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77 3\4 fuel sending unit question

Discussion in 'Chevy Trucks' started by jrd426, Aug 19, 2006.

  1. jrd426

    jrd426 Junior Member
    from canada
    Messages: 5

    hi there, can someone tell me what the voltage should be at the fuel tank sending unit? The truck is originally a 77 but i've changed the cab to a 81. I tested the fuel sending unit that was in the tank as a 77 and its defective and will replace it. Both the original 77 cab and the now 81 cab has the dual tank switch. I want to have just the one tank on the truck but when tracing the wires from the switch to the fuse block there is a relay of some sort there. i've been told that this relay in the newer cab drops the voltage being sent to the tanks sending unit. The voltage at the sending unit right now is 12v and am thinking that the relay under the dash near the fuse block is cooked..I also dont want to cook the new sending unit i'm going to buy feeding it with 12v if its supposed to be low voltage. The fuel gage right now sits way past full at around the 3 o'clock position. I'd like to bypass the dual tank switch all together if i can easily as again i'll only ever have the one tank on this truck..sorry to be so long..anyone have any ideas on the voltage, relay, etc
    thanks in advance, Jeff
  2. jrd426

    jrd426 Junior Member
    from canada
    Messages: 5

    oops, made a typo..the newer cab now on the truck is a 80 not 81...anyone???
  3. 75gmck25

    75gmck25 Senior Member
    Messages: 119

    I have a '75 truck, but the fuel switch and extra tank came from an '81. I'll assume your main tank is on the passenger side and the aux is on the driver's side. In later models (not sure which year) the driver's side tank became the main tank.

    There is no relay under the dash in the stock '81 dual tank setup.
    - There is a 12 volt, ignition on, connection from the fuse block to the tank switch. One half of the dual pole, dual throw tank switch applies 12 volts to the solenoid/switch under the truck when the dash switch is in the aux position (if the aux tank is on the driver's side). If you don't want to use dual tanks you don't even need this 12 volt connection.

    - The other half of the dual pole switch switches the dash gauge from one tank to another.

    The stock gauge wiring is overdone, but probably made it easier to add the aux tank at the factory. The wire to each tank sending unit runs from the dash switch, through the firewall, and along the frame to each sending unit. The wire from the gauge also goes through the firewall and along the frame rail, then hits a junction near the right side tank, and runs back along the frame rail and back through the firewall to the dash switch. In other words, it loops back on itself.

    If you disconnect the junctions inside the frame rail near the right side tank you can access the wire to the right side tank sender and the wire that at goes back to the gauge. If you connect them together you have bypassed the dash switch.

    The sending unit is a 0-90 ohm GM sending unit. They are very sensitive to good grounding. The ground for the tank runs from the sending unit through the frame rail and grounds inside the frame.

  4. jrd426

    jrd426 Junior Member
    from canada
    Messages: 5

    thanks Bruce..this is very helpful info. ideally i want to bypass the switch on the dash altogether with. Just one set of sending unit wires from the tank direct to the gauge. The solnoid (or tank switching unit) that is located in the frame on the passenger side looks like an original and pretty rough looking..I wouoldnt be surprised if it doesnt work anyways. I called around to get a price on a sending unit and told a few places that the frame wiring and tank is 77 and the cab is now a 80 (not 81 i goofed) and they tell me there is a different part number for a 77 sending unit and 80 sending unit..I'm thinking of just going with one listed for a 77 as maybe the only difference is tank size but not positive. Any idea what voltage i might or should see on the connector that plugs onto the sending unit using a volt meter? Just trying to figure out how to make sure my wiring for the sending unit is ok. When i buy my $100 sending unit (yup 100 bucks here) i have a sneaky feeling that my fuel gage in the cluster will not move off of way past full (3 o'clock position)..I do have the original instrument cluster from the 77 i kept which i tried already before determining the sending unit was defective and it performed pretty much the same..no movement at all with ignition on\off..
    thanks, Jeff
  5. 75gmck25

    75gmck25 Senior Member
    Messages: 119

    I would suggest doing a careful check of the wiring before you buy a $100 sending unit, unless the truck has been idle for a long time with no gas in the tank. The biggest problem with used sending units is that they have been exposed to air in an empty tank and the variable resistor has rust on it. The gauge sender is just a metal bar that sweeps up and down along a variable resistor as the tank float goes up and down. If the sweep portion has rust on it, the connection is poor and you don't get a reading from the sending unit.

