fuel gauge voltage?

[BerniniCaCO3]

Hi--

My fuel gauge won't read more than 3/4 full
I'm operating on the theory that a low voltage sent to it, could result in lower fuel readings than there in fact are.

For this theory to be true, it implies that the fuel gauge float/sending unit would have 0 resistance when full and higher and higher resistance as the float drops down. In that case, low voltage going to it in the first place would result in a lower fuel level reading.

If in fact it's the opposite, that 14V=empty and 0V=full, then the gauge would always read higher, and my idea is wrong.

Know how it works?

[turbo2256b]

The gage itself works on a pulsating 5 volts. There is a box above the glove compartment most all the gages and indicator lights run through.
The sending unit it self is tested by ohms ... think its 20 empty and 73 full. might have that backwards. Havent got into it for a while someone in here might have more info than I remeber without looking it up.

[GM_Guy]

The guage voltage regulator is on the back of the instrument cluster.

Check your guage fuse and its corresponding socket for corrosion (swap out the fuse things look clean). Based on what you said in the other thread about reading 7 volts on the w/p wire.

8 ohms full - 86 ohms empty according to 'the book'.

The box above the glove box is the low fuel warning module. The ones below, if equipped are low oil and a heated windshield module.

Alex.

[86VickyLX]

The guage voltage regulator is on the back of the instrument cluster.

Check your guage fuse and its corresponding socket for corrosion (swap out the fuse things look clean). Based on what you said in the other thread about reading 7 volts on the w/p wire.

8 ohms full - 86 ohms empty according to 'the book'.

The box above the glove box is the low fuel warning module. The ones below, if equipped are low oil and a heated windshield module.

Alex.

the 90 is in the cluster for low fuel module

[BerniniCaCO3]

OK, looked at it partially today.
It has, from long experience, ~2 gallons left in the tank.

I unplugged the 2-wire harness going in to the back of the sending unit. Hot-ground read 8 volts and change. Unhooked, the gauge reads ~3/4 full. Although, it's always read around that when there's no power to it: the needle just hangs something not quite vertical from gravity when there's no magnetic force on it. So meaningless. The 8volts was odd, as you're right, the book (p100) says that the hot for the fuel gauge sender passes through the voltage regulator and should be at 5 volts. Bad voltage regulator? EXTRA voltage = reading high, since low resistance equates to a full tank. That might explain why my fuel gauge has always always lived in the 1/4- 5/4 full range. It'll never show empty; just under 1/4 tank means about 1 gallon and a gas station asap. It wouldn't necessarily explain this new 3/4 tank cap that I'm experiencing: it used to have the needle swing all the way out of sight, then count back to 1/4 tank.
Now it sits at 3/4 tank for 100 miles, and only then counts back to 1/4 tank; as if it works normally below 3/4 tank but won't move past that when truly full.

I had a colleague in the car and the key in position II, engine not running. I grounded out the 8V (0 ohm in effect). Gauge didn't move, he said. Dunno what that means for my diagnosis.
I tested the resistance on the sending unit itself. Again, @approx. 2 gallons. Read around 60ohms. Doesn't sound far off. Truly empty tank is 86ohms by the book.

I'm not sure if I want to refill the tank completely tomorrow and test resistance again, because if I determine it IS the sending unit, I'll want my near-empty tank!

I have not, as of yet, tested the voltage AT the guage. Anyone know a good harness point that doesn't require removing bits of the dash and the instrument cluster, to save a little time?

When I last had the instrument cluster out, just a week and a half ago to re-replace some shitty chinese light bulbs that came burnt out, the needle does swing with no perceptible resistance to my merely human fingers. Doesn't mean that it's not hanging up past the 3/4 mark, but, it's at least not hanging up to any degree that I can in the least perceive. A physically sticky needle was a possibility, and if it is, I can't tell.


That's where I'm at.
Let me know what the above measurements might mean, if you have a diagnosis for me based on the above, or if you don't, what I should be doing tomorrow evening to narrow it down.