my fuel gauge is significantly inaccurate, where the fuel is nearly empty and it reads about a quarter of a tank. How do you adjust or calibrate the gauge so it reads correctly.
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
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#2-1985 Bright Red/Carmine Cp.L-98/auto
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Depends...does the gauge show to be accurate up until that point, or is it off all across the board? If it works until the fuel gets low, I would suspect a bad sender unit. If it is wonky from top to bottom, it could be a bad resistor, or the gauge itself.
____________________________________ Joel Adams C3VR Lifetime Member #56 My Link
Honestly....this was my dad's car befor he handed it down. He told me the gas gauge was off and to refill at 125 miles.
Might experience is at 125 miles the car is at 1/4 tank...he is a physicist so maybe his impreaaion is off by a factr greater than the rest of us.
Is there a calibrating technique that I should know?
Vette(s): 1981 Blue Ice Met. w/pearl ghost flames
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I agree with Joel. I'd bet the sending unit in the tank is kaput. Regardless, you'll have to remove the sending unit from the tank to either test it with a Ohm meter or replace it. There are little brass contact strips that the float arm contact constantly swings up and down as the fuel sloshes around. There could possible be a bad spot the a couple of those brass strips.
If the sending unit shows good then you'll want to connect the sending unit to the wire harness and have someone in the car to watch the fuel gauge move as you slowly move the float arm through the full range of its travel.
Did you fuel gauge ever exhibit a rapidly fluctuating needle as you drive?
____________________________________
GEN III 5.7L "LS6" Engine swap TKO500 5 spd.
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'69 "N11" Sidepipes Borgeson Steering Box Born 8/1981
Sequence #3975
Hi Sarge....
When I fill the tank completely it reads full....as I use gas the guage goes down but (according to my dad, I have not ran it put of gas tomprove or disprove the veracity of my physycist/engineer father's description) when the gas gauge hits 1/4 the car is actually on fumes.
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
Am-Fm/map light National/Regional/Chapter NCRS "Top Flight"
#2-1985 Bright Red/Carmine Cp.L-98/auto
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It's possible that the bladder inside the tank is deformed, and the float is hanging up on it at about 1/4 tank, if the gauge appears to read properly until it gets to that spot. Don't think you can look down into the tank on a '75-'82 car, like you can the older ones, to see if the float is hanging. Ya may have to just pull the sender out(from the top) as Darryl stated, and have a look-see from there. Afaik, there are no "calibrations" to be done on the sender/gauge unit...they either work, or they don't. Not sure if the later cars have the resistor on the gauge itself, but those can cause a fluctuating/bouncy reading on the gauge, as well as incorrect readings. At this point, I think I would disconnect the gauge wire AT the sending unit, and ground it. If the gauge goes to EMPTY then, you'll know the problem is with the sender unit/float. If the gauge does NOT go to EMPTY, and stops at 1/4, then your gauge itself is wonky, and will need to be either replaced, or repaired.
____________________________________ Joel Adams C3VR Lifetime Member #56 My Link
Vette(s): 1980 blue w/ oyster leather, 4 speed, engine from a '73, modified
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I just rebuilt the sending unit in my '80 with the help of an article from Corvette Enthusiast from October 2006. According to the article, the sending unit should have a smooth 0 to 90 ohms resistance as the float arm moves from empty to full. Might have been the other way around. In any case, you should see my point. Remove the sending unit and measure the resistance as you move the float back and forth. If it jumps or doesn't reach the full range, you should open it up and clean it with some carb cleaner. Check the quality of the contact.
|UPDATED|6/11/2015 3:53:21 AM (AZT)|/UPDATED|
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1980 with the heart of a '73, because more horsepower is always better.
hi sarge81 is right that's is to only way to check,many factors are at work no gas tank that I have worked on is perfect some will say 1/2 tank when only 1/4 tank,my company built fire trucks we made are own tanks some 50 gals or more,using factory sending units we bent arms to make some what acurant 1/2 tank does not ment 1/2 tank left not true do as sarge81 sezs than you can replace this many not change gauge many factors remove is the only way dwa
Vette(s): 1975 C3 Red, T-Tops, Black Interior.
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0 ohms is empty. 90 ohms is full. disconnected it will read over (past) full.
45 ohms should read 1/2. You can use different resistors if you want to check or try to adjust the sending unit when you have it out.
But as Joel said, ground the signal wire and see if it reads empty. It already reads full okay. If it does not read empty, either the gauge is bad, or there is a bad ground connection between the gauge and the tank, providing the power and ground to the gauge are both okay. But if you ground the signal wire and it reads empty, forget everything from there forward. It's in the tank, OR you have bad ground connection for the tank unit.
Bad connections are not likely in this case due to the fact that they tend to get worse over time, and this one has been consistant for years.
Also as Joel stated the bladder in the tank is a common cause for this problem. When they deform they can cause all kinds of problems. The bladder is there because it's a sports car. At one time cars on some race tracks had to have a bladder in case of an accident that punctured the tank, fuel did not leak out. Our cars were raced in these venues often enought the bladder was a factory item. If the outer shell of the tank is good the bladder can be removed without a problem.
