The Stovebolt Site Forums The Shop Area The Electrical Bay Gas guage (tank always full) Don't I wish

With the price of gas these days, I wish my tank truly was always full, but thats not the case. I recently replaced the fuel guage in my 1953 3600 (converted to twelve volts), and now when the guage gets power, the needle shoots way past full. I've tried everything I can think of and even checked it with another guage, (it does the same thing). Does anyone have the solution to my problem?

Either:

1. The line between the gauge and gas-tank sender is bad; or,
2. The Gas tank sender is bad; or,
3. The Gas tank sender is not grounded.

Did you use a 12V guage. If not you need a power reducer.

What Tim said, too.

Oh man!
Now the price of gas is going to go up.

The default position of the gauge is full when there is no current to the gauge. You have a bad ground or a bad circuit to the gauge. Your sending unit is a 30 ohm unit. 30 ohms resistance is a full tank. With no current you are getting full resistance therefor a full reading on your gauge.

I think that when there is no current to the gauge, the gauge shows "Empty" (such as when the ignition is turned off).

When there is current to the gauge and the gauge is not connected to the sender (and thus not connected to ground), the gauge shows "Full".

These diagnostic notes are from the Shop Manual:
[img]http://ares.cs.siena.edu/~lederman/truck/Documents/Fuel%20Gauge/Fuel%20Gauge%20Diagnostics_files/image002.jpg[/img]

I have an extra sending unit and got to thinking about this the other day so I got the trusty multi-meter out to verify. 30 ohms resistance is in the full float position. If you look at the "Gauge Shows Full at All Times" section, #2 shows a broken wire which would be no current.

I'm starting to think it's the sending unit.
*New 12v guage
*new wiring
*original sending unit
But I had this question, Does the sending unit itself have a ground, or is it grounded to the tank? When I restored my truck there was only the one wire coming from the unit going to the guage.

The sending unit grounds to the tank and the tank is normally grounded to the frame.

On a panel body trucks, the sending unit is grounded to the frame through the medal gas line that is part of the sending unit and that is grounded to the frame at many places with metal hold-down clips.

On cab-body trucks, some people run a ground wire from a sending unit attachment screw to a clean ground on the frame (you could at least try this quickly see if that is the problem).

tclederman,
i have been trying to get my fuel gauge to work and am having no luck. in your reference from the service manual it notes "dash unit improperly grounded on instrument panel", this is the only thing I have not tried. Do you have any clearer instruction other than "properly ground dash unit"?
tired of carrying around an extra 5 gallon gas can!

cgresham1212,

I know what you mean. Fuel gauge problems seem very hard to trace. On one of my trucks, I had to resort to taking all parts out and bench-testing each part separately, and then testing them together on the bench, and then still having grounding problems back on the truck.

I have heard/read different opinions about the gauge housing having to be grounded. It does not hurt to test this by running a test-wire from a clean ground (from the battery neg. terminal, if possible) to and clean part of the gauge case (a clean mounting screw/bolt/hole. I have a long test wire for this purpose it has a small alligator-clip on one end and a large battery-termial-sized spring clip on the other end (I will be using this same set up in a few minutes to check out the grounding of a taillight that is giving me problems).

Good luck,
Tim

Along the same thread, I have a question about my '58:

My gas gauge is "nervous." The needle moves when you turn the ignition to ON, and it appears to read fairly accurately, but it tends to swing quite a bit. It isn't a jolt or a jump like you would expect from a grounding issue, just a sporadic swing. Any thoughts? Is grounding the culprit in this case?

Bill

I had those same problems in my 66 when I rewired, It took awhile to figure out, lot's of testing and other gauges.

continued : My gauge reads in the middle when no power, it's magnetic, but any way it turned out to be a bad ground.

Does it move when you are stopped or when you are moving? These old sending units show a change in level instantly. As the gas sloshes around in the tank the gauge shows it by moving up & down in real time. Chevy tried to minimize the sloshing & gauge movement by chambering the gas tank. If the gauge moves up & down as you drive & the tank isn't full it's probably normal. If it moves when the truck is still you need to look elsewhere, most likely a ground issue.

You make me think it is a grounding issue because it happens right after I start it up. The ignition gets power, the gage reads in the range I expect, but at idle it acts a little nervous, swinging around a ±1/8 tank range (1/4 tank total.) I need to be more observant when I power up, but before starting.

I doubt there is that much slosh at idle. I can imagine the gas surface dancing a bit, but not sloshing around until I get it in gear.

Thanks for the confirmation- time to look at grounds.

Bill

To test a gauge for an AD truck, take the wire that goes to the sending unit off the gauge and attach a clip lead or wire to that post. With the wire dangling in the breeze (open) the gauge should go to full. Then ground the wire and the gauge should go to empty. If this works, your issue is with the sender or the wire to it. If it doesn't you have a gauge problem. If you have an original 6 volt gauge operating on a 12 volt system you must have a special voltage reducer installed. Most vendors sell them. You must also have your gauge well grounded. You can test this by touching a wire from a known good ground to the chrome bezel. Come back if this doesn't help.

