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Questions On Gas Sending Units And Gauges

Discussion in 'Early CJ5 and CJ6 Tech' started by SoCalNickG, Jun 20, 2018.

  1. Jun 20, 2018
    SoCalNickG

    SoCalNickG Member 2023 Sponsor 2022 Sponsor

    Whittier, CA.
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    I have a 71 CJ5 that PO installed the under seat tank as an auxiliary. There are two fuel gauges mounted in dash to left of steering column. The Jeep has an manual valve mounted between the front seats. The valve has three positions labeled: 'Off", 'Aux.' and 'Both'.
    [​IMG]
    [​IMG] I pulled the under seat tank to clean it an check sending unit.The brass float has been etched through at the fluid level of the gas that was left in the tank (for years). Tank is dirty but seems solid. I tried to check the sending unit with a ohm meter and I saw no change in the meter when the float arm is moved. It appears to be an earlier tank and sending unit (six screws in sending unit, fuel pick up from under the tank, no vent). The inside of the tank was coated in a white chalky material that was flaking off. You can see the stuff on the sending unit. I will be cleaning the tank, repainting and reinstalling with a new sending unit.
    [​IMG]
    The dimpled hole on right is drain plug and the hole on left is a flared fitting for fuel delivery. The tubing in the next pic is the fuel line, steel line with flared fitting both ends plumbed to the valve.[​IMG] Inside of the tank the flared fitting has tubing arching up, curving over the drain hole back towards bottom of tank. Is this normal? You can see some of the chalky stuffing this picture, tank is partially cleaned inside.[​IMG]
    Can I use the sending unit wires to easily isolate which fuel tank is controlling which gauge?
    Why would you have the valve draw off both tanks?
    This last picture is just to make you rust belt guys cry a little. This is the tub under the tank and the driver's foot well. Not too bad for a 47 year old jeep. It's shiny because i had a mishap with some Lucas fuel system cleaner. Note to self--move open oil containers BEFORE lifting tank out of tub.
    .[​IMG]
     
  2. Jun 20, 2018
    ojgrsoi

    ojgrsoi Retired 2024 Sponsor 2023 Sponsor 2022 Sponsor

    Weatherford, TX
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    I think I would visually trace the wire from the underseat tank to the gauge. It's the shortest. Tag with masking tape. Or am I missing the question?

    Based on the descriptions a fuel line goes from each tank to the selector. Depending on which tank you select the fuel would go on to the fuel pump. Two sources and one output to the fuel pump. Is that right?
     
  3. Jun 20, 2018
    Focker

    Focker That's a terrible idea...What time? Staff Member

    WA
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    Do you have the need for 2 tanks?
     
  4. Jun 20, 2018
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

    Medford Mass USA
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    When I had my '73 CJ-5, I installed an under-seat tank to extend the range. It worked great to add extra fuel, but the location of the filler neck on an under-seat tank means that the Jeep always stinks of gas, and you're always smelling gas that's sloshed out of the filler neck. The filler neck is so short that gas always splashed up against the back side of the vented gas cap. A small amount of that gas makes it out of the cap.

    If you need the capacity, it's a better option than carrying gas cans ... maybe. One option is to eliminate the under-seat tank, if you don't need the capacity.

    Pretty sure the only steel under-seat tank available today is the one with the 6 screws. The lock-ring tank used later is available in plastic from MTS. Thre's also an extra-capcity rear tank from MTS, but you'll lose some rear break-over angle since the tank hangs down more than stock.
     
  5. Jun 20, 2018
    SoCalNickG

    SoCalNickG Member 2023 Sponsor 2022 Sponsor

    Whittier, CA.
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    I probably won't NEED the extra capacity of the under seat tank but I don't want to start ripping mods out of the Jeep just yet. The Jeep was running last year when I bought it and now that I have time to work on it the fuel isn't getting to the carb. I decided to start with a clean fuel tank and work my way to the carb. The underseat tank was the starting point in this project as it was easer to get at. The tank is good condition and when I finish cleaning it the tank should be fine.
    I was hoping to find an easy way to check the fuel gauge for function and to identify which tank works which gauge. With the key on the gauges light up, one gauge reads empty constantly and the other goes to 3/4 with the key on. neither tank is 3/4 full. The underseat tank is removed and the rear tank was drained and I put 2 gallons of fresh gas in it. When I scratch the sending unit wire (from under seat tank) to ground neither gauge moves.
    The manual valve does appear to have two inputs and one output. I am not sure why it is labeled "both" on the third position. The valve is mounted almost directly above the overdrive so it not real easy to see all the connections. I plan on shelving that question for now assuming I get the Jeep running on the underseat tank.
     
