Fuel Tank Sending Unit Info

agent20x

agent20x

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Joined
May 14, 2008
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52
Location
Oxnard/San Luis Obispo, CA
Does anyone know what the operating range is for the potentiometer on the Fuel Tank Sending Unit (in ohms I'm guessing)? Also, would the higher end mean full and the low end mean empty?

Oh and the car is a 75 if it makes any difference
 
0-90 ohms is standard GM for the period. I don't recall if it is direct or reverse action.
 
agent20x said:
any ideas why its currently reading about 1300 ohms? I am measuring between ground and the wire coming from the sending unit (at least I thought I was)
Click to expand...

Lots of them.
1)Bad sender
2)Bad wire
3)Wrong wire
4)Bad connector at sender
5)Tank has bad ground (Check resistence from tank to frame)
 
Real easy way to test--

Plain standard model testlight with an 1894 bulb, disconnect the sending unit at the tank. Connect the testlight between the wire to the tank and a good ground. Turn the key on. Testlight should glow (not very bright, but it should come on) and the gauge read 1/2 tank. Will work for the temp gauge too.

:idea:
 
TimAT said:
Real easy way to test--

Plain standard model testlight with an 1894 bulb, disconnect the sending unit at the tank. Connect the testlight between the wire to the tank and a good ground. Turn the key on. Testlight should glow (not very bright, but it should come on) and the gauge read 1/2 tank. Will work for the temp gauge too.

:idea:
Click to expand...

well ill give that a shot and see what happens
 
Ok so does anyone know how the gas gauge works? Is it just an ohm-meter or does it display the voltage as a result of the voltage drop across the resistance of the sending unit?
 
The simple answer is that it's a gauge that reads voltage across a calibrated resistance circuit. The reading is converted by the scale on the front to read quantity instead of volts.

BAT+ ---------gauge---------sender---------ground


Now that is a simple schematic!!
 
It's 90 Ohm full, 0 Ohm empty. Make sure you have a proper ground on there, the sender is grounded properly, there's a little tab on the right side of the flange, the ground wire connects there, the sender wire obviously connects to the sender itself with a little elbow style socket.
 
well I was just able to determine the sending unit is working correctly, but I am still trying to figure out what the circuit looks like so I can try to adapt it to the new meter set that I got. The stock gauge picks up a ground somewhere between the gauge and and the sending unit, but I cant figure out where it could be in the circuit without shorting the sending unit
 
agent20x said:
well I was just able to determine the sending unit is working correctly, but I am still trying to figure out what the circuit looks like so I can try to adapt it to the new meter set that I got. The stock gauge picks up a ground somewhere between the gauge and and the sending unit, but I cant figure out where it could be in the circuit without shorting the sending unit
Click to expand...

I'm sorry, but you totally lost me.:huh:
 
Well I think I got everything figured out that I needed to. I am working on developing a circuit that will allow the use of the stock sending unit on aftermarket gauges (at least ones that operate in the 20-230 ohm range like auto meter). As soon as I have it ready I will post the schematics for anyone that might be interested.
 
You got the wrong autometer gauge, they have them in 0-90 also. The 33-240 ohm is reversed even. 0 empty 90 full vs 33 full 240 empty. You need to reverse the scale and change the output. Better get a different gauge.
 
Twin_Turbo said:
You got the wrong autometer gauge, they have them in 0-90 also. The 33-240 ohm is reversed even. 0 empty 90 full vs 33 full 240 empty. You need to reverse the scale and change the output. Better get a different gauge.
Click to expand...

They only offer the quad gauges in the 33-240 range, but I am an electrical engineer and I like doing these kinds of things :beer: Now that I have all the right info it wont be too hard.
 
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