fuel guage insanity

[robert_63]

just a quick question guys, i replaced fuel sending unit and been checkin wires, when i disconnect the wire guage reads full, and when i ground the wire(from the guage) to the frame it goes to empty, is this the correct reading? or should it read empty when unplugged and full when grounded? ive searched the fuel guage threads and just cant seem to figure it out. any help would be greatly appreciated.

[old Rusty C10]

when my ground was not making contact properly the tank would read way past full once the ground was tight it would read whatever the level of the tank was

[robert_63]

It stays around a 1/4 or little more its new sendin unit and have the ground from the top of unit attached to the frame
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[joe231]

Grounded it should read empty
Open it should read past full
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[hatzie]

The gauge has a calibration resistor on studs at the back of the small can housing the movement coils. Sounds like you have a problem with yours.
More here...
http://67-72chevytrucks.com/vboard/s...02#post5241602

[joe231]

Originally Posted by hatzie

The gauge has a calibration resistor on studs at the back of the small can housing the movement coils. Sounds like you have a problem with yours.
More here...
http://67-72chevytrucks.com/vboard/s...02#post5241602

How did you arrive at that conclusion?
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[hatzie]

Originally Posted by joe231

How did you arrive at that conclusion?
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The gauge reads lower than it should at full sweep.
Roberts' gauge reads F when the sender is unplugged. That's NOT OK. The needle should sit way past full (roughly 3:00) with the key on and the sender unplugged.

The 73-91 sender is a wire wound 0-90 ohm potentiometer with some mild swing damping. It can be tested with a multimeter set to the 100ohm scale. Attach one lead to the sender pin and the other to the round sender top.

The meter should read near 0 ohms with the float hanging.
The meter should read close to 90 ohms when the float set for a full tank.

I'd test the original GM sender if it's not pitched out yet.

Attaching a 90-100ohm resistor between the frame and sender wire should put the fuel gauge pointer near the F. The OP already proved that grounding the sender wire moves the gauge to E.

[joe231]

Originally Posted by hatzie

The gauge reads lower than it should at full sweep.
Roberts' gauge reads F when the sender is unplugged. That's NOT OK. The needle should sit way past full (roughly 3:00) with the key on and the sender unplugged.

Oh
Gotcha

I missed that the first time.

Thanks
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[robert_63]

One thing i have found on here is guys posts sayin when the wire is off or "open" the guage goes past full some sayin it goes to like the 3 0'clock position mine just goes to maybe a hair past the full mark. i have checkd the wire all way down the frame and to the crossover solenoid and when unplugging it the gauge does the same to just a hair past ful or e when grounded out. beginning to wonder about the oreilly's sendin unit.

[tucsonjwt]

Are you saying that your fuel gauge does not read correctly after replacing the sending unit?

[Palf70Step]

My gauge was doing the same thing yours is. I replaced the gauge and everything is good now.

[robert_63]

yeah when hooked up it goes to 1/4 thanks 70step i will try get me a guage and see

[Palf70Step]

I got lucky and found a decent cluster in a pick and pull. Several of my gauges got replaced with the ones from that cluster.

[1425956]

Mine jumped around a lot too so I replaced the sending unit. It doesn't drop around much now but it still reads way past full and stays there for about 100 miles; then, once it starts to move, it drops fairly quickly.

[RPOZ11]

Good Post.

I have had trouble with my 73 C/20 ever since Ive owned it.
It has never read correctly the amount of gas in the passenger's side GM tank.

Would you guys suggest starting there and working forward, or at the Gas Gauge in the dash back?

Can a bad printed circuit also cause a bad reading as well ???

How would the stock gas gauge read dual tanks; being a 73 and not a factory dual tank option...but an added driver's side GM tank?

[motornut]

my 78 gauge jumped till i swapped over to tach cluster
I added grounds behind the cluster where they all go down to one
now no probs
bout a foot from the cluster plug

[E.TexasTwister]

I just got this truck Sunday. (72 c10) I drove it about 400 miles and every time I stop for gas it would show way past full. Like some said above around the three o'clock area.
It would run about a 100 miles then drop pretty rapidly.
What fix did anyone find out on this?
Thanks

[motornut]

Originally Posted by E.TexasTwister

I just got this truck Sunday. (72 c10) I drove it about 400 miles and every time I stop for gas it would show way past full. Like some said above around the three o'clock area.
It would run about a 100 miles then drop pretty rapidly.
What fix did anyone find out on this?
Thanks

I'd check tank ground,sender wires to start....
if the sender wire is cut it's usually over full @ 3,
shorted out would be stuck E.

[hatzie]

Originally Posted by E.TexasTwister

I just got this truck Sunday. (72 c10) I drove it about 400 miles and every time I stop for gas it would show way past full. Like some said above around the three o'clock area.
It would run about a 100 miles then drop pretty rapidly.
What fix did anyone find out on this?
Thanks

Since it moves with the fuel level... That sounds like a gauge range problem not an open sender wire.
More resistance than there should be in the sender wire would cause this. A bad gauge or bias resistor could do it as well...

When she gets to empty does the gauge read E or is it reading 1/4 or more?
With the tank filled what is the resistance measured from the TAN sender wire on the cluster plug to a good ground?

This might be better answered on the 67-72 section of the boards.