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1968 GTO 461 TH400 Auto
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Discussion Starter · #1 ·
I have 1968 Pontiac that my fuel gauge (stock) is not working properly. I purchased a new sending unit and installed, but I am only getting a reading of little over ¼ tank when full. I tested the sending unit off the vehicle and get zero ohms when empty and 90 ohms when full. With sending unit hooked up at empty I get empty, but at full I get little over ¼ tank on gauge. Tested gauge by unhooking OHM (brown) wire at sending unit and gauge went to full, then grounded brown wire and it went to empty. I am now confused I have read that 90 ohm sending unit is correct for this vehicle, I also read that the resistor on the back of the gauge may be bad. Is there a way to check the gauge for proper resistance? Do I need a 30 ohm sending unit? Can you purchase resistor for this gauge and if so what resistance?
 

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Gauge sounds like it is fine.Sounds to me like you have a 30 ohm unit , not a 90. That would explain it.
30 Ohms would register around 1/4 -3/8 tank
Ohm the unit in the tank. Or pull it out and ohm it. Check to be sure that the float can also rise to the top and is free of mechanical obstruction in the tank
 

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Also, just throwing this out there, but did you verify the swing of the unit? Just because you're getting a full swing on your bench, doesnt mean you are in the tank.
 

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1968 GTO 461 TH400 Auto
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Discussion Starter · #5 ·
Gauge sounds like it is fine.Sounds to me like you have a 30 ohm unit , not a 90. That would explain it.
30 Ohms would register around 1/4 -3/8 tank
Ohm the unit in the tank. Or pull it out and ohm it. Check to be sure that the float can also rise to the top and is free of mechanical obstruction in the tank
Checked unit it measures 90 ohms when lever is at full on sending unit but gauge measures about 1/4.
 

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1968 GTO 461 TH400 Auto
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Discussion Starter · #6 ·
Also, just throwing this out there, but did you verify the swing of the unit? Just because you're getting a full swing on your bench, doesnt mean you are in the tank.
I have tested the sending unit hooked up outside the tank it is a 90 ohm at full position and 0 at empty position, so it is a 90 ohm sending unit. Thinking maybe I need a 30 Ohm sending unit?
 

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I have tested the sending unit hooked up outside the tank it is a 90 ohm at full position and 0 at empty position, so it is a 90 ohm sending unit. Thinking maybe I need a 30 Ohm sending unit?
Yes, I understand what you did... However, when I was younger, I built fuel tanks and I was the one who installed the senders. Each sender had to be custom fit to each tank. So... If your sender has a 12' swing, outside the tank, and it reads 90 at full swing... That's all well and great... but if it can only swing 6" when it's installed in the tank, then it's not going to read properly.

Many/ most senders need to be adjusted/ calibrated, prior to installation... but some are a direct replacement. It's likely that yours was a direct fit, but this is a very common thing, so it's worth verifying. A picture of the sender would help.

Here's a picture of a GM sender. which is adjustable between 6"-24"! Not only is the height adjustable, but the float arm length must also be custom fit to each tank!
142471
 

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1968 GTO 461 TH400 Auto
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Discussion Starter · #8 ·
Yes, I understand what you did... However, when I was younger, I built fuel tanks and I was the one who installed the senders. Each sender had to be custom fit to each tank. So... If your sender has a 12' swing, outside the tank, and it reads 90 at full swing... That's all well and great... but if it can only swing 6" when it's installed in the tank, then it's not going to read properly.

Many/ most senders need to be adjusted/ calibrated, prior to installation... but some are a direct replacement. It's likely that yours was a direct fit, but this is a very common thing, so it's worth verifying. A picture of the sender would help.

Here's a picture of a GM sender. which is adjustable between 6"-24"! Not only is the height adjustable, but the float arm length must also be custom fit to each tank!
View attachment 142471
With the sending unit out of the tank but hooked up when I manually swing the arm to full it reads 90 ohms, but the fuel gauge only reads little over a 1/4 tank. When I swing to empty it does read empty and 0 ohms.
 

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1968 GTO 461 TH400 Auto
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Discussion Starter · #10 ·
OK not sure what exactly fixed it but I ran a ground from the block to firewall. Then I was going to run another ground from firewall to instrument cluster, but noticed the connections on back were little dirty so took nuts off cleaned put nuts back on and ShaZam it worked
 

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1967 Lemans 1967 GTO parts car
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OK not sure what exactly fixed it but I ran a ground from the block to firewall. Then I was going to run another ground from firewall to instrument cluster, but noticed the connections on back were little dirty so took nuts off cleaned put nuts back on and ShaZam it worked
I like ShaZam!
 
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