Weird Fuel Gage Problem???

[PurpleBeeper]

70 Road Runner - My gas gage hasn't worked in many years, so when I worked on my tank I put in one of those $25 repops so I could go to 3/8" line. When the tank is full, the gage reads 1/2. When the gage reads "empty" I have about 4 gal. of fuel left. I "figured" it was the cheap sending unit, so I bought a used OEM sender from a '68 Coronet.

I tested the '68 sender by grounding it to the body & putting a jumper wire from it to the sending unit "L" connector of the car. Turned the key to "on" (with sender in "full" position) and the gage reads 1/2 full, just like the cheap repop that's in the tank now.

Can a bad fuel gage cause this? Is it even possible?

I "thought" the gage reading was completely controlled by the sender and the gage is either "good" or "bad". What the heck is going on?

 

[Speedbird]

Could be several things maybe.
You know that circuit works on voltage and resistance.
Do you still have a mechanical 5 volt limiter?
Is it consistent? I've put the RTE Engineering solid state one in some of my cars.
What about the associated connections to the tank unit? Are they old and adding resistance?
I don't know but this was a balancing act when the cars were new.
Now?

http://rt-eng.com/rte/index.php/RTE_limiter

 

[Belle66]

I had the same problem. I suspect it's in the dash. All gourd and continuity checks were just fine. Not yet ready to pull the dash, I opened up the sending unit and attached some long clip leads so I could sit in driver's seat and watch the gauge. I then opened up the sending unit and re adjusted the wiper arm of potentiometer until I got FULL on the dash. Then adjusted float arm to get full swing from EMPTY to FULL. Re-assembled and put in tank. That was 1 1/2 years ago so far so good.

 

[gtxno1]

Lol working on mine now same thing take your sender wire short it to ground should read over full if good like belle said will need to adjust the arm on the sender not looking forward to guesstimating where it should be just adjust the float arm do not putz with the internals run the risk of damage

 

[mmissile]

Mine would read at less than an 1/8th, when the tank was over-flowing[new sender and ground]. When we rebuilt the cluster and new dash harness....it instantly went to 3/4, yesterday[probably close]. All these 50 year-old parts are suspect.

 

[PurpleBeeper]

You guys are firing on all cylinders.
Onlyone - Good idea, but I have the RTE voltage limiter
Belle66 - I understand what you're saying, but I've never gutted a fuel sender before. Do you have any pictures or can you walk me through it?
gtxmo1 - Thanks...duh, I forgot to run that test & only tested the sender. I will try it. So, if the gage pegs "full", then it's good, right?
mmissile - Dear God, I hope I'm not going wire by wire & circuit by circuit. The dash harness is new, but the body wiring is original.

 

[PurpleBeeper]

Update - I grounded my sending unit wire & my gage went to past full. It sure "seems" like I've got two bad sending units and/or a borderline ground wire going to it/them. Am I right? Any other ideas?

 

[gtxno1]

Your gauge and wiring are fine. You can take the one unit in your hand attach a ground to it and the other to the wire to the sender and do a sweep test if fine you must perform your own calibration I must do the same thing to mine to get it to read correctly I will be bending the float rod in the direction of the bottom of the tank to read it a little sooner

 

[gtxno1]

if you suspect a ground issue just connect a ground wire directly to the fuel nipple coming off the sender if it now reads correctly then you would be right a ground issue if still doing the same thing you are not calibrated correctly

 

[gtxno1]

I suspect you are just not calibrated

 

[miller]

Just a suggestion, since ground seems to be the issue.

Any chance you've checked ground, from the metal fuel line, to body/frame?
On my 64, the fuel line coming off the tank, of course uses the metal strap, to cross over the rubber hose. But, from there, the steel line goes directly up to the body, and takes an all steel clamp. Believe that makes positive ground for the fuel line, instead of relying on the other clamps.