["QUICK-SILVER"]

Originally Posted by Greg Reid

What you're describing is kind of stramge. You ground the wire while the circuit is complete and it goes empty but you ground it with the wire unhooked from the sending unit and it stays full?
I'm thinking it should have gone to empty either way.

Unless the guage IS bad, but then the first empty reading wouldn't have happened.
Unless another possibility, a broken wire in the ribbon running from front to back and wiggiling the harness/connector had it making and breaking contact. But it doesn't really sound like it though.

Think of the fuel guage as an ohm meter, thats only capable of reading 0/zero (dead ground) to 90 ohms. And 0 to 90 is all the sending unit can produce. 0=empty and
90=full.
With the connector unplugged, on the back/trunk side, you can test the sending unit with an ohm meter. Sending unit wire to ground should read 0 to 90 ohms if it's good.

You can perform the same test on the spare sending unit and move the float up and down to see the 0 to 90, if it's good. If it is, hook it to the car harness and see if the fuel guage works.

[Greg Reid]

Originally Posted by "QUICK-SILVER"

Unless the guage IS bad, but then the first empty reading wouldn't have happened.
Unless another possibility, a broken wire in the ribbon running from front to back and wiggiling the harness/connector had it making and breaking contact. But it doesn't really sound like it though.

Think of the fuel guage as an ohm meter, thats only capable of reading 0/zero (dead ground) to 90 ohms. And 0 to 90 is all the sending unit can produce. 0=empty and
90=full.
With the connector unplugged, on the back/trunk side, you can test the sending unit with an ohm meter. Sending unit wire to ground should read 0 to 90 ohms if it's good.

You can perform the same test on the spare sending unit and move the float up and down to see the 0 to 90, if it's good. If it is, hook it to the car harness and see if the fuel guage works.

We're exactly on the same page as far as how it works. I haven't looked at a fuel gauge in a while and don't remember the colors of wires, etc. but it's a very simple, one line series circuit. 12v, gauge, variable resistor {sending unit}, ground.

Keep going Jay. You're close. As Clay said, clip the ohmeter leads to the sending unit and move the lever up and down. You should read a variable resistance from 0 at the bottom to 90 at the top.
OL means it's out of range. I don't know what meter you have but I wouldn't think that 90 ohms would be out of range.

[jims68ragtop]

I was getting ready to ask the same original question about my fuel gauge, and then I found this thread. I read through all of the posts, and I think I understand where my car stands. It is a '68 Lemans. Over the last few months, I installed a new engine and rebuilt trans. I painted the underneath side of the car with the engine and trans out. I removed the gas tank and cleaned it out. I had to install a new strainer on the fuel sending unit, because the old one fell apart in my hand when I touched it. I cleaned and reinstalled the sending unit, painted the tank and put it back on the car. I reused the thick tar paper mat on top of the tank. The first time I started the car, the fuel gauge was showing around 3/4 full (that's probably about how much gas is in the car). The next time I started it, the needle was nowhere to be found. I assumed it was past full, but could not see it. I took the tan sending wire off of the connector where the license plate light also connects. I then grounded the tan connector wire, and the needle dropped to empty. When I took the temporary ground wire off, the needle slowly went back to past full. From this thread, I think this means that the gauge is good, but the sending unit is bad. Please let me know if I am stating this correctly, as there is a ton of info in this thread. Thanks.

[Greg Reid]

Either it's bad or you have a bad connection to, or integral to the sending unit. I can't remember without going through the entire thread again...but I think that's what Jay had. Like a bad crimp connection or contact integral to the sending unit.

[Patina Tiger]

Yes,

It is the little retainer cap that is on the underside of the sending unit, electrical current gathers corrosion there due to air exposure in the tank over the years. Carefully pry off the cap and clean the brass fitting and thin metal strap. After done, set the unit in the trunk, (for ease of testing), and ground with key in the RUN position then move the float arm up and down for reading. Should be good, then install into tank.

Alex

[Greg Reid]

And from what I've been reading here on the forums, it's worth fixing. I understand that a lot of the new units bought at the parts stores are failing within a year or two.
These originals last 40 years easily.

[geiser]

I have been doing quite a bit of reading and have a simple question. Does the center trunk body plug line up with the sending unit?

I put in a new sending unit and tank early in my rebuild process (before the dash was in.) I found the harness was not connected on either end in the trunk after the dash was put in. I currently have a full tank. I really don't want to drop the tank to connect the harness and was hoping to pry the body plug open to connect/fish the harness to it.

[lazlo]

Originally Posted by geiser

I have been doing quite a bit of reading and have a simple question. Does the center trunk body plug line up with the sending unit?

I put in a new sending unit and tank early in my rebuild process (before the dash was in.) I found the harness was not connected on either end in the trunk after the dash was put in. I currently have a full tank. I really don't want to drop the tank to connect the harness and was hoping to pry the body plug open to connect/fish the harness to it.

Two hoses in the filler neck, at least one down in the fuel. Seal the two hose with a rag to make air tight as possible. Set a couple of 5 gallon buckets on the floor to catch the gas. Stick an air hose in one of the two hose, give it air and watch the gas come out the other hose. This very effectively causes a siphon that will empty your tank, without the nasty aftertaste...