wire something that temporarily switches to the subtank position (if necessary) then switches back.

-there is probably a function that auto-switches tank if you ignore the "e" light and empty a tank. i think i can live without this.

-i need a dash light that tells me when the subtank is live and the fuel gauge is reading the subtank sender. first step is to look and see if there is an unused light position in the instrument panel, hopefully near the fuel gauge. if not, then the easiest solution is likely drilling the sub switch for a light .
 
Is your CAPS key broke?

Like that's helpful. You can't even wire driving lights, why are you posting in a subtank thread? It's also called the shift key not CAPS key.


Semlin, I'd step back inside the guild and check with the group...this place is only for shock and trim painting threads lately. I believe when switching to your sub tank on the OEM setup you turn the vehicle off, then engage the switch and then turn the engine back on but I am not completely certain. What tank setup are you going with and which style filler neck?
 
Wiring! I can't even change the fuse.

And my Mac has a CAPS key!

Check with mobi-arc on this oem setup.
 
it's caps lock, just to be pedantic.

There was a thread on here discussing about installing/retrofitting a subtank into a USA LC/LX. Have you read that one? There's a bucket load of info in there and it discusses the difficulties etc of doing it.

If you can't find it in the seach post back and one of us should be able to dig it up for you.
cheers
peter

Sent from my iPad using IH8MUD
 
perhaps i have missed a subtank thread, but as far as i can tell none of them address wiring the tank the way i have described or the issues raised above.
 
I assume my '99 doesn't have the subtank light/symbol otherwise we wouldn't have used the diesel air filter change symbol/light...

Joe at Slee set my OEM style subtank electrics up...sorry I didn't take notes. There are a couple idiosyncrasies that couldn't be worked around: Using the OEM subtank switch via relays, etc., I can switch from the main tank to the sub tank without issue. However if I want to switch from the main tank back to the sub tank feed then I need to cycle the ignition switch, cutting power to the ECU, to get it to function. Hasn't been more than a minor annoyance.

The other feature that he couldn't figure out: How to get the sub tank OEM fuel pump, via the ECU, to pump via three speeds like the main tank pump. I think we settled on using the middle speed. The only issue I've noticed, and again its not been a compromising issue, is when feeding from the sub tank and WOT it doesn't seem to have quite the same hp as doing the same from the main tank...

The OEM fuel gauge reads from whichever tank/pump is being used/energized: Nice!

Other than that its certainly great to have a redundant fuel tank/fuel pump set-up.
 
Semlin, I'd step back inside the guild and check with the group...this place is only for shock and trim painting threads lately. I believe when switching to your sub tank on the OEM setup you turn the vehicle off, then engage the switch and then turn the engine back on but I am not completely certain. What tank setup are you going with and which style filler neck?

i have purchased a stock oz tank set up from swcruiser which comes with a stock filler neck, pipes and dash switch. i have not heard of having to shut down to switch tanks in oem configuration. i will look into it.

i can't think of a safety issue. i can see not doing it at w-o-t to avoid a momentary lean condition, but intuitively the fuel flow rate does not seem to be great enough. for the two seconds downtime the risk would be a check engine light not engine damage.
 
I assume my '99 doesn't have the subtank light/symbol otherwise we wouldn't have used the diesel air filter change symbol/light...

Joe at Slee set my OEM style subtank electrics up...sorry I didn't take notes. There are a couple idiosyncrasies that couldn't be worked around: Using the OEM subtank switch via relays, etc., I can switch from the main tank to the sub tank without issue. However if I want to switch from the main tank back to the sub tank feed then I need to cycle the ignition switch, cutting power to the ECU, to get it to function. Hasn't been more than a minor annoyance.

The other feature that he couldn't figure out: How to get the sub tank OEM fuel pump, via the ECU, to pump via three speeds like the main tank pump. I think we settled on using the middle speed. The only issue I've noticed, and again its not been an issue, is when feeding from the sub tank and WOT it doesn't seem to have quite the same hp as doing the same from the main tank...

The OEM fuel gauge reads from whichever tank/pump is being used/energized: Nice!

Other than that its certainly great to have a redundant fuel tank/fuel pump set-up.

the two tanks have identical fuel pumps. it sounds like joe tapped into the factory harness and ran into some roadblocks getting it to function as full oem. with a lexus harness i think that will be a non starter. i have gone at this in the opposite direction wiring it from scratch using oem parts and only tapping into the oem harness to get what i need (and what i know). it means i have to buy some toyota connectors and build a small harness.

what i am planning is basically to fork off the main tank harness with a relay wired completely outside the ecu system. the ecu will think it is running off the main tank the whole time. the subtank pump will receive the main tank pump signal when it is active.
 
