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1974 620 "new" fuel tank installation considerations: Hose Selection & Venting

Cardinal Grammeter

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Got a beautiful tank from Bud's from OK.




I don't have the expansion tank, nor the canister (although I'd like to get one to go with the plumbing that is still intact.)


I'm thinking:


*) Block or connect together the 2 top tubes that go to the exp tank

*) Connect the remaining (aft most) to the line at the top of the filler neck.  EDIT:  put a "T" in this hose near the top and let this be the vent - maybe some hose hanging from it

*) Use a vented cap



NOTE:  My current tank has some of those top fittings unconnected so maybe just hook up the way it is now since it doesn't seem to be a problem.  The filler neck tube is unconnected too.




Concerned about the PITA supply line side fitting up against the frame.  If that hose goes bad, have to remove tank to fix.  


In concerned about longevity and alcohol.  Maybe use Gates fuel injection hose?  There are Marine and Aviation hoses that are better.  The PTFE lined hose is great but should use special fittings - I don't want to mod the end-bulged tubes coming out of the tank. 


EDIT:  Just talked to tuner friend and NAPA sells an E85 "rubber" fuel injection hose.  I'll use that.


These are the clamps I want to use but might use the screw type considering how access is (if any):  


Briggs & Stratton Hose Clamp for Models 95162, 93053 and 805187


SPLASH GUARD:  I might make something to keep spray off the forward 2 bolts on RHS that rot out.


DEATH:  I laugh!  I'm doing this job like I want truck to last 40 years - I'm 67!  hahahaaaaa


Edited by Cardinal Grammeter
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Got it out only broke 2 screws, torched out the stufs.  


TANK WAS SCRAP  AND it was NOT THE ORIGINAL TANK!  So I'm installing at least the 3rd tank...


There was mud piled up on the sending uni and the side flanges where the 2 bolts are.


I'm thinking of making a splash shield that will be held in place by the 2 bolts.  I have a 8.5' siding brake and am thinking how to design it...



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UPDATE;  Dust shields not possible since brackets do same thing.


Old tank failed because rust starts with water getting between flanges due to capillary action.


Tank was perfect in that regard, but I should have ran a fine weld sealing those edges.  


I have to laugh!  Desert junk yards ship tanks all the time since a few days in the sun and they are totally fumeless.  It takes forever for a tank to "fume out" in non-desert areas - so they do't ship.


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I found that road dirt accumulates on top of the flange and is damp all the time. The tank halves are welded between rollers for a continuous seam. Mine perforated on the top half just behind the welded seam. I tinned the tank and cut a patch out of a radiator brass.  Worked fine and I made a habit of hosing the dirt off this area for the rest of the time I had my tank.

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I cannot remember and do not feel like searching.  Does the 620 have an access door in the bed  like the 521?  I am convince thats the main spot the 521 tanks get boned in.  All that water running and falling on top of the sender setting in that bowl.  When my tank goes back in my truck, I am going to redo that door somehow.

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NO DOOR inthe 620 bed.


And OF COURSE, being as meticulous as possible, new tanks is in, various maintenance done, exhaust repair done, more rust proofing done, and....


The   @#$*(&#*(@   fuel gauge is dead on EMPTY!


I checked resistances of my old gauge in the new tank and got readings.


I'm pretty sure I have the two wires on the right terminals.


I was thinking if I put the SIGNAL wire to the GROUND terminal, I would get a FULL.  However there is so much paint on the tank, it is entirely possible it is not grounded.




Are the wires on the right terminals?  (YELLOW is SIGNAL and that is the post with the black insulator)

Is there voltage on the SIGNAL wire?

Is the GROUND wire grounded?

Do I get FULL if those two wires connected?



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The flow is battery to ignition to fuse box to gauge cluster voltage regulator through the gauge to the tank through the sender and to ground. A full tank has 10 ohms resistance or less. An empty tank has 80 ohms or more. If you ground the Yellow wire the gauge should read FULL. Any ground will do to prove that everything to the tank is working. When plugged in, if it doesn't work then the black wire is not grounded or the sender in the tank isn't right. The black wire also grounds the tail lights so if they are not working then the ground isn't. If tail lights are working then the ground for them is working at least. 

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Thanks for the tip about the tail lights.