If you take a known, good sending unit, hook it up outside the tank and move the float manually, you should see the gauge respond in kind. If not you either have a problem in the wiring to the gauge or the gauge itself is bad.

I know you folks get tired of hearing this, but I have a gas gauge problem. Tha gauge shows past full all the time. I removed the sender lead off the back of the gauge and the reading did't change,I Then grounded the same lead and the reading stayed full. I have a reducer on the 12v side of the ignition. I read 6v on the E side of the gauge, and 12v on the F side of the gauge. I have read 18-ohms on the sending unit lead coming off the tank with approx 12gal in a 16gal tank.
Somebody please help me with a little or a lot of advise on this dilema.

Thanks too much

It sounds like either a broken wire between the sending unit & the gas gauge or a bad ground. Your sending unit is a 30 ohm unit which will read 30 ohms +/- when full. The default position with no reading from the sending unit is full. If your's is showing 18 ohms, the sending unit is probably good. It's just not getting the signal to the gauge.

mickey deville,

Good troubleshooting. Hope you did those tests with the key ON! If you turned the key on and the gauge read full, then you pulled the sending unit wire clear, and the gauge read full, then you grounded the sending unit wire to a known good ground, and the gauge read full, you have a bum gauge.

If the gauge is good, and you ground the sending unit wire with the key on, the gauge should swing to empty.

This is presuming you did all these tests with a good wire attached directly to the sending unit side of the gauge near the dash.

The part of your post I don't understand is the reference to 12 volts on the F side! There shouldn't be 12 volts anywhere but on the line side of your voltage reducer.

The gauge probably has two terminals..one for your reduced 12/6 volts, and the other for the sending unit wire. There shouldn't be anything higher than 6 volts on this gauge.

If you go back and read some of the above posts, it is pretty straigtforward how to isolate a bad gauge, or bad sending unit.


Stuart

Maybe I wasn't clear enough in what I'm trying to say. Looking at the back of the gauge from under the dash, the right terminal reads 12v, the left terminal reads 6 volts. I read 12 volts across both sides of the reducer. This is to second reducer because it didn't make sense to me that both sides of the reducer would read 12v.
If it is not to much trouble, and I should know how to trouble shoot something as simple as this, being a retired phone tech, please try to help me figure out what I am doing wrong.

thanks too much

mickey,

I guess I am not clear on the problem. Your truck/car is now 12 volts and you have a voltage reducer for the fuel gauge.

This setup worked in the past, but has failed now??

This is a new installation that you can't get working??

I think the proper wiring for the voltage reducer is 12 volts in on one end, and 6 volts out the other. Voltage would be read with a meter between either end and ground.

The stock fuel gauge should have only two terminals on the back, neither marked 12 volts!


A switched, fused, 12 volt source will go to one end of the voltage reducer, the 6 volt output will go to one side of the stock fuel gauge, and the other terminal on the gauge will go through the vehicle to the tank sending unit.

I don't know if this clears anything up or not.

Stuart

Both sides of the reducer could read 12 volts if the reducer is nothing more than a resistor and no current is flowing through it or the gauge, just as both side of a ballast resistor read 12 volts until the points close. As a tech, you would know this, but it might help others.

my gas gauge is always pointing like a 1/4 tank past (toward the full side) where the true gas level is. any idea of how to fix this?

The reducer would have to have a ground connection as well as a power connection to truly regulate the voltage.

Reading high could be from a high resistance connection from the gauge to the tank or a high resistance ground at the tank or a needle that has slipped or defective movement in the gauge. Where does the gauge go if you ground the terminal that is wired to the tank unit?

Not all sending units are 30 ohms, the one in the tank on my 68 shortbed is 90 ohms. Many of the suppliers list diffrent sending units for inline 6's and V8's not sure why??? May be a guage diffrence ?? A fuel guage is just a variable Resistor. with a float arm. About as simple as it gets.

Shop Shark,
OK here's the deal of the gas gauge. I finally understood what you're saying about grounding the sending unit. I did that with a ground from the gauge side of the unit, and the gauge went to empty like it is supposed to. I think the whole problem is gonna be the gauge. I am reading 18 ohms at the tank sending unit with a new wire, and the unit is new, so its hopefully good. When I first started fooling with this thing I found a burned wire on back of the gauge due to too much voltage. The little coil was black and apparently charred. I did manage to solder a broken wire on the coil and get continuity at the gauge. So I really want to thank you guys for the help and I am sure I will be hollering again for help. Unless you disagree about my solution I am now gonna try and find a gauge.

Thanks too much

When mickey deville is happy, we're all happy!


S. Shark