    GM has several parts numbers for the sending units, but most of the differences are with the number of fuel ports, not the electrical characteristics. Some very early sending units may have only one port (maybe?), later units added a gas return line from the fuel pump, and most later ones also have a vapor return line that connects to the canister in engine compartment. If you look up the parts numbers it will usually show 1, 2, and 3 port sending units. All the saddle tanks (either 16 or 20 gallon) and sending units mount the same on all carbureted trucks of this vintage.

    I need to do some troubleshooting of my own gauge and tank switch, so it would be a good excuse to take a look at everything. I was messing around with my underdash wiring to replace my A/C controls, and now my fuel gauge reads completely full for both tanks. I assume I either knocked a tank switch wire loose or grounded a wire that is not supposed to be grounded.

    I've never checked the voltage at the sending unit, but assumed it was just 12 volts. I've always just checked the resistance. I will try to check the voltages on my truck, but probably can't get to it until over the weekend.

  6. jrd426

    jrd426 Junior Member
    from canada
    Messages: 5

    hi Bruce, thanks for the info..I'm going to do some wire tracing next. If you do check the voltage on the sending unit plug i would be interested in what the reading is you get..have a good weekend.
    ty, Jeff
  7. 75gmck25

    75gmck25 Senior Member
    Messages: 119

    I never got back to you after I checked the voltage. The line from the gauge to the sending unit has about 8.5 volts on it. I assume its a 12 volt line, but the resistance of the gauge drops it to a lower voltage.

  8. sweetk30

    sweetk30 PlowSite.com Addict
    Messages: 1,588

    73-87 trucks and 91 blazer/burban/crewcabs were all same for gauge setups.

    the switcher only flips back and forth the wires to the gauge and power to the line valve.

    vary comon problem is the sender ground to frame is bad. hardly ever see bad main wire. unless you have switched or removed the tank and for some reason the little tab inside the lug is bend out of shape or the tab broke and is still on the old sender.

    if we cant help ya over hear try this site. www.ck5.com best chevy site out there. in the garage section this has been covered before lots of times. do a quick search for this topic. good luck. :salute:
  9. jrd426

    jrd426 Junior Member
    from canada
    Messages: 5

    thanks for the great info and help guys..I really appreciate it!

  10. fast68

    fast68 Junior Member
    Messages: 17

    in 77 the pass side tank was default

    this was through 79 then in 80 it went ot driver side default

    i took all my pass side tank and parts out of my 77 it is factory dual tank

    if you stil need any specific info on wires and colors and such then lemme know i removed all this stuff from mine since my pass side tank was rusted out

    the hot side is always hot full 12V

    the ground side is the variable and is what the guage runs off of, from the tanks directly

    the dual tank switch cuts off one tank and lets the other through ot the guage- whichever you select of the two possible positions

    its real simple basic

    0 to 90 ohms resistance in the ground side of the sender units for the guage wire from them each

    the green wire is the dual tank switch selector wire

    default is pass side,

    blue is one tank and other is pink

    pink is driver i believe and blue is pass i believe, but i would have to look again

    what i did when i rmeoved my pass side tank permanently is i took the wire from the driver side tank to the tank switch in the dash and i juntioned it to the pink wire to my guage cluster plug behind my dash, the pink one

    pink is the one that goes to the guage from the switch where it gets the 0 to 90 ohms range from whichever tank you have the switch selected to

    understand ?


    77 was still first design of dual tank system

    by 81 it was hcanged to the second design and was a bit differently wired up

    i jut stripped an 85 k20 with dual tanks of all the wiring and switch valve assembly too

    so i have them around here somewhere too

    they changed them alot

    lemme know if you need more info