  6. Jun 20, 2018
    Howard Eisenhauer

    Howard Eisenhauer Administrator Staff Member

    Tantallon, Nova...
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    Nov 22, 2003
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    Yup, that's factory- leave's about 1/4" of fuel in the bottom of the tank so you'r not sucking crud.

    Probably an aircraft valve- on small planes with fuel tanks in the wings it's normal to draw from both at the same time to maintain balance.

    You can wire a single pole double throw switch to read both tanks with one guage, was discussed here recently if you want to try a search.
     
  7. Jun 20, 2018
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

    Medford Mass USA
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    The TSM covers testing the gauge and sender pretty thoroughly. If you don't have the '71 TSM, the '74 TSM at www.oljeep.com is close enough for this component.

    That the one gauge goes to 3/4 with the key on is odd. The gauges should respond with E-1/2-F when you connect a 72-23-9 ohm fixed resistor in place of the sender. They aren't designed to operate with no resistance (grounded), so use of a fixed resistor for testing is recommended.

    Also note that the fuel gauge has an internal voltage regulator for both the fuel and temperature gauge, and it does not work if the fuel gauge is not grounded. So don't test with the dash removed, or with the gauge removed, unless you ground the case of the fuel gauge.

    Replacing the internal voltage regulator of the fuel gauge with a 7805 IC is a fine upgrade.
     
  8. Jun 20, 2018
    Focker

    Focker That's a terrible idea...What time? Staff Member

    WA
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    There's info on the Technical Index page too.
     
  9. Jun 20, 2018
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

    Medford Mass USA
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  10. Jun 21, 2018
    Wenaha

    Wenaha Member

    West Coast
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    The in-tank fuel pickup that goes up and curves over is pretty standard for a Jeep.

    I had my 80 series Landcruiser set with two tanks. IMO this is WAY better than carrying gas cans - the weight is lower to the ground and it is easy to fill and access, no gas cans and spouts on the side of the road!

    Mine was set up with a dash-mounted selector and a switch next to the injector pump that switched both the fuel flow and the sender readings - so I could monitor fuel levels on the same gauge. I had two parallel fuel lines into the engine compartment with a large in-line Mallory canister filter.
     
    Norcal69 likes this.
  11. Jun 21, 2018
    Norcal69

    Norcal69 Out of the box thinker 2024 Sponsor 2023 Sponsor 2022 Sponsor

    Northern California
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    I started off with an under seat tank. 10 Gallons didn't cut it. Jerry cans either in the jeep with you, or mounted on the rear are a PITA if you ask me. I installed a YJ tank in the rear, 15 gallons just like a cj tank but with a drivers side fill. This is the slickest setup if you ask me. 25 gallons of fuel on board tucked up and out of the way and 5 more gals onboard than if you had a underseat tank and two 5 gal jerry cans.. I have a brass 3 way valve from Napa mounted in between the seats. Both tanks run to the valve, then one line to the front . I have a 3 position switch wired to the fuel gauge to tell the level in either tank. I don't rely on the gas gauge too much anymore as it is not that accurate with either tank. On long trips I top off with fuel at the last available gas before venturing out of civilization. When the 15 gal rear tank starts to sputter out, If I haven't already gotten gas at some point, then I know that I have 10 gals to find a station.
    Addition:
    I would not run both tanks together, but isolate them so you can pull from one or the other. I have gotten used to the feel of either tank starting to run out and can easily go from one to the other on the road without loosing much speed.
     
    Last edited: Jun 21, 2018
    ojgrsoi likes this.
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