Topic: 1980 Fuel Gauge Calibrating
in Forum: C3 Fuel, Emission Control, and Exhaust Systems
Already a Member?
Click Here to Login
Not yet a Member?
Click Here to Register for Free!
SPONSOR AD:: (Our Sponsors help support C3VR)
Lifetime MemberModerator
Duncanville, TX - USA
Joined: 11/8/2003
Posts: 20214
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
Member: NCRS, NCRS Texas, Corvette Legends of Texas
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.
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 
(click for Texas-sized view!) NCRS
"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"
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?
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?
TKO500 5 spd.
Borgeson Steering Box
Born 8/1981
Sequence #3975
Click here to see more pics of my Vette on CarDomain.
Lifetime Member #26
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.
Lifetime MemberModerator
Duncanville, TX - USA
Joined: 11/8/2003
Posts: 20214
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
Member: NCRS, NCRS Texas, Corvette Legends of Texas
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.
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
(click for Texas-sized view!) NCRS
"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"
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|
|UPDATED|6/11/2015 3:53:21 AM (AZT)|/UPDATED|
1980 with the heart of a '73, because more horsepower is always better.
http://www.c3vr.com/member_uploads/32401_32500/32417/20140914_17190920156365050r.jpg
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
0 ohms is empty. 90 ohms is full. disconnected it will read over (past) full.
|UPDATED|6/11/2015 7:46:54 AM (AZT)|/UPDATED|
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|
in Forum: C3 Fuel, Emission Control, and Exhaust Systems
SPONSOR AD: (Our Sponsors help support C3VR)