If you are doing this on a newer GM, the reading from full to empty is 30 ohms to 120 ohms. Still a 90 ohm spread, just a slightly different range. These cars have a carbon printed resistor in the sending unit instead of a wire wound resistor.
|UPDATED|6/11/2015 7:46:54 AM (AZT)|/UPDATED|
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On the "Tail of the Dragon" (some day, no strike that, October 2008 it turned red, still in progress!) (click either image to see a bigger version)
Sarge81 said: Let me see if I have this straight and correct me if I'm wrong.
'77 and earlier vettes had a bladder that was pliable?? (I've never looked in one)
'78-'82 vettes had a thick hard plastic liner. More like how modern plastic fuel tanks are today. (I've looked in a few of these tanks)
And no... '82 Cross Fire Injected vette tanks do not have a baffle in them for the in-tank fuel pump. This I know for fact.
I had wanted to ask someone knowledgeable about this because when I read Knudsen's "Star Spangled Sports Car", he had said in there that the 78-82's had a hard liner and that 77 was the last year of a bladder. I was confused/surpised because I had always heard that all C3's had a bladder, and I have never had my tank off/out to check. I guess mine works the opposite since it was my understanding the tanks were 24 gallons, but when mine reads empty the most I have ever put in it is about 18 gallons and I always thought 6 gallons was a lot to have for "reserve".
just thinking [yits hard] if car gauge shows 1/4 tank of gas,than fill it up and see how much you put in that could tell 1/4 how much fuel you do have just a thought dwa
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
Am-Fm/map light National/Regional/Chapter NCRS "Top Flight"
#2-1985 Bright Red/Carmine Cp.L-98/auto
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rastafford3164 said: I was confused/surprised because I had always heard that all C3's had a bladder, and I have never had my tank off/out to check. I guess mine works the opposite since it was my understanding the tanks were 24 gallons, but when mine reads empty the most I have ever put in it is about 18 gallons and I always thought 6 gallons was a lot to have for "reserve".
There were NO bladders or liners in the tanks until the '75 model. Earlier cars were nuttin but steel. The tanks are listed as 20 gallon for most years, but the '68-'74 cars have a filler tube extended down into the tank, that will keep you from being able to put any more than 18 gal in them. The '75-'77 cars had a little less fuel capacity due to the bladder taking up space. The '78-'82 cars had different tanks to start with, and I think most of those will hold 20 gal when totally empty...not 100% sure on those. The '75-'77 bladders can be removed fairly easily thru the filler neck hole, once the neck/sending unit has been removed. The later cars with the less flexible liners can be removed, but not very easily.....usually not worth the effort. Even the rigid type liners can deform over time, and cause problems.
____________________________________ Joel Adams C3VR Lifetime Member #56 My Link
Vette(s): 1980 Corvette White and Black with Black interior. 350 V-8 Auto
Joined: 7/18/2015
Posts: 32
Hello all. I just bought a 80 vette that needs a lot. First thing to fix is the gas gauge:) then speedo. I've read this post and I'm not sure what to do next. When I disconnect the ground to the sending unit the gauge goes past full. As soon as I reconnect it drops to around a quarter full but nothing else. There is probably 15 gal in the tank and no change. I have a limited budget and a little knowledge(which is dangerous) so I just don't want to throw money at parts wishing it works. I'm looking for any and all advice I can get. Thanks
Vette(s): 1975 C3 Red, T-Tops, Black Interior.
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Ground the wire to the sending unit and see if it reads empty. If so your 80 either has a bad ground from the sending unit in the tank, or a bad sending unit. if not, it's back up to the dash gauge or a bad connection between the two.
|UPDATED|7/20/2015 6:00:18 AM (AZT)|/UPDATED|
____________________________________
On the "Tail of the Dragon" (some day, no strike that, October 2008 it turned red, still in progress!) (click either image to see a bigger version)
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OK ...... it may be Aliens but I have to tell this story.....
I have been
having a problem with my 1980 fuel gauge for 5-6 years.....
The gauge would not go to full when topped off, went to 3/4 or less. When fuel burned past that, it would read correct and the low fuel light would come on when at that point in the red on the dash gauge. I removed wires as both Joel and Ken have instructed and still did not correct. Spring of 2014 I removed and replaced fuel tank sender, left lots of blood around tank opening for future DNA testing if needed. Still no change.
This spring 2015 I purchased a new fuel gauge and was ready to replace but did not find time before gathering.
OK here is the Aliens part.....We have a local quick stop fuel station that offers non ethanol fuel. I started running that in the vette last month. I filled the car 2 weeks before the gathering and it sat until that trip.
As we are on our way I noticed the gauge is reading full again, during the gathering there was no non ethanol fuel to find..... the gauge would not go all the way to full but very close. I came home close to empty and filled with the non ethanol and its back to the full mark on gauge.......doo doo doo doo......doo doo doo doo.......you explain that one to me......Kyster......Adams Apple........??
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