Hi everyone, I am troubleshooting my fuel gauge as it will not go past 1/4 of a tank. I figured i would replace the sending unit as it was original and probably coated with junk. So i did, and my gauge still reads, 1/4 tank. It will go from 1/4 to empty, but not above that with the sending unit connected. When its disconnected, the fuel gauge does jump all the way past full. Am I looking at a gauge problem here, or a gauge cluster circuit board problem, or something else?

Thanks for any info,
---Chris

 

The only thing I can think of..... with a full tank of gas, try looking inside the tank. Is the sending unit arm floating on top of the gas or is something in there holding it down? Alternatively, (this is what I did to test my fuel sending unit), with the key in the "on" positioin, fish into the tank with a wooden dowel. (I recommend using a wooden dowel, no metal rod... I am just supersticious that way!) Push the float thing down and see if the guage goes towards empty. Lift it up and see if you can get the guage to read above 1/4 tank. I would use a flashlight but I wouldn't get an incandescent bulb too close. As I said, I am just a little scared around an open gas tank!! That's all I can think of for starters.

 

Before I installed my new sending unit, I tested it by hand and watched the gauge move, when i put it all the way down, it would read empty. Then when I pulled it all the way up, it would still only read 1/4 tank. I went ahead and installed the new unit even though it didnt fix the problem just cause I figured it was a good idea to have another new part.

--Chris

 

OK, I see you're already ahead of me! In that case, it looks like a resistance mismatch. What do you have, a 78? I wonder if you have the sending unit for a different year. You either need a different sending unit or need to install a resistor in the circuit. What you definitely need right now is advice from someone who knows this sheeat a lot better than me!!

 

I never purchased a sending unit. Did it come with a sheet showing it's specifications? I would assume it output would be close to 0 volts when the tank is empty and another voltage when the tank is full. You can try and fill the tank and take a voltage measurement somewhere near the gauge; preferably unhook the gauge wire and read the voltage there. If the voltage is correct, then either a bad gauge or possible ground issue. If the voltage reading is low, then perhaps you a wiring problem.

 

I had a similar problem on my 82. On my 82 the gas tank has a plastic inner liner. The liner or bladder as sometimes called had pulled away from the steel in the area that is closest to the taillamps. It didn't pull away much, but just enough for the float to not be able to swing free in full range. This may be why you can get a partial range reading. What I did is on the float arm there is a S bend. Make this bend a little more pronounced which will shorten the arm length.
Hope this helps.
Steve

 

Could be an incorrect resistor on the gauge or a bad connection at the gauge/resistor/flex circuit.

There's a resistor on the backside of the gauge connected to the gauge and flex circuit. It is a piece of white ceramic with a color. The color tells the resistance. It is part of the calibration. You might have the wrong one for the year.

Each gauge has a resistor. They are easy to mix up if you take the flex circuit off. You'll end up with some strange readings on all the gauges until you sort them out.

Here are my notes on resistors:


There are color coded calibration resistors associated with each gauge. The first time I took mine apart I didn't recognize them as resistors. They don't look like a traditional resistor. I thought they were just insulators. There are a lot of older vehicles out that and I bet people have mixed them up over the years. They are flat ceramic rectangles that connect between two posts. They are color coded for value. From what I've determined, here are the values:

Disclaimer: I have never found this documented anywhere. It is not in the electrical troubleshooting manual. The color codes are not in the assembly manual. This is by trial and error testing and comparing other known unmolested gauge clusters.

Colors / Measured Values:
Light Blue - 125 ohms
Green - 90 ohms
Dark Blue - 83 ohms
Red or Orange - 85 ohms

I have also seen Pink on the oil gauge and Yellow on the temperature gauge in an original 1980 Vette. I don't know the values.


Here's the results of changing or mixing up the resistors:

Volts - Less ohms = higher reading
Gas - Less ohms = lower reading
Temp - Less ohms = lower reading
Oil - Less ohms = higher reading

The Light Blue (125 ohm) is used on the Volt gauge. If you use something with less ohms, the voltage reads higher. With a Green (90 ohm) resistor, the reading jumps to 18 volts from a normal 13.5V.

Red (90 ohms) is used on the temperature gauge. If you lower the resistor to 40 ohms (test) the reading will lower 25 - 40 degrees.

Dark Blue (83 ohms) is used on the oil gauge. If you lower the resistance to 40 ohms (test) the reading will jump from the normal 30 to 60.

Green (85 ohms) is used on the 77 Gas Gauge. Green on a 77 causes the low fuel light to come on with slightly over 4 gallons in the tank and it will show Full with a full tanks of gas. Red (90 ohms) will cause the light to come on at 2 gallons but the gauge never quite reaches full. Red MIGHT be correct for later C3's that have the 24 gallon tank. I haven't verified this.

 

I memory serves me right, the gage works on resistance. 0 ohms being emtpy and 90 being full. Or is it the other way around ??? I do remember that damn near every GM gage runs a 0-90 ohm range.

 

I have checked the resistors, my gas gauge does have a green resistor. I checked resistance at my sending unit again, got 92 ohms. Then I took the center cluster apart and checked resistance to ground from the sending unit wire in the connector for the cluster, got 92.1 ohms. So the sending unit and wiring to have to be good. Which I can only assume means, bad gauge or a bad printed circuit. Does anyone know how to go about diagnosing either of these and what the resistance values should be? I measure resistance of the ground for the gauge and the sending unit wire, and got 62ohms without the main connector plugged in. After plugging in the connector though, i get only 33ohms, any suggestions as to which part is bad?

---Chris

 

Fuel guage read only 1/4 tank when full. I replaced sending unit-no change. Replaced fuel guage-fixed problem