Fuel gauge only goes to half-full - what's the problem? Please help!!

[professor-patrick]

Ok, this is frustrating.  I am working on a 68 that had a fuel gauge that only went to half full.  I got a new sending unit, hooked it up to the tank wiring (while lying under the car), moved the float arm through it's full range of motion, and the gauge worked correctly from empty to full.  So I put this new sending unit in the tank, filled it up, and it the gauge again only goes to half.  I took the old sending unit, put it in a new tank not installed in a car, looked through the fuel filler hole, and checked to make sure the float lever would move uninhibited - no problems.  Why does my gauge only want to go to half-full?

[Ghoste]

Bad ground?? 

Is the ground strap hooked up?

[professor-patrick]

Grounded and double grounded.  Grounded like a drunk pilot.

[JimShine]

My 1969 Charger has the same problem. New sending unit, double grounded, fully functional cluster regulator (been checked and another functional regulator installed just to see if the problem couldn't be read) and it only reads half full (or I am thinking 50% of what it should). My guess is my gauge needs to be recalibrated.

[Ghoste]

I don't know Prof.  If it worked outside the tank and the only variable is inserting it into the tank, there has to be something there.

[Johnny SixPack]

My 1969 Charger has the same problem. New sending unit, double grounded, fully functional cluster regulator (been checked and another functional regulator installed just to see if the problem couldn't be read) and it only reads half full (or I am thinking 50% of what it should). My guess is my gauge needs to be recalibrated.

All three of my MoPars have that very problem.

3/4s = full

Empty = 1/4 of a tank (5 gals.) left.

I remember someone saying that it be that the arm on the sending unit may need to be tweaked?

I don't know if that is true or not though, as I've not done anything about the problem with mine.

[Ghoste]

I don't suppose you were able to notice if there was anything inside the tank that is mounted on the car? 

[Plumcrazy]

Does the guage go to full if you ground the wire that attaches to the sending unit?

[professor-patrick]

Plum - do you mean take the wire off the sending unit that goes to the gauge, and ground it instead - see if the gauge goes to full?  Haven't tried that but I will - what will that tell me?

[Ghoste]

That is what he means.  It'll tell you which end of the car holds the problem.

[professor-patrick]

How's that work Ghoste?  Will it mean it's the sending unit or the gauge?

[Ghoste]

Basically, yes.  It may not necessarily mean that one of those two things are the fault, but it will be on one side of where you break the connection to ground out the circuit or the other.
Does that make sense?

[professor-patrick]

I'll give it a try.  Thanks.

[Plumcrazy]

Plum - do you mean take the wire off the sending unit that goes to the gauge, and ground it instead - see if the gauge goes to full?   Haven't tried that but I will - what will that tell me?

Make sure you use a clean area of metal   on the body for ground.   Or better yet, take about a 20 foot piece of wire and attach clips to each end. Attach one end to the battery negative terminal and use the other for ground.   Then you will be sure you have a good ground.

If the gauge goes all the way up, you don't have a good ground at the sending unit, the sending unit is bad or something is interfering with the float.

If it still only goes halfway, there is too much resistance in the wire to the sending unit, the gauge is defective or the needle is sticking on something.

I'm assuming all the other gauges are working normally, did you check them?

[MikeD]

I have the same problem and a few of my friends with mopars have the same problem. One think I do not get if the ground is bad the gauge should not work at all??

Thanks MikeD

[Ghoste]

A ground can be poorly connected and result in a guage that doesn't work properly because of the increases resistance.