Our recently purchased '95 at first had an intermittent fuel gauge reading, now it just shows full all the time. I bought the Hayes book (big mistake - I should have bought the Chiltons, which I'm now going to do!).

Anyway, my experience tells me that most sending units pull closer to ground the more empty the tank is. And since the gauge in my '95 shows full (even though I know the tank is less than half full), that I should be able to pull the correct wire in sending unit/fuel pump harness to ground through varying resistance to simulate the sending unit. This will allow me to determine if the problem is the sending unit, gauge or wiring. Only thing is, I don't know which wire is the correct one.

Anyone know?

 

Hey KenHorse, can't help on wire color on the newer cars but most fuel gage issues are from poor grounds, clean those up first and check the weatherpack connectors along the whole length of the harness, they all need to be clean, shiney, and tight.

 

Good point. But I have to assume the grounds are OK as the fuel pump runs and shares a common ground on the harness (in addition to chassis ground of course)

 

If the sending unit is showing full all the time, the variable resistor in the sender has an open circuit. Most likely, the wiper in the circuit is open to ground. No resistance will show empty reading. [8D]

 

I asked about this very same thing for my '87 L98,in another post but this one is farther along and perhaps more people will benefit if we all converse here. I hope it is OK to jump in. Mech259, Can you please elaborate onyour last postas I am not quite familiar with resisters. I am aware of it's position on the sending unit/fill assembly where the float arm is attached but can this be fixed or is a new unit needed? I have checked all the connections and the grounds and they seem to be fine. Mine happened when I reattached the ground cable on the battery after installing new coil, rotor and cap. the entire dash wigged out for a second when I turned the key onthen the gas gage settled on half for just a split second and jumped to full. Still there now. The pump and rest of the dash work perfectly.

 

The sending unit is a variable resistor. The older units used to use a wire wrapped coil that a wiper slid along and contacted the resistor at all the points along the resistor. All your gauge is basically a ohm meter. The newer ones use a circuit board that essientially does the same thing as the wire wrapped resistor, except they are more prone to failure with time and constant movement of the wiper arm. So if you gauge is reading full all the time then there is an open circuit, whether it be a wire or a wore out sender. You can test it by grounding the sending unit wire and if your gauge is reading correctly, it should show empty.

 

Mech259, Thank you for the explanation. I will remove the unit from the tank and investigate. Perhaps when I last replaced the pump I did not fasten the wires fully. I hope it is just a loose wire and not the whole thing. I will post my results as soon as I do this. Thanks again. Cheers

 

I pulled the sending unit/fuel pump assembly this weekend and found a little corrosion where the sending unit's resistive winding is grounded (it is done through a spring). A careful disassembly and Scotchbright cleaned it right up. I reinstalled and voila! A working fuel gauge again!

 

[sm=smiley16.gif]Glad you fixed it.

 

TRy this it could help
If your C4’s fuel gauge reads full regardless of how much fuel you actually have, the sending unit
in the tank is most likely at fault. If you have priced a new sending unit, you realize that they run well over $300. In many cases, however, they can be repaired in about an hour with very little expense. This repair is relatively simple if you have decent mechanical aptitude, and doesn’t require any special tools. I did this repair on my ’84, but the procedures should be the same for all C4’s You should purchase a new fuel tank gasket prior to beginning. They run about $15 at any Chevrolet dealership. Also, have a fire extinguisher handy just in case. Here is how to proceed.

1. First, you must bleed the pressure out of the fuel system prior to removing any fuel lines to avoid being sprayed with fuel. Remove the fuel pump fuse from the fuse box, and crank the car. If it starts, allow it to run until it dies. The fuel pump fuse is clearly marked. On the 84, it is the bottom fuse in the third column of fuses.




2. The second step is to remove the fuel filler door. There are 4 phillips screws holding it to the body of the car. Once the door is removed, remove the gas cap and the rubber boot surrounding the filler tube. It has a drain hose that slides off as well. The boot is held in only by pressure, there are no screws or fasteners. You now have access to the fuel lines, wiring, and the fuel tank cover.





