Gas gauge reads 1/4 tank on full tank of gas

[tucsonjwt]

My 83 C20 dual tank 454 has me stumped. The gas gauge reads only slightly above 1/4 full on both tanks when they are full. Once the tanks actually get below 1/4 tank full, then the gauge seems to read accurately.
I just installed new fuel tank sending units in both tanks (Spectra brand) and made sure that the grounds to the frame were good.
The tank switch works properly.
What should I check next?
(P.S. I am neither a mechanic nor an electrician, so I need gas gauge troubleshoot instructions for dummies.)
Thanks

[knightowl73]

Probably the guage itself then if you have changed the sendng units and checked the grounds. Mnes does somethng similar, havent changed the guage, but i have done the rest too with no luck, so id assume the guage since there is nothng else to check. If im wrong someone correct me.

[TimQ]

Mine does the exact same thing as well. Would love to hear if swapping the gauge fixes it for you.

[old Rusty C10]

sounds to me like a wiring issue... start at the grounds

[tucsonjwt]

I checked the sending unit grounds at the frame - look O.K. - tight and clean - new wire and connector from new sending units. Are there other grounds? I am assuming that the ground to the instrument panel cluster and printed circuit are O.K. , since all other gauges are working properly.
If the gas gauge is getting current from the sending units enough to register properly when the tanks are actually 1/4 full, then how could this be a ground problem? (Obviously, I know nothing about automotive electronics, just my dumb question.)

[kris's66lwb]

did you install the sender in the right position ? it could be hitting the side of the tank causing it to be psyhiclly limited to reading 1/4 tank

[tucsonjwt]

The sending units can only be installed in one direction, since the 3 outlets on each unit must point inward in order to connect via short hoses to the hard lines mounted on the frame rails. Since the functioning of the fuel gauge reading only 1/4 full with a full tank, then reading accurately below 1/4 tank is essentially the same as with the old sending units - I think the sending units are installed in the correct orientation. The only difference with the new sending units is that the gauge reads just slightly more than 1/4 tank with a full tank of gas, whereas the original sending units read almost exactly 1/4 tank when full. I am debating taking this to a shop for analysis or just installing a new fuel gauge instead (about $60 from Classic parts, w/S&H). My experience with several shops in Tucson, even specialty auto electronics shops, is that they also guess a lot - at $70 + per hour and often don't fix the problem the first time. (rolling dice in my hand right now)

[Red_69]

Mine did the same thing. I replaced the gauge and it fixed it.

[camshaftgsxr]

wiggle the gas gauge and see if it changes..

[tucsonjwt]

Thanks red and camshaft. I be wigglin' and jigglin' this afternoon, then I'll buy the gauge if it doesn't work.

[camshaftgsxr]

my gas guage was reading about a half tank off and it was caused by the metal tabs coming out of where the guages sit being bent off the guage poles so make sure all the tabs are contacting the poles on the back of the guage then it should read right.. if not have fun buying new guages bro...

camshaft

[tucsonjwt]

I wiggled and jiggled and took the gauge out. The gauge has three studs (threaded) coming out of the back of the gauge, which contact three metal tabs in the instrument panel (sort of like a spade connector). The studs are making good contact with the tabs, and I even used some needle nose pliers to squeeze the tabs together a little to make it more snug. The gauge still moves up to 1/4 tank only. So, the consensus is a new gauge - correct?

[Red_69]

Hit the salvage yard. Should be able to find something. Good luck.

[tucsonjwt]

The only gas gauge I found is at Classic Parts. It shows a 76-87 gauge which has a different appearance than mine. My gauge reads E through F with 1/2 in the middle and a gas pump symbol at the bottom of the needle. The Classic Parts gauge says "Unleaded fuel only" below the needle, and uses full, 1/2 full, and empty circle icons to represent fuel level. The mounting hole patterns look correct. Do you think this is a generic replacement for my OEM gauge? Thanks.
http://www.classicparts.com/1976-87-...tinfo/24-842C/

[knightowl73]

well i still say ts the guage, and im sure they are the same generic guage from the place u are talking about. i got me a parts truck today so i got one for mine, but its pegged out past full so i dont thnk its worth swapping anyway but i am goona soon. mine never reads past 1/2 tank with a fillup.

[TLoz]

Check the sending unit. I got a new one from Classic. Installed it and the gage read full. Started checking everything, grounds, wiring and gage. Finally I dropped the tank back out of the burb pulled the sender and found that the new sender would read full after moving a 1/4 of an inch. Get to your sending unit with a VOM meter and see if you have 0 to 90 ohms-from empty to full. Good luck!

[tucsonjwt]

I guess I should have checked the new sending units before I installed them. However, they are acting the same way the original sending units did before installing the new ones. So, I guess I will install the new gauge when I get it before I go back to checking the sending units.

