Gas Gauge issue

[MY1978]

I had my original sending unit replaced about 3 years ago because I had needle bounce when driving.  Everything was fine, at least I thought, until today.  I decided to drive my truck around today running as much of the gas out currently in the tank out without running out.  I started at 1/2 tank and when I filled it after running around and doing errands ( a total of about 50 miles) the needle was registering at just about 1/8th tank.  Went to "fill" my tank and I only had to put 8 1/2 gallons in.  I know this is a 15 gallon tank.   So if the guage registered 1/8th above empty, shouldn't I have had to put at least 12-13 gallons in my tank before it registered full?  Because of how low the gas fill inlet is, I always get some splash back.  I was manually holding the trigger and it splashed back.  I waited about 15 seconds until things settled and then started pumping again at low volume and again got splash back.  I do not!!!!  want to take this someplace again and have the tank dropped again.  This would explain how I can drive 20 miles and burn over 1/4 tank gas.  The gauge is not registering.  Is there anyway to calibrate the gas gauge? 

[bd]

It's not realistic to calibrate the gas gauge.  What did the fuel gauge indicate after the 8 1/2 gallon "fill-up?"

[MY1978]

It moved up immediately to 3/4 full and then moved to "F"

[bd]

Disconnect the fuel gauge wire at the tank sender.  It should simply unplug.  With the ignition switched ON the fuel gauge needle should rotate to about the 3:00 o'clock position and stay there.  Does it?  Now, temporarily ground the fuel gauge wire that was unplugged from the tank.  Make sure that you ground the wire to shiny bare metal.  With the ignition switched ON the fuel gauge needle should rotate and point directly at empty.  Does it?

[MY1978]

Disconnect the fuel gauge wire at the tank sender.  It should simply unplug.  With the ignition switched ON the fuel gauge needle should rotate to about the 3:00 o'clock position and stay there.  Does it?  Now, temporarily ground the fuel gauge wire that was unplugged from the tank.  Make sure that you ground the wire to shiny bare metal.  With the ignition switched ON the fuel gauge needle should rotate and point directly at empty.  Does it?

Disconnecting the fuel gauge wire at the sender is going to be tricky.  My truck is a short bed and the fuel tank is tucked up under the cab and front part of the bed.  I cannot even see the sender.  Any suggestions to make this easier?  I don't have a lift so I have to shimmy under the truck and I cannot sit up under it.

[bd]

The fuel gauge wire connecting to the fuel tank sender is 18-gauge tan.  If you're fortunate, that tan wire runs down the inside of the right (passenger side) frame rail to a single-wire harness plug located somewhere in the area between the front of the transmission and the transmission rear cross member.  If you can find that plug, disconnect it and perform your tests there.  The wire you're working with runs forward.

[JohnnyPopper]

Most wiring under the carriage is covered with (fill in the blank).

Use brake cleaner to dissolve gunk to ID the color.

Then use the same to blow off the gunk and debris off the connector bd speaks of before disconnecting and testing.

Use the same to find a shiny metal surface to conduct your test, although you may have to burnish a location in addition to cleaning.

Curious to see how this turns out, I too have a bouncing needle on both tanks.

[bd]

A little sidebar....  Fuel gauge needles bouncing in sync with bumps, turns, acceleration, and braking (in other words, fuel sloshing in the tank) are often a result of dry needle bearings.  Fuel gauge needles are mechanically dampened from the factory using viscous fluorocarbon gel, or dampening fluid, that fills the narrow gaps between the needle axles and their support bushings.  The dampening fluid gradually dissipates over many years leaving the needles instantly responsive to signal noise as the sender floats bounce around in the tanks.  Various solutions are to install a new gauge, have the gauge professionally refurbished, or attempt to recharge the fluid yourself using a syringe and 30,000+ centistoke (cSt) silicone or fluorocarbon gel dampening fluid.

[VileZambonie]

If getting under the truck is too difficult for you, pop the fuel gauge out and using your meter, measure the resistance at the gauge post. Then compare that to the reading at the cluster connector.

