Bought the car as completely trashed out. Had been sitting for 6 years without the doors ever opened. Too much labor and parts to list here, but I have one BIG remaining problem and that is; I repaired the instrument panel, but since day One, the gas gauge has always pointed to "Full". Most advice I get is that the problem could be the sending unit in the tank has gone bad OR there is a problem with a ground wire.
A new stock sending unit for a '77 runs at about $250which could add up to about $1,000 when I'm told that the labor involved would be removing the spare and the spare rack, removing the gas tank and then trying the $250 sending unit. I bought one of those "vertical" sending units, but I'm told they don't work well and are more trouble than they're worth.
Someone also suggested that there might be a way to access the sending unit from the top of the tank at the filler spout. This is my first rebuild and my lack of experience is really showing in trying to resolve this problem. I hope someone on this forum might have some "happy scoop" to send me. I'm retired on a fixed income, so I have to work carefully with a budget on the car. I've been using the trip odometer to estimate when I need gas, and always afraid that I'm going to run out.
Thanks, Mick Carey Newport, RI
Ret. USMC

 

I too just bought a 77 vette. My problem was a little different ( my fuel gauage wobbled) but diagnosis is not too hard. First there is no "easy" way to access the top of the tank. If the tank is very loose, you can get enough of an angle to look in the fuel door and tell if your wires are connected at the top. Here's the diagnosis process. you will need a multi-meter to check resistance and I found a couple of long pieces of wire to use as jumpers comes in handy.

This process can be applied to all recent GM cars except those with computerized gauges.
1) Locate the fuel sender feed wire near the fuel tank. on a 77 the tan wire and the black ground go into a rubber plug that connects to the wiring harness in the driver side rear fender (look above the muffler). Seperate the plug. This will disconnect both wires that go to the sending unit. Use a jumper wire from the black wire on tank side of the plug the to re-ground the car. there's a ground nearby for the radio antenae. With the tan wire still disconnected, the fuel gauge should read past full with the ignition on. Wait a few moments as some fuel gauges take a long time to respond.
With the ignition still on, attach the other jumper to the the tan wire from the harness (that was connected to the tank sending unit) to any convenient ground and the gauge should read empty. If not, you have a wiring problem or a bad gauge.

2) If the gauge responds correctly, the gauge and wiring are OK. Next use a multi-meter to measure resistance to ground of the sender wire connection on the top of the fuel sender or the tan wire from the top of the fuel tank. Measurements should track the amount of fuel in tank.

Full - 84-88 ohms
Half - 40 ohms, give or take
Empty - 0-2 ohms

If this doesn't check, then sender or wiring on top of the tank is bad or the sender not adequately grounded. Senders are typically grounded by a black wire which is welded to the sender and attached to the body with a sheet metal screw. I did not find this on my 77.

3) If the sender checks OK but gauge and wiring don't, clean the connections, reconnect the sender wiring and separate the Fisher connector (plastic connector with several wires – look for the tan one) near the fuse block under the dash). With the ignition on, the gauge should then read past full.
Next, with the ignition still on, ground the tan wire in the dash side of the Fisher connector and the gauge should read empty. If not, you probably have a bad gauge or possibly a dash wiring problem. Go to Step 5.

4) If the gauge checks OK, then make the same resistance checks to the tan wire in the body side of the Fisher connector. If the readings are different than those at the sender, body wiring has a problem and requires detailed inspection. If they look OK, then the Fisher connector is probably dirty.

5) Clean and reconnect Fisher connector, pull the connector off the back of the gauge and make the same resistance checks to the tan wire. If they don't check, you have a dash wiring problem. If they check OK, your gauge is bad. Gauges can be bench-checked but this is best left to a specialist.

Sending units are $150- $250 and new fuel guage is $60.
Hope this helps. Good luck.

 

My friend Doug has done his home work and is as knowledgable about this problem as anyone. I have to throw one quick thought in there. I talked to a guy from Hawaii whose gas gauge wouldn't go below 1/2. I took his problem to a guy who is much more knowledgable than I, and he immediately said there is a bladder in the tank for those years and sometimes it bulges out a little bit and the float won't go down because it hangs up on the bladder.
He bent the rod that holds the float out about 1/2 inch and it cleared the bulge and works perfect now. Just a thought.

 

An always fuul tank gauge is usually the ground wire off the sending unit to the frame, or in a vette's case, where ever a ground wire can touch metal.

 

To Tim's point, since the 77 does not ground to the fame as other years do, I spliced into the ground wire on the tank side of the plug in the rear fender well and took an extra wire directly to ground where the radio antenae grounds to the frame. This did not solve my problem as I needed a new guage but this guarantees a good solid ground and gives me piece of mind.

 

which could add up to about $1,000 when I'm told that the labor involved would be removing the spare and the spare rack, removing the gas tank and then trying the $250 sending unit. I bought one of those "vertical" sending units, but I'm told they don't work well and are more trouble than they're worth.
???
If you have some basic hand tools, a jack with jack stands this job is really simple. I have taken out my gas tank in my 77 5-6 times, with it out you can remove the shield and clean, paint and restore the whole area, plus if you get all new nuts/bolts it will come out again so much easier. I can remove my tank in 15 minutes because I have the process down and have all new bolts (I also have air tools) and side pipes.

To do this first look real close as to how the gas spill over rubber neck is installed around your filler neck, I didn't do this and I still haven't figured out how it go's back on, next remove all the gas from the tank, jack the car and place stands under the frame but away from the tank, Remove the exhaust bolts from the frame and converter and drop it down or pull it out the rear, then remove the spare tire by unlocking the lower half, remove the tire and unbolt the upper half, this is really straight forward and obvious, remove the two bolts attaching the gas tank straps to the rear tank support, remove the four bolts attaching the rear tank support to the frame, the tank will fall into you lap after removing the wires and rubber hoses connected to the upper side of the tank, I had to cut the rubber lines off the tank because they where old and brittle, if you do this cut them as close to the tank as possible so you can re-use them. The shield on our 77 vette's is about a dozen or so bolts and comes off easy.
When I removed my tank I removed the sending unit and the bladder, the bladder was in great shape so I reinstalled it along with my sending unit, I was trying to figure out how to install a in tank fuel pump. This really is not that hard plus the rear of my Vette is now cleaner and painted and looks better that new, I didn't reinstall my spare tire because I feel I don't need it, I do get complements on the rear all the time.
Reinstall is reaverse but make sure that the rubber hoses and electrical wires are not run under the straps, this will sinch the fuel lines and will cut fuel flow. Pease.