3. There are 3 fuel lines. The upper line is the pressure line that runs to the throttle bodies (crossfire) or fuel rail (TPI). The lower right line is the return line, and the lower left runs to the charcoal canister. Clamps hold on each fuel line. Loosen the clamps and remove the hoses.



4. There is a single three wire harness that services the fuel pump and sending unit. There is a small, white plastic clip that must be removed prior to pulling the connectors apart. Compress it with a pair of needle nose pliers and remove it. Now the harness will separate.



5. Remove the nine 10 mm bolts that hold the cover onto the tank. Carefully lift the fuel pump / sending unit assembly out of the tank. It is a good idea to clean any debris from the surrounding area prior to removing the assembly to prevent it from falling into the tank. Place some old rags over the painted surfaces of your car to protect it from any fuel that drips. You will need to gently maneuver the assembly back and forth to get it to clear the opening, but it will come out with relative ease.

6. Gently lay the assembly on your workbench. Now use some duct tape strips to cover the opening of your fuel tank. Fumes can accumulate rapidly, so do this at the earliest opportunity. DO NOT use a rag to cover the opening. This makes an excellent Malatov Cocktail out of your car. If you’re working in a garage, keep the door open for safety!!


7. Now that you have the fuel pump / sending unit assembly on your bench, it’s time to disassemble the sending unit. It is basically comprised of two parts; The float arm and contact, and the body with the windings. It works like a potentiometer (volume control) in that moving the arm changes resistance, thus telling the gauge how much fuel is in the tank. Malfunctions occur when either the contact or the windings become dirty, rusted, or otherwise don’t make good contact. Once this is corrected, your gauge will once again read correctly.

To disassemble the sending unit, GENTLY pry the three small tabs back just enough to allow the cover with the windings to separate from the assembly. These can break easily, so use as little force as possible.



8. Now that it’s apart, use some 600 grit sandpaper and gently sand the CONTACT. DO NOT sand the windings or you will ruin them. The contact is mounted on a thin piece of springy metal, so use a finger behind it to support it as you sand. Sand it until it has a shiny, new surface. Next, soak a Q-Tip in WD-40 and GENTLY clean the windings. Move the Q-Tip with, not against, the windings. They are very delicate, so use only very slight pressure.



9. Gently reassemble the sending unit and bend the tabs back into place using only necessary pressure. Ensure that the float arm moves smoothly and doesn’t bind.
10. Connect an Ohm meter or multi-tester as follows: Using an alligator clip, attach the negative terminal to the metal body of the sending unit. Press the positive tip against the wire that leads from the sending unit to the top of the assembly. Move the float arm back and forth and note the changes in resistance. Unfortunately, my digital multi-tester quit in the middle of this job, so I used an old analog tester. It still showed the resistance rise and fall with the movement of the arm. If this happens, your sending unit is now functioning normally again. Time to put it back together again!

11. Use your new gasket on reassembly. You will notice a notch cut into the gasket. This notch goes in the upper right corner. The gasket will fit the other way, but it won’t seal and you will have a nasty fuel odor (and safety hazard). Ensure that this gasket goes back in correctly.



12. Position the assembly back into the tank with the new gasket. Again, you will have to do some gentle maneuvering to get it to go back in correctly, but it will slide back into place with relative ease. Ensure that the gasket lined up correctly, and put the bolts back in. Snug them down by hand, and then tighten in a star pattern to about 15-20 ft lbs.
13. Reconnect the wiring harness and remember to slide the plastic retainer back in.
14. Reconnect the 3 fuel lines. Replace the fuel pump fuse, the gas cap, and start the vehicle and check for leaks.
15. Shut the car off. Remove the fuel cap, replace the rubber boot and reconnect the drain tube. Replace the fuel cap and the fuel door.
16. Enjoy once again having a working fuel gauge. Total cost less than $20 and about 2 hours of work.

black 88