[tucsonjwt]

knightowl73 appears to be correct. I installed the new gas gauge, turned the key to on, and the gauge went to full on both tanks - and both tanks are, in fact, still full of gas. Time will tell if the gauge reads correctly as the tanks start to empty, but so far so good. Thanks everybody.

[camshaftgsxr]

congrats on fixin' your truck

[ebgwd]

Gas gages for dummies.
I found this overview and troubleshooting very informative.
http://www.chevytrucks.org/tech/gasgauge.htm

Click Here

BTW.... my truck has exactly the same problem.
Thanks to you, I will check the gage rather than replacing the sending unit.
I will meter it too.... before getting the parts.

[tucsonjwt]

Yeah, I read this information a few times and tried the diagnosis listed there. Unfortunately, the diagnosis (as I interpreted it) led me to replace the sending units - so, this is what I mean about a lot of guesswork involved in this exercise. I'm not saying that the information here is wrong, just hard to pin down a problem like this with so many variables affecting the malfunction.
Bottom line, it is a lot easier to pull the instrument panel and gently wiggle out the fuel gauge than to drop the tank(s) and start checking wiring.
If an auto electric shop has a means of bench testing resistance of the gas gauge out of the truck, then that sounds easier than buying a new gauge. However, I have not yet found a shop that works for free - so $50 for a new gauge, or $50 to test a suspect gauge - which would you choose. In fact, all of the auto shops around here have a minimum $70 charge for any work. What bothers me is that I have paid a lot of money over the years for mechanic's wrong guesses - I can make wrong guesses myself for free.
(I have driven new trucks and let the dealer work the problems, but then I never used them as trucks because I did not want to get them dirty or banged up. So, an old beater makes the most sense if you actually use the truck as a truck.)

[joe231]

Originally Posted by knightowl73

well i still say ts the guage, and im sure they are the same generic guage from the place u are talking about. i got me a parts truck today so i got one for mine, but its pegged out past full so i dont thnk its worth swapping anyway but i am goona soon. mine never reads past 1/2 tank with a fillup.

Pegged past full just means the wire to the sender or the sender itself is open(ie. Unplugged or cut wire), that gauge should be good

[ebgwd]

Originally Posted by TLoz

Check the sending unit. I got a new one from Classic. Installed it and the gage read full. Started checking everything, grounds, wiring and gage. Finally I dropped the tank back out of the burb pulled the sender and found that the new sender would read full after moving a 1/4 of an inch. Get to your sending unit with a VOM meter and see if you have 0 to 90 ohms-from empty to full. Good luck!

So, would it be possible to pull the fuel gauge and check the voltage there?
My tank is full and the voltage is just under 12 vdc.
Seems like the voltage should be lower.
Mine is a 86 C-10 and pulling the full gas tank is harder but appears to be necessary. Do you agree?

When I plug in the gauge and only have 2 of the 3 pins on the back of the gauge inserted into the plugs that are in the dash, the gauge will read full. When I seat all 3 pins into the plugs the gauge reads just above 1/4 tank, as shown.

Finally figured that to take resistance readings, switch needed to be OFF.
Negative lead is connected to hard ground on frame of truck.
Left side connector is 12vdc when switch is on.
See photo and give advice. The more I read the more I don't really know about 12vdc circuits.
Thanks

[tucsonjwt]

I am not sure what this means, but you obviously need all 3 pins making contact for the fuel gauge to work correctly, so I don't know what checking 2 pins means. Why not just get a good gauge and plug it in? This is a long shot, but if you don't want to buy one online for $50, then maybe a local shop will take a gauge back as a return if you buy one and it does not fix the problem. (not likely, because there is usually no return on electrical.)
My thought is that if you buy a new gauge and it turns out that your existing gauge is good, then you can sell the new gauge or keep it because eventually your existing gauge will crap out (sell the new gauge for $30 on ebay or craigslist? - $20 diagnosis). I would not have the patience for dropping the tanks to check the lines to do more guessing unless I was sure that the gauge was good.

[78-5.3]

Originally Posted by tucsonjwt

I am not sure what this means, but you obviously need all 3 pins making contact for the fuel gauge to work correctly, so I don't know what checking 2 pins means. Why not just get a good gauge and plug it in? This is a long shot, but if you don't want to buy one online for $50, then maybe a local shop will take a gauge back as a return if you buy one and it does not fix the problem. (not likely, because there is usually no return on electrical.)
My thought is that if you buy a new gauge and it turns out that your existing gauge is good, then you can sell the new gauge or keep it because eventually your existing gauge will crap out (sell the new gauge for $30 on ebay or craigslist? - $20 diagnosis). I would not have the patience for dropping the tanks to check the lines to do more guessing unless I was sure that the gauge was good.

not to steal this thread but i have a similar problem mine reads below the empty to about 1/8 tank i replaced the sending unit when i did the 5.3 conversion strange??? oh if i stop quickly the needle moves ????

ive replaced the gauge with new and used ones same thing!!!