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Fuel guage problems

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BIGCHEVY4X

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i have an 80 blazer with a fuel guage that reads 1/2 tank when full and its driving me nuts.....i disconected the fuel sender wire at the frame and when i ground it it goes to E but when i open it it only goes to 1/2 tank so that means that its in the wiring or the guage right? Is there any way of checking the guage or terminals with an ohm meter? I was also thinking of running new wires but im not sure which terminal is which?
 
Grounding the sneder wire should make it read full not empty. If that's the case then the sender is bad.
 
BIGCHEVY4X said:
how could that be if the sender is disconected?
Click to expand...

GM uses ground to complete a circuit. Grounding the guage wire completes the circuit and will show as a full tank on the guage. If that happens then you know the guage and wiring back to the sender is OK and your sender is the problem. There are very few times that the actual guage fails and more often the wire grounds out giving a false "full" reading on the guage.
 
Grounding the gauge will read E.
An open in the circuit will read over full.
If your gauge never reads past 1/2 then the crystal attached to the 2 posts on the back of the gauge has a crack in it. Get a new gauge.
 
I took the guage out of the dash and tried to hook it up to a power and a ground and the sender and i got it to read past full but cant get it to read about 3/4 full where its supposed to be
 
Because that is not how it is wored in the truck.

Check the resistance of the sender in the tank. It should read smoothly from around 10 ohms to around 90 ohms as you move the wiper.
They advertise it as 0-90 ohm, but most I have seen are around what I posted.
 
i havn't had time to test my fuel gauge but i have the same problem. my gauge will read random things, more so empty then anything else. but if i tap on the gauge it goes up. i think my problem is the gauge without even testing it.
 
readymix said:
0 Ohms = Empty
90 Ohms = Full
Click to expand...

From the GM Service manual for 10-30 series 1987 Light Duty Trucks
page 8C-4 Instrument Panel
Gauge Reads Between 1/4 & 1/2 with 90 OHMS from J-24538-A (their tester)

page 8C-2 Instrument Panel
Fuel Gauge
The fuel gauge is an electrical, current sensing type of indicator. It has two coils in it. One coil sets up a constant magnetic field. The other coil has a varying magnetic field which is varied by the reostat attached to a float in the fuel tank. A magnet, attached to a pointer, is located between the two coils. The magnet will establish a position which is controlled by the magnetic fields of the two coils. Refer to figure 2 for diagnosis.

I will scan and put up Sunday night. It just says to take wire of check this and check that. The only major thing is what I listed above (page 8C-4 Instrument Panel) it all basically says if all else fails replace gauge.
 
readymix said:
0 Ohms = Empty
90 Ohms = Full
Click to expand...



gauges read full when grounded... end of story... I only test dozens a year...
 
Ford sytle gauges or Chevy?

I don't remember mine being that way when I burnt the rheostat out in the sender. It also doen't make any sense if the sender has a low resistance when the tank is empty. :thinking::thinking:

If I get a chance I will ground mine today. Not sure if I can though....the engine is in pieces right now.
 
I am seeing it written both ways when I search through Google.
Now I really need to figure this out. If they do read full when grounded, why are the senders advertised as 0 Ohms = empty?
 
If your gauge never reads past 1/2 then the crystal attached to the 2 posts on the back of the gauge has a crack in it. Get a new gauge.
Click to expand...

Mine stays on less than a 1/4 tank all the time,but I contributed that to it originally being a 24 volt system.
 
  1. Using Gauge Tester (J 24538A). Disconnect feed wire from fuel gauge tank terminal and connect one test lead to wire and ground other lead. Turn ignition on. If gauge responds but not accurately, proceed to step 2). If gauge does not respond, go to step 3). If gauge responds accurately, go to step 5).
  2. Remove gauge and check for loose nuts at gauge terminals. If nuts are loose and gauge reads between 1/4 and 1/2 with 90 ohms from tester, tighten nuts and install gauge. If gauge is inaccurate in other ways and/or nuts are tight, replace gauge.
  3. Disconnect front body connector. Connect tester to lead that goes to gauge. If gauge responds accurately, check wiring between rear compartment and front body connector. If gauge does not respond, go to step 4).
  4. Remove gauge. Check for bad connections at gauge terminal or instrument cluster connector. If connections are good, replace gauge. If bad, repair connections and install gauge.
  5. Check rear compartment connector and wires to sender. If okay, replace sender. If not okay, repair wire or connector.
 
Here is how I see it working. If I am wrong someone please correct me with facts. I really would like to get this whole thing straightened out. Looking over the internet I see it posted more than 3 different ways. Someone has to have a good understanding of this circuit.....Dorian?

Refer to the schematic below.
The sender works like this:
0 Ohms = Empty
90 Ohms = Full

When the tank is empty the sender is at around 0 Ohms. This causes the maximum current to be applied to that side of the gauge windings....creating a large magnetic field....drawing the needle armature towards the "E" side of the gauge.

When the tank is full the sender is at around 90 Ohms, causing less current to be on that side of the gauge windings. This will cause the needle to move toward the "F side of the gauge since that side is attached directly to ground.

I am not sure what the valuse of the windings are, but would love to know.

Anyone else have any insight?

fuel tank sender.jpg
 
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