Gas gauge solution?

[Isaiah92260]

So my gas gauge isn’t working, when the key is off the needle goes down to around 3/4 a tank, but once I turn the key, it shoots way past full where I can’t see the needle. I’m assuming the gauge isn’t working since it is showing that it’s moving when it gets a signal, messed around with the wires and seems like they are working since the needle is getting a signa and when I ran I test light the power wire at the gas tank it lights up, I even replaced the ground wire at the gas tank and still nothingl. I’m thinking the wiring could be grounding out somewhere? Or maybe the float inside the tank has gone bad?

[ray_mcavoy]

The gas gauge pegging beyond full is usually due to:

(A) A bad ground at the sending unit. I see you've already addressed this by replacing the ground wire at the tank.

(B) A break in the wiring or bad connection somewhere between the gauge & sending unit. A quick way to check the integrity of the wire is to temporarily ground out the end at the sending unit. If that makes the gauge read empty (with the key on) then the gauge & wiring are okay. But if it still pegs beyond full, you'll need to check the sending unit wire and it's connections.

(C) A bad sending unit. You can check the sending unit with a multimeter set on the resistance scale. Temporarily disconnect the wire from the sending unit and place one of the meter's probes on the sending unit terminal. Connect the meter's other probe to the body of the sending unit. You should get a reading somewhere between 0 and 30Ω depending on how much gas is in the tank. If it reads open circuit the sending unit is bad.


Note that a sunken float would cause the gauge to stay on or near empty (just the opposite of what you're seeing). Similarly, a short to ground in the sending unit wiring would also cause the gauge to read empty instead of peg beyond full.

[Isaiah92260]

Originally Posted by ray_mcavoy

The gas gauge pegging beyond full is usually due to:

(A) A bad ground at the sending unit. I see you've already addressed this by replacing the ground wire at the tank.

(B) A break in the wiring or bad connection somewhere between the gauge & sending unit. A quick way to check the integrity of the wire is to temporarily ground out the end at the sending unit. If that makes the gauge read empty (with the key on) then the gauge & wiring are okay. But if it still pegs beyond full, you'll need to check the sending unit wire and it's connections.

(C) A bad sending unit. You can check the sending unit with a multimeter set on the resistance scale. Temporarily disconnect the wire from the sending unit and place one of the meter's probes on the sending unit terminal. Connect the meter's other probe to the body of the sending unit. You should get a reading somewhere between 0 and 30Ω depending on how much gas is in the tank. If it reads open circuit the sending unit is bad.


Note that a sunken float would cause the gauge to stay on or near empty (just the opposite of what you're seeing). Similarly, a short to ground in the sending unit wiring would also cause the gauge to read empty instead of peg beyond full.

That’s great info thank you! I’m going to do these 3 procedures later tonight

[mongocanfly]

And you shouldn't have any power at the the gas tank...not sure how you got the test light to light up..the sending unit sends a grounded signal to the gauge...
Like Ray stated...wiring or sender....

[Isaiah92260]

Originally Posted by mongocanfly

And you shouldn't have any power at the the gas tank...not sure how you got the test light to light up..the sending unit sends a grounded signal to the gauge...
Like Ray stated...wiring or sender....

Thank you boss! Yes I got the test lite to work off of the lead wire going to the float with the key turned on

[Isaiah92260]

So I checked the terminals at the gauge and the right terminal lights up strong.
But the terminal on the left is very dim.

Is it just that I have a bad gas gauge ?

[Isaiah92260]

The terminal on the left that is showing dim, is the brown wire which runs to the float

[ray_mcavoy]

Originally Posted by Isaiah92260

So I checked the terminals at the gauge and the right terminal lights up strong.
But the terminal on the left is very dim.

Is it just that I have a bad gas gauge ?

No, that doesn't indicate a bad gauge at all.

The terminal on the right (pink wire) is the +12V feed to the gauge. So a nice bright test light there is perfectly normal.

The terminal on the left (brown wire) goes to the sending unit. The coils inside the gauge have a fair amount of resistance that limits the amount of current that can flow to this terminal and results in the very dim test light. So that doesn't indicate a problem with the gauge.

[Isaiah92260]

Originally Posted by ray_mcavoy

No, that doesn't indicate a bad gauge at all.

The terminal on the right (pink wire) is the +12V feed to the gauge. So a nice bright test light there is perfectly normal.

The terminal on the left (brown wire) goes to the sending unit. The coils inside the gauge have a fair amount of resistance that limits the amount of current that can flow to this terminal and results in the very dim test light. So that doesn't indicate a problem with the gauge.

Ohhh okay I didn’t know that.
I did the the procedures you said to do and I wasn’t getting anything from the sending unit... I think getting a new one will fix the problem

[timbuk]

could use a solution with my 65 gmc gas guage truvk is on a 85 frame and has dual tank setup from the 85 and is hooked up to factory 65 guage when key is turned on it moves off the full mark slightly that's it im thinking wrong ohms from guage to sender any thoughts? haven't been here for years but still got all my stuff!

[ray_mcavoy]

Isaiah92260, Yes, if you're not getting any resistance reading (open circuit) on the sending unit, that's likely your problem.


timbuk, Yes you do have an ohms mis-match between the 65 gauge and 85 sending units. The 65 gauge is designed to work with a 0 - 30Ω unit while the 85's are 0 - 90Ω units.

Although, if you let the fuel level get down below a third of a tank, it will put the 0 - 90Ω sending unit in a part of it's range where it should allow the 0 - 30Ω gauge to start dropping below a full reading. And since 0Ω corresponds to empty with both, letting the tank run empty (or close to it) should allow the gauge to read empty (or close). If not, you might also be dealing with some other issues.

[timbuk]

thanks ray I thought so im not sure if the guage shoots down to empty when I ran outa gas I never noticed! hmm how to rectify this one!

[ray_mcavoy]

Originally Posted by timbuk

thanks ray I thought so im not sure if the guage shoots down to empty when I ran outa gas I never noticed! hmm how to rectify this one!

You're welcome. This thread is about going the other way around (making a 90Ω gauge work in one of these trucks that originally came with a 30Ω sending unit) but some of the ideas discussed about modifying the sending units might still work in your case. --> http://www.67-72chevytrucks.com/vboa...d.php?t=779678

[timbuk]

http://www.tanksinc.com/index.cfm/pa...cat/cat159.htm found this bit if info hope it helps someone I think the solution for me is to get the correct ohm aftermarket guage