The Stovebolt Site Forums The Shop Area General Truck Talk Fuel gauge issue . . .

Hello again Stovebolters . . I have a fuel gauge issue.

Yes, I have read through most of the fuel gauge questions/answers from older posts. I did not see my issue addressed . . . when I turn on the dash key switch, the fuel gauge needle drops below empty, it does not register any fuel? I know there are at least 8 gallons in the tank.

I am guessing that the gauge is original (?) to the '51 2-ton truck. Old posts usually said that electrical issues and not the gauge was the typical problem. It was not working when I bought it because most of the truck wires had been shredded. After removing the seat today I checked voltage at the tank. It was only 1.53 volts. Battery, headlight and starter all showed 6.54 volts. ALL wiring on the truck was replaced with a new harness and anytime a wire goes through the firewall, floor or other metal area, the wire is encased with a rubber grommet. I read that when the gauge trim ring is painted (as I have done) the fuel gauge may not be properly grounded . . . I attached a temporary grounding wire to one of the threaded studs holding the gauge group to the dash to make certain the gauge is grounded.

So, I am at a loss . . the voltage drop at the tank, the needle drop when the key is turned on and the fact that this is the only electrical issue I have experienced since replacing the harness?!

Any suggestions will be appreciated . . . . thanks.

Dennis in Austin

My gauge ,was reading lower than what was in the tank and it turned out to be the float was half full of gas ,new float and was accurate again . I think in one of the past posts it explains how to check if it is the gauge or if it is the sending unit .

The ground that is normally the issue can’t be reached from outside the gauge cluster. The cluster bezel and glass must be removed. I have a spare gauge and took a picture to show you.

I wouldn’t jump the gun and take your cluster apart until you test everything else. for example I installed a new gauge, new sending and new wiring and it failed after a few weeks. When I turned on the ignition it went right to empty and I knew I had at least a half tank of gas. The brass float was full of gas. I replaced it with a plastic float and now it works.

I’m going to leave the troubleshooting to the experts.Good luck!

It's a fairly simple circuit,
Power from key to the gauge ( battery voltage )
ground wire from gauge to the sending unit center stud ( wire harness )
Sending unit has variable resistor ( changes with fuel level )
Resistor to sending unit frame ( must be out of tank to verify )
Sending unit mounting screw to frame rail of truck wire ( must have clean metal to metal contact at tank and frame rail)
and now you can ad internal ground inside gauge as shown above.

Work your way back and you will find the problem.

At the sending unit, disconnect the wire that goes to the gas gauge. Now when you turn on the ignition, the gauge should go past full.

Now touch the wire to a “known good ground”. The gauge should go to empty. Don’t assume a ground is good. If you have to, take a jumper cable from the battery ground to make sure. All of these test are dependent on a good ground otherwise we will be chasing our tails.

If both of those test passed, the gauge and wire going to the gauge are good and its on the sending unit side. Now turn off the ignition.

Now reconnect the wire to the sending unit. Take a test lead and connect it to that ”known good ground” and connect the other the the ground (or body) on the sending unit. With the ignition on does the gauge show relative gas level?

If not, remove the sending unit. First check to see if the float is fill of gas. If it is replace either the float or the sending unit.

If not, with a multimeter, set it on ohm’s and connect the leads to the wire going tho the sending unit and the other to the body of the unit. Now swing the float and the meter should show 0 to 30 ohm’s. A few ohms one direction or the other is not a problem, but it should be even across the entire movement of the float and not erratic.

Report back with your findings.

Thanks . . hope that's not the case . . . brand new tank, float assembly . . the whole 9 yards!!

As soon as the rain passes, I will get out to the truck and try each of the suggestions. THANKS!! This is frustrating . . keep thinking it's the gauge because everything else is new. BUT, the voltage at the tank is my driving force now . . . and I appreciate the detailed analysis help!! Always glad for the knowledge base nere . . .

One other possibility to be aware of, as this happened to me. I also had a new replacement fuel level sensor, but discovered they aren't that well made and the possibility exists for the body of the sending unit to ground out on the gas tank. I discovered this while I was troubleshooting and loosened the screws holding the sending unit, which resulted in the gauge working correctly. If I remember, all I did was be careful about centering the sending unit in the hole while tightening the screws, ensuring it didn't touch the tank.

"After removing the seat today I checked voltage at the tank. It was only 1.53 volts." I will assume by "tank" you mean the sender. There is no voltage at the sender. So I don't know what you "read" on the meter and at what meter setting.
You say you have read old posts and most of the problems are electrical not the gauge. The gauge IS the bulk of the "electrical". Actually the most typical problem is ground. You need a wire from a sender screw to the chassis. You need to verify that you have a hot wire to the proper gauge terminal and that the other terminal has the wire that goes to the sender center tap.