Looking forward to this install. when I originally received 80s tanks a few years ago they sent the hard lines also to do a factory install (duplicated system) but it looked to complicated for my needs. It would be nice to see someone be able to do this on the 100 series... :cheers: to you determination.
 
Reading your summary reminds me why I DID NOT set up my subtank like an OEM unit. Set it up like a giant jerry-can and simply transfer fuel from the aux. tank to the main tank. No additional return-lne plumbing. No additional fuel filter necessary. Way less braindamage but then again, if you're a project guy, this should keep you busy.


looking for comments and feedback

i have been researching how to wire and plumb a subtank in the factory manner fror a 100. that means the dash subtank switches between tanks. there will be no transfer between tanks.

going full oem is impossible or at least impractical unless your harness is subtank ready. oem includes separate relays in the rear quarter panel for the sub fuel pumps and the fuel pump selector that are controlled by the ecu. in fact, there may be a separate fuel control computer for some models although it shows up in the 1999 uzj oz epc as "not applicable".

anyway here is my tentative plan for "near oem".

1. disconnect the main fuel line and return line from the main tank behind the tank. these parts are fully compatible with the subtank harness. you join the subtank line assembly in series into these parts.

2. install the oem fuel line-solenoid 77720-60070 for subtank vehicles. this is a "t" solenoid that switches line flow between the two tanks, apparently on the return line.

3. install the oem y pipe for subtank vehicles on other line (appears to be main fuel line) 77205-6A060

4. connect everything either with proper factory lines or flexible fuel line.

4. install the oem subtank stub wiring harness 82414-60010. this will reach from the subtank fuelpump/sender and the solenoid up into the rear quarter panel. in the oem configuration it would connect there to a relay for the solenoid and a relay for the sub tank fuel pump. for reasons explained below i think neither is needed.

5. make up a harness that interrupts the main fuel tank stub harness at the connector under the driver's side passenger door sill and also connects to the sub tank sub harness and the subtank dash switch.

to do this you need

-harness wire
-male and female connectors to match the main tank harness stub plug
-connector to match the subtank harness stub plug
-connector for subtank switch
-a single pole double throw (spdt) relay (oem part suggestion welcomed)

the harness is then rigged as follows

-12v power for both pumps comes from main tank fuel pump relay at all times so the ecu does not know of the existence of the second tank. this power goes to the spdt relay which is controlled by the subtank switch.

the subtank switch operates the relay which switches between
a) main tank and subtank pump
b) fuel line solenoid position "main" or "sub"
c) fuel line sender wires from main and sub

This leaves a few unknowns

-do i need a momentary overlap of power from both fuel pumps when switching? it looks as if the factory system has an open y where the fuel lines join. as long as the power switch is instantaneous i think that the fuel pressure regulator and the volume of fuel in the fuel filter will cover for me.

-do i need a separate power source and relay for the fuel line solenoid or can i piggy back it off the fuel pump relay using just one spdt relay? the factory setup includes a small efi style relay for the subtank fuel pump (oem parts are 90987-02004/85999-12050) and a larger relay for the solenoid 28380-50020. i do not need these to control these devices. i also assume i do not need these for voltage drop since the main tank fuel pump is powered from engine bay to middle of the rear by a 14 gauge wire (i will use 12 gauge wire to run power down to the subtank stub harness). however, perhaps i should find a separate power source for the solenoid and rig two spdt relays from the subtank switch.

-in the oem setup the oz websites indicate the ecu will redirect the return line fuel on shut off to the subtank so it gets the dregs and does not become completely empty. i will lose this function. not sure how important it is to tank function. i cannot figure out how to wire something that temporarily switches to the subtank position (if necessary) then switches back.

-there is probably a function that auto-switches tank if you ignore the "e" light and empty a tank. i think i can live without this.

-i need a dash light that tells me when the subtank is live and the fuel gauge is reading the subtank sender. first step is to look and see if there is an unused light position in the instrument panel, hopefully near the fuel gauge. if not, then the easiest solution is likely drilling the sub switch for a light .
 
the two tanks have identical fuel pumps. it sounds like joe tapped into the factory harness and ran into some roadblocks getting it to function as full oem. with a lexus harness i think that will be a non starter. i have gone at this in the opposite direction wiring it from scratch using oem parts and only tapping into the oem harness to get what i need (and what i know). it means i have to buy some toyota connectors and build a small harness.

what i am planning is basically to fork off the main tank harness with a relay wired completely outside the ecu system. the ecu will think it is running off the main tank the whole time. the subtank pump will receive the main tank pump signal when it is active.
Any updates?
 

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