Quality gas tank sending units?
 

Need advice here. In the last year, both gas tank sending units in the '70 & '73 TA's died. Lucked out & got a NOS unit for the '70, but had to settle for a repro bought through Bob's Chevrolet Parts for the '73. But I have similar problems with both. They stay on full or near full for way too long and then I've actually run out of gas with them still reading above 1/4 full. I can drive almost 150 miles before the needle starts to drop.

I've cleaned the grounds more than once, no improvement. I can remove the senders from the tanks, hook them up as normal, and test them on the cars moving the float arms through the full range and they read properly. What gives?

Hmmmmm, no educated guesses even?

OK I'll bite. Is the Tank an Original or repro? If repro what type. Sounds to me
if the low reading is the same on all of the senders then the first place I would look is the
drop on the float. If it reads full scale on a bench test, then perhaps its the drop
available in the tank. If it fails a bench test, you have the issue.

Original tank, no dents to limit travel.

OK, that eliminates the tank as an issue. Next bench test one or all of the sending
units with a multi meter. This test is to insure your getting a reading on the full
scale of the potentiometer within the sender. If this test is successful, and tells you
the units are capable of full scale readings, this then becomes an issue of adjusting the
float drop to provide full scale on your gauge. Repro's or for that matter many
Auto store parts have very basic instructions, and will not provide you data on the
part modifications needed for the unit to perform in the Car you have. A simple bend on the float rod may do the trick for you. Before you do any of this make sure your
grounds are both clean and tight. A loose ground will give all types of crazy results,
so be sure to do this fist. The Tank ground on a second gen is especially bad for this
as it's in a spot that can easily get rusty. These are newly replaced units, so you should
have no issue at the sending unit for ground, but for sure check where that wire attaches to the Car for it's ground, and that the body to frame ground is good as well.
this is likely a single flat braided wire under the hood connecting the Block to the Firewall.

It is also possible that the production run of these parts (regardless of year) has an issue
and this can be in the potentiometer or how it's mounted on the sending unit. Are you
able to check the old against the new? for the float drop this would be especially helpful.