fuel gauge accuracy 1975 280z

[Dave WM]

I know its been covered in the past, I did a search and found an old thread, but what the heck here is what I found.

Mine would go well be low E and still have 4.5 gallons in the tank, and the low fuel light would not come on until I was about 2 gallons left IIRC from prior testing.

1st you can remove the sender without dropping the tank, just remove the one holding strap on the filler side (undo the j hook and let it hang out of the way.

Remove the filler neck guard plate, then twist the retaining ring, remove and then fiddle the sender out. No need to jack or remove tire (removing tire makes it a bit easier but not needed).

1st I check the ohm range and made sure had free movement, all good. then I noted the bend angle (draw on top of the bench some ref lines while holding the wire on the bench), bent it just a tiny amount down, reinstalled now I have nearly 1/2 tank, yikes.

so bent it back and try plan B, just this time rather than remove it I loosely attached the retaining ring and twisted the unit slightly clockwise. Voila now I am at about 1/8 tank (needle cover the vertical of the "F" in fuel) which is a lot better than completely below the E mark. This is with about 4.5 gallons, so say 2.0 gallon to spare before sucking air and having a bit of a safety margin I really have 2.5 gallons which is pretty close to 1/8. I tried to start out with it perfectly vertical (the sending unit) but if you are off only a few degrees it matters.

Anyway just thought I would mention.

 

[Dave WM]

Yesterday the low fuel light came on, I drove the gas station, indicator right at about 1/8th, 5 gallons to get to 1/2, 10 gallons to get to 3/4 and 14 gallons to fill (after restarting twice after 1st shut off.

So I looks like the gauge tracks pretty well and give me prob 2 gallons left (say a gallon that never gets used) when the low fuel light comes on. I feel pretty good about the calibration now. Now that I trust my fuel indicator I am one step closer to a road trip.

[jlenownnab]

I noticed yesterday after getting my gauge working from missing fuse that it is like a fishing bobber. My needle had a lot of back and forth movement when the gas was slushing around. Old skool, gotta love it!

[Captain Obvious]

Dave, I took some sender resistance vs needle position measurements off a spare fuel gauge I just picked up. Granted, this is a sample size of one, but here's; what I got.

Here's "running on fumes" empty at 100 Ohms:

Here's "just topped off" full at 10 Ohms:

And here's a whole bunch of points along the scale:

 

[Dave WM]

Looks perfect on the scale (10-100). I recommend you drain the tank, then put back 1 gallon (you might want to start the car and take it around the block just to make sure 1 gallon still allows the pickup to work), and reinstall. Shoot for E exactly at that point. You can adj by bending the float arm wire but a little there goes a LONG way. I found just minor tweeking a few degrees of vertical on the install of the sender worked perfect, but that was with my side mount, a top mount will be different of course.

Edited July 18, 2016 by Dave WM

[siteunseen]

I've never seen my light come on, maybe it's inop?  On the wonderful scale you have made for us where do you think the light should come on?  63 OHMs?  I might try and see if i can get it to come on, drive around the gas station's parking lot. 

And are the lights constant or do they get brighter the lower the level gets?  Thank you Captain Obvious.

[Dave WM]

the empty light comes up slowly as the thermistor warms up without the cooling effect of being submersed in a liquid. The thermistor resistance stays high limiting the current flow (and thus light from the indicator) current is always flowing thru the circuit but the high resistance of a cold thermistor limits the flow below what would cause the light to be seen. Current flow thru the resistance creates heat, with out the liquid to carry that away the thermistor begins to heat up, the resistance drops and the light comes on.

Thermistors are resistors that change value with heat. You can test the resistance with a simple meter. typical will be in the hundreds of ohms IIRC, FSM will tell you. best test is to drain the tank and give it time (couple min even). If it comes on then all you have to do is make sure its set at a level that gets it out of the fuel before you run out of gas. the light comes on rather slowly. One thing to check is make sure the light is not burned out AND its not pushed out of the bezel (mine was the plastic socket had fallen out or was pushed out so even if it came on I would not see it). Test it by shorting the wires of the plug where the sending unit attaches. this will check the wiring, bulb and placement.

Edited July 18, 2016 by Dave WM

[wheee!]

Lots of good info here!

[siteunseen]

When I shorted my 240's it went to FULL.  I may try and empty the tank, always need gas for the mower and I'm leery of running my $250 Bosch pump low of fuel.

[Dave WM]

the 280z with the indicator light has 3 term, a Black common and two others, shorting one to black will indicate full on the gauge, short the other to Black and the light will come on.

[siteunseen]

15 minutes ago, Dave WM said:

the 280z with the indicator light has 3 term, a Black common and two others, shorting one to black will indicate full on the gauge, short the other to Black and the light will come on.

Thank you.  That's an easy to do project, mine is accessible in the hatch.  From there I can see the light. 

[Captain Obvious]

Glad to help. Just remember that this is a sample size of exactly one gauge so far. No guarantees that others will be exactly the same. 

I'm not going to pull my fuel gauge out of my dash to do the same measurements, but I should be able to power it up through the dash connectors above the passenger's knee. I'll do that one when I get a chance to see how similar it is to the one on my bench.

My fuel gauge in the car isn't accurate, but I haven't even determined yet if the issue is in the dash or in the tank.