Age old fuel sending unit question...

[msbaugh]

I've tried and tried and I feel like an idiot!
My fuel gauge (when the tank is full) pegs out at 1/2 way on the gauge. I'm trying to correct that..

Upon grounding the sending unit wire directly to - battery the gauge will go to full where it's supposed to, so the gauge is fine.

I pulled the sender unit and verified resistances in all readings. It's a linear relationship between float position and the resistance on the wire.

I've got around 78 ohms on empty
Around 31-35 ohms halfway
Around 7-8 ohms on full tank

At the full position I still only have 1/2 the gauge travel that I need. Come on electricians how can I bend/add resistors to this thing so that it reads correctly?

It seems to me like this unit needs to have half the resistance it does at full in order to peg out. I'm i thinking about this wrong?

[edl94]

If it was the other way you could add a resistor to correct it, but there is no way to remove ohms. Quick question though is there resistance in the wire between the gauge and the tank I would think there should be near 0 resistance. Ground the tank connector and check the gauge. If it reads full then the wiring is ok, if not time to trace the wire and look for bad connections corrosion etc... It sounds like the sendor is good.

[Strawdawg]

bend the arm

[edl94]

I may have been to quick to respond rereading your post I think you did ground at the sender. You still need to check resistance from the gauge to the sender. Say you find 12 ohms resistance in the wire, grounding it would still show full but add 8 ohms in the sender and now you have 20 ohms and the gauge is off. Also check resistance between the sender body and the tank, and tank to body, resistance here will also add to the total and throw off the gauge. It also could be the float may not make the full swing I bought a brand new sender and the float had a leak and slowly sunk after 2 weeks the gauge would only read 1/2 with a full tank. Remove and shake the float any liquid inside and it leaks.
Good luck
Ed

[soundcontrol]

Also check the ground clip that is connected to the fuel line and the inlet of the sending unit. That is supposed to ground the sending unit to the body and its very common to have a bad connection there, on some cars I had the clip was missing.

[msbaugh]

I can't figure it out.. The gauge pegs out with the wire connected straight to the battery. The only resistance in the circuit is through this sender unit.

I've verified that the float isn't sinking by checking the reading all the way up with the sender out of the car. It's the same only half reading. There's no way that I can get this float reading to change unless I added resistors, but that would only make the gauge read lower, not higher! Hmmm is there just no way at all I can get a correct reading with this POS sender? It's just simply designed wrong unless I'm missing something

[msbaugh]

Also check the ground clip that is connected to the fuel line and the inlet of the sending unit. That is supposed to ground the sending unit to the body and its very common to have a bad connection there, on some cars I had the clip was missing.

Where does the fuel line/tank ground? I can't find what you're talking about

[Bullitt-]

Where does the fuel line/tank ground? I can't find what you're talking about

Sending unit originally came with a strap that bridged to the fuel line..often missing not reliable at best, many clamp a wire to the sending unit & find a screw to ground to the frame/body

[Bullitt-]

oe

[edl94]

Without this the sender will not have a good path to ground.

[msbaugh]

Tried the ground strap but to no avail!!! I put the sender back in and filled the tank up. The float is rising all the way up but it will still only go to 1/2 way on the gauge. Is there a resistor in these circuits near the back of the gauge? I trace the wire from the sender up to the driver seat and nothing there 

[Bullitt-]

With the sender out & held to the top does the gauge register full?

[msbaugh]

With the sender out & held to the top does the gauge register full?

No it only registers half! The only way to go to full is to ground the sender wire

[Bullitt-]

IF I'm understanding you correctly the sending unit does not work correctly out of the tank...Then why do you think it could in the tank?
If there was much resistance on the sender wire I can't see how it would register full when it is grounded. 

 

[Bullitt-]

Thought of a couple of things to check
Check the resistance from the installed sending unit to any good ground on the frame/body...there should be no resistance, if so the ground is poor.

The sending unit wire I suppose could have up to 10-ohms resistance and still register a full tank when grounded...find a 10-ohm resistor and substitute for the sending unit, ground one end and connect the other to the wire & see what you register, if 1/2 tank you know the wire has an issue.