[MY1978]

The fuel gauge wire connecting to the fuel tank sender is 18-gauge tan.  If you're fortunate, that tan wire runs down the inside of the right (passenger side) frame rail to a single-wire harness plug located somewhere in the area between the front of the transmission and the transmission rear cross member.  If you can find that plug, disconnect it and perform your tests there.  The wire you're working with runs forward.

Thanks, will try this today and report back. 

[MY1978]

A little sidebar....  Fuel gauge needles bouncing in sync with bumps, turns, acceleration, and braking (in other words, fuel sloshing in the tank) are often a result of dry needle bearings.  Fuel gauge needles are mechanically dampened from the factory using viscous fluorocarbon gel, or dampening fluid, that fills the narrow gaps between the needle axles and their support bushings.  The dampening fluid gradually dissipates over many years leaving the needles instantly responsive to signal noise as the sender floats bounce around in the tanks.  Various solutions are to install a new gauge, have the gauge professionally refurbished, or attempt to recharge the fluid yourself using a syringe and 30,000+ centistoke (cSt) silicone or fluorocarbon gel dampening fluid.

FYI - I do not have bouncing - solved that with the sender being replaced.  Now I have it showing I have less fuel in the tank than I really have - by 4-5 gallons.  Might seem like a dumb issue, but just like to know how much is TRULY there.  I do not have a trip meter to set to remind me of how many miles I have driven on a tank of gas.  I don't even know if they had them back in the day - at least on trucks.

[MY1978]

Most wiring under the carriage is covered with (fill in the blank).
Use brake cleaner to dissolve gunk to ID the color.

Then use the same to blow off the gunk and debris off the connector bd speaks of before disconnecting and testing.

Use the same to find a shiny metal surface to conduct your test, although you may have to burnish a location in addition to cleaning.

Curious to see how this turns out, I too have a bouncing needle on both tanks.

I did clean and undercoat my truck myself which was an ordeal in itself with not lift.  See my pic of how I accomplished this!!!.  Couldn't use a spray gun because of the angle I had to shoot at.  Days spent power washing and scraping old undercoating off (as much as I could), using rust convertor in a small pressure sprayer, using Eastwood rattle can rust encapsulator and then finally rattle can undercoating.  Days!!!!!!!!! of being up under this beast.

Anyway, I think, not positive till I get out there, that I tried to be smart and cover, or protect, any wires from the undercoating coating.  I am hoping I did, but no guarantees.  So will have to go on a hunt.

Yep, brake fluid cleans everything off, even stuff you don't want it to (don't ask me how I know      )  Makes sense about cleaning the connector, don't want any junk in the connector.  I have electronic cleaner spray I will use before reconnecting.  Also thinking of dielectric grease for a better connection.

Only way I will know is when I go to fill up.  So we shall see how this goes.

[MY1978]

If getting under the truck is too difficult for you, pop the fuel gauge out and using your meter, measure the resistance at the gauge post. Then compare that to the reading at the cluster connector.

The idea of disassembling the dash gives me chills and stomach flips  it is a PIA getting the intrument bezel off over the steering wheel and shifter as well as having to pull the headlamp and radio knobs not to mention the screws that are in 40+ year old plastic instrument cluster housing.    

[JohnnyPopper]

I would take the instrument bezel over the gravel 

Pull straight and fast on the radio knobs.

The headlight knob can be worked through the bezel with some patience.

I wonder if dropping the column to the end of the threads would help?

My first time ended up breaking the gear indicator landing on the plastic column ring. 

[MY1978]

Thanks, I don't mind the gravel.  Hint......Use a yoga mat makes great cushion over gravel.  I have 2 that I used when I skooting around under truck while doing the undercoating ordeal.  I will start low and work up. 

Although, I was thinking maybe not such a bad thing to have it register less than in tank.  By my calculations, I still had 5 gallons to go so what should be able to carry me to a local station to feed the beast. 

I need to pull the lower column bezel cover as my gear indicator is not indicating and I know why.  Just need to get that fixed.