We need to settle down and begin a trouble shoot. As usual we are jumping all over the place with ideas and explanations. You are not up on electrical yet. No problem. Make sure that if there is a fuse associated with the gauge, that it is checked. Make sure you check voltage at the gauge terminal.

You need to go back and review the "old" posts more carefully. We have gone over every possible issue and tests in those old posts. There are dozens of posts to study. It will take time but it's all there. Your issue IS addressed many times. So I see you have not read much.

There are some red flags here:
1. Gauge not working when truck purchased means you don't know anything about it I assume. Could be the wrong gauge or is bad.
2. Everything else is "new" so points to gauge as you say. but we don't know who installed the new?
3. Thinking there is supposed to be voltage at the sender.

Hello Phil - Phak 1 . . .

Line 1 ... gauge did not move, did not go to full.

Line 2 ... again gauge did not move.

Line 3 ... something is amiss!

Line 4 ... again nothing.

Line 5 ... half a tank of gas . . . used my torch to look into the tank. First time I worked on my '67 TR6 the book said "insert your torch into the hole on the tank top to verify fuel level". Took me more than a minute to decide the Brits meant flashlight!! Imagine my red face embarrassment . . .

Line 6 ... Ohm meter set at any of its 5 positions registered 1 . . there was no movement whatsoever!! I think the problem is obvious . . a bad (new) sending unit!!

I will be ordering a new unit today and after installation, I will go through your multi-step procedure again and report my findings. One other thing that I thought of was, although I followed the wiring harness installation correctly (I checked often), perhaps the two leads were backward? After I receive the new sending unit, hopefully all will be right in the world?!

Many THANKS Phil for your detailed analysis.


Greg Brown . . . I looked at the assembly while taking it out of the tank and I'm pretty certain the sending unit was adequately isolated by the rubber ring. Thanks for your input, I will be doubly careful when installing the new one.

One other thing I should add . . . after removing the sending unit, I again took the voltage measurement at the sending wire/my grounding lug . . it was 5.87 volts and 5.88 volts between the sending wire and the negative post of the battery. I think that, along with the 0 ohms measured, shows that the sending unit is the issue. I am moderately hopeful that a new unit will clear up my problems?! Fingers crossed . . .

Well, we did not get past step one, so we haven‘t verified if the gauge or the wire going from the tank to the is good. Replacing the sending unit more that likely will not solve the problem. The next step it to test the gauge and wiring.

We test, not replace parts to solve problems, so we don’t spend money on parts we really don’t need.

It’s not going to help having two people trying to diagnose the problem and I see that bartamos has joined the post. He is much better at diagnosing these issues, so I’m going to leave you in his expert care. Good Luck and I’ll be watching.

51 2 Ton, I don't know if you've read my latest post or not at this point. Please just let me know if you have.

From what I think I am reading, your testing is disturbing and possibly damaging to the components. Not sure.
If you want to move on because you don't think we are "clicking", that's OK too. Please advise. Thanks

Thanks Bartamos, I have been in the garage and was just replying to your earlier note when I had the misfortune of accidentally closing down my computer and losing everything I had written. I need to go back and, as you say, review all of the old posts. I did my research a couple months ago and have slept since then. I will try to do that research of past posts in the next couple of days and will then perhaps be better equipped to talk apples and apples with you?

THANKS for your help with my issue, talk again soon,
Dennis

When you get time, I would like to go over your tests and we can try to redo some in a different way. Unless you get it fixed.

Well . . THANKS again everyone . . the fuel gauge is now working properly.

After rereading many of the older posts, I came across one in 2013 by LonnieC. Although the discussion was good and even though none of the pictures indicated were available, there was one little piece of information that was very helpful . . . a web address for fifthaveinternetgarage.com. Many useful tech tips listed there and one specifically for fuel gauges. Following their recommendations, I was about to start the testing procedures when, just because I wanted to start from the very beginning, I squeezed myself back under the dash to check the wiring. Lo and behold, the simplest of fixes . . . put the sender wire and the hot wire from the ignition switch on their proper posts!! Then check to be sure! Obviously, I had not.

I feel like the such a DUMBA$$ . . . KISS: Keep It Simple Stupid . . start from the beginning and check/recheck everything. Then check again!!

Sorry I wasted your time and talents . . but I do appreciate the responses.

Now, if I can only stop that pesky drip from the Schwartz hydraulic bed lift!!

Glad it worked out for you. Sometimes we are the problem. Don’t feel bad, its happened to me and I’m sure to many of our fellow “bolters”!

That's why you saw voltage at the sender. Hot was feeding the sender rheostat. You are lucky, that should have fried it. Maybe you used another new sender....or 6V was not enough to fry.

Yes, it was fortunate . . . I would have thought, like the red tag on the back of the gauge says: "connecting the hot wire may destroy the sender unit". Fortunately, I was lucky . . this time!!