Are you saying the black wire to the sending unit is the ground for the tail lights?  So the tail light ground is through the fuel tank sending unit?  OR the black wire at the sender is also the tail light ground and the proper chassis ground is somewhere else (not the sending unit.)  The latter makes more sense since if the tank was to be the ground, there would only be a single wire to the sensor.  So the assumption is the tank is not grounded and the sensor ground is carried to the proper chassis ground.


If that's the case, due to the painting and greasing of the mounting bolts, the tank itself may not be grounded to the chassis.


JUST CHECKED:  Have tail lights so the tank is grounded as is the fuel sensor ground terminal.


Since I have TEMP, that means I should have regulated cluster voltage at the yellow.


Tomorrow I'll touch them together.


I do not see how I could have damaged that sending unit.  Perhaps when I etched the exposed top to get rid of the rust I somehow ruined the connection to the fine ni-chrome wire on the inside...

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Looks like it is corrosion or a mechanical connection problem:


Drove to lunch, no gas gauge.


Drove to dinner GAS GAUGE WORKED!


Really perplexing because yesterday, I seriously wigged the connections on the T-posts on the sender.  So what other thing could have a dodgy connection?

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I use a Krud Kutter metal etch to clean the sender contacts. Be careful, it will remove plating, but if you spray it on and let it soak for a couple minutes then rinse it off, it should do the trick.


Likely the contacts aren't actually dirty though. If possible, remove the cover and look at the swiper. Maybe it came loose or just needs to be bent a bit towards the coil. I do this on older cars/trucks all the time.


BTW - EFI hose is very stiff and can be hard to get on the nipples. There is hose out there that stands up to the new gas that isn't so stiff. Do a google search for "E85 fuel hose SAE ratings" and you'll find a couple good articles explaining hose tech.

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I sort of kept the acid off the T terminals which looked like brass.


Bending the arm in the rheostat is an old "scratchy" volume control trick.  I inspected it and it looked less worn than the sender out of the new tank from OK - so I didn't mess with it.


The Gates hi pressure E85 hose is not that stiff, was easy to put on.  More room between tank and frame than I expected.


So far, the fuel gauge is working fine.



On REDUNDANT GROUNDING:  The black wire connects the Sender and Tail lights an is rounded somewhere on the chassis.  However, there is a ground path to chassis though the ground terminal of the Sender.  The path is Sender chassis > Lock Ring > Tank > Tank Brackets > Frame.  If that proper chassis ground fails, ALL the ground current will go though the Sender ground terminal.


SAME PROBLEM occurs with the Voltage Regulator:  If you lose the proper chassis ground, ALL the current in that black wire goes through the small ground wire to the VR > Inner Fender > Chassis thus melting the insulation off and even oxidizing the copper until no metal is left - it breaks then.  


CHASSIS GROUND LUGS:  It would be nice to know where these are.

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A Toyota Land Cruiser I built for a customer last year had a similar problem -  the gauge would work intermittently. Drove me nuts, because, like you, I tested every part of the system at least twice including the gauge. I went back to the tank for a third time and when I wiggled wires around, I noticed the gauge jump. The wires looked fine, but as it turns out, the wire to the sender was broken inside the insulation. I tested the continuity by poking through the insulation at varying points, moving closer and closer to the sending unit, until I found the break.


The back of the vehicle is hot and messy, with exhaust heat and grease flying around, so it makes sense that the break in the wire was near the tank.


Have you checked the wire continuity? If you have, try again while wiggling the wires.


Found this discussion online -


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I looked in my '73 and '78 FSM and can't find any ground connections on the rear harness only the single black wire that works to ground everything. The frame and the box are poor returns as they are rubber mounted (shocks, leaf springs, engine and transmission mounts etc.) The '73-'77  tanks are mounted on the box, '78-'89 are bolted to the frames.

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Finally.... FIXED!


Was going to put the signal wire on the sender ground terminal just to demonstrate a "Full"  reading gauge.


As I was sliding the signal wire of the "nail head" terminal, it immediately got stiff and "scratchy" when rotated.  


Oh Snap!  The terminal, when slid all the way over the nail head was actually loose.  Sliding it not even 1/8" moved the terminal to where it clamped strongly.


I knew it was immediately solved.


And it was.


Took 30 seconds.



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