I am trying to verify the accuracy of the fuel gauge on my 91 I recently purchased. I suspect it may be stuck at Full or, at the least, very erratic. It showed Full when I pumped 5 gallons in. Hmmmm...not right.

Not a lot of info in the factory manual, but I did find the following on corvette web:Fuel Quantity Sender C4 Corvette Sensor

• 0 Ohms @ Empty, 45 Ohms @ 1/2 Full, 90 Ohms @ Full. Top of fuel tank, beneath filler pipe escutcheon (shield) panel.

I am changing brake lines, so the car is on jackstands and leaning a little forward. Gauge read Full. I did not take readings at first. My bad. I siphoned 5 gallons from the tank and got 68 Ohms, read 2.5 volts across black and purple when started the car, and the gauge was down one indicator bar. Okay...so there is life in there. I don’t expect accuracy with it leaning, I just want to see if the system is alive.

I adjusted the car on the stands and leveled it off a little, still leaning forward a few degrees. I took new readings for curiosity. Gauge pegged at Full again, 75 ohms, and 3.2 volts.

Questions for you smart guys:

1. Is the computer taking continuous readings or a periodic sample of the voltage across the tank sensor? The analog gauges I am used to will move with sloshing gas. I don’t know how digital gauges work.

2. I can find references to other GM fuel gauges having a max voltage of 5 when full and an increase to empty as the resistance drops. At empty, the sensor is pretty much going to ground with no resistance, so voltage flow is at max. Does anyone know what voltage I should read across purple and black when the tank is full? How about empty?

This car has been sitting for a while, so the sending unit may be dirty, corroded, etc. I plan to pull it for inspection.m, but I want a baseline before I do that.

If this information is in the factory manual, anyone can point me there. I can’t find it.

DWA

 

The processors in our ECMs are pretty old with rather sluggish refresh rates.
For example the tach does not keep up with RPM increase at WOT in 1st gear.
I'm sure the ECM does periodic sampling so suspect you won't see an effect from rapid fuel "sloshing" on your display.

Recently replaced the pump/gauge on my '88 but didn't measure resistance nor voltage outputs.
Fuel level reading seems rather accurate based on fuel added at fill up and prior display reading.

FYI: '88 FSM: the fuel level resistor should read 90ohm-full, 0-ohm-empty; the voltage is read by the inst. cluster.