Fuel Tank Sending Unit problem
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From: Egg Harbor Twp, NJ NJ
Fuel Tank Sending Unit problem
I have a 1982 Collector Edition and just replaced the Fuel Pump and Sending Unit assembly with one from Eckler's. The pumps OK, but the fuel gauge only reads 1/4 when the tank is full. When I move the float down, the gauge goes from 1/4 to empty smoothly. The float returns to all the way up when I release it. Eckler's says the sending unit is bad, but I suspect the resistance of the sending unit is wrong. The original unit measures about 1.2K, but the new one only reads 900 ohms. If I ground the lead going to the gauge, the gauge reads empty. With an open circuit the gauge pegs past full. When I installed a 1K resistor in series with the sending unit, the fuel gauge reads a little over full. Does anyone know what the correct resistance of the sending unit should be? The service manual does give this information. Any ideas? Thanks, Tom
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From: Myrtle Beach SC
Team Owner
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From: Crossville TN
Well, if you put a resistor in the line, it reads FULL. What does that set-up do when the tank is nearly empty? Maybe you just leave it like that.
{Can you just push the float down with something to 'simulate' an empty tank?}
{Can you just push the float down with something to 'simulate' an empty tank?}
Racer
Joined: Sep 2009
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From: Alvin Texas
I have the same problem with my '82 CE. The liner has developed a wrinkle on the forward side and when the float falls below that point, appx at the 1/4 tank, it will not come up when fueled, but can be pulled up with a bent coat hanger only to hang up when the gas is used to the 1/4 level. Old fuel gauge/ pump did the same thing. When I have time I am going to modify the float arm to clear the wrinkle.
Former Vendor
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From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
It could be the liner hitting the float... You might check this page out too! I don't think you need the page but you might upload it anyway.
http://willcoxcorvette.com/repairand...lp.php?hID=277
I think the ohms reading were more like 90 than 900 and some factory units will read higher than 90. But.. If you grounded out to empty.. and you create an open circuit and the gauge goes full.. then the gauge and all the wires to the gauge are good.
If you tested the sending unit at the tank for ohms and you got 90 when full, then something is wrong inside the tank.
http://willcoxcorvette.com/repairand...lp.php?hID=277
I think the ohms reading were more like 90 than 900 and some factory units will read higher than 90. But.. If you grounded out to empty.. and you create an open circuit and the gauge goes full.. then the gauge and all the wires to the gauge are good.
If you tested the sending unit at the tank for ohms and you got 90 when full, then something is wrong inside the tank.
Last edited by Willcox Corvette; Jan 11, 2012 at 08:36 PM.
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From: Bryan Texas
Former Vendor
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From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
Yes.. I was tired and should have completed the sentence..... you can add..
If you tested the sending unit at the tank for ohms and you got 90 when full, then something is wrong with the inside the tank or the liner.
With the simple test he completed he has eliminated the dash unit and the wires as the problem. If the sending unit puts out 90 ohms full and 0 ohms empty.. then then about the only other thing it could be is something blocking the float.. or possibly a ground on the sending unit.
Willcox
If you tested the sending unit at the tank for ohms and you got 90 when full, then something is wrong with the inside the tank or the liner.
With the simple test he completed he has eliminated the dash unit and the wires as the problem. If the sending unit puts out 90 ohms full and 0 ohms empty.. then then about the only other thing it could be is something blocking the float.. or possibly a ground on the sending unit.
Willcox
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From: Bryan Texas
I've been working on a 69 camaro. We put a brand new GM (made in china) senderr in it. Had to bend the float rod down an inch or so because we put 6 gallons in it and it barely registered on the gauge. We got it to read about a quarter tank. Then we filled the tank up and it only read 1/2 full. Pulled the sensor and checked it out of the car, tested fine. Then we put it in a bucket of gasoline and the float only came half way up. The float is good, just too much drag in the sensor itself. We have a new one coming.. never had so much trouble with a sender before.
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From: Egg Harbor Twp, NJ NJ
Thanks to all that provided information. Yes the sending unit is 90 ohms, I miss read my good old Simpson 260. I should have used a digital meter
Well the problem is NOT in the fuel tank. There's also a 90 ohm resistor across the fuel gauge. It appears that this resistor is open. These resistors make up a voltage divider circuit. When the tank is full (sending unit at 90 ohm) this circuit provides 6 volts to the gauge that equals Full. When the tank is empty, the sending unit outputs "ground" and the gauge moves to empty. Now I'm just waiting for the "new" assembly from Eckler's. I didn't think the original sending unit was bad, I just didn't know about the other resistor. As I don't want to open my console right now, I'm going to add a 90 ohm resistor to 12V to create a new circuit. I tested it out and the gauge works correctly.
Well the problem is NOT in the fuel tank. There's also a 90 ohm resistor across the fuel gauge. It appears that this resistor is open. These resistors make up a voltage divider circuit. When the tank is full (sending unit at 90 ohm) this circuit provides 6 volts to the gauge that equals Full. When the tank is empty, the sending unit outputs "ground" and the gauge moves to empty. Now I'm just waiting for the "new" assembly from Eckler's. I didn't think the original sending unit was bad, I just didn't know about the other resistor. As I don't want to open my console right now, I'm going to add a 90 ohm resistor to 12V to create a new circuit. I tested it out and the gauge works correctly.
Former Vendor
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From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
Thanks to all that provided information. Yes the sending unit is 90 ohms, I miss read my good old Simpson 260. I should have used a digital meter Well the problem is NOT in the fuel tank. There's also a 90 ohm resistor across the fuel gauge. It appears that this resistor is open. These resistors make up a voltage divider circuit. When the tank is full (sending unit at 90 ohm) this circuit provides 6 volts to the gauge that equals Full. When the tank is empty, the sending unit outputs "ground" and the gauge moves to empty. Now I'm just waiting for the "new" assembly from Eckler's. I didn't think the original sending unit was bad, I just didn't know about the other resistor. As I don't want to open my console right now, I'm going to add a 90 ohm resistor to 12V to create a new circuit. I tested it out and the gauge works correctly.
Well the problem is NOT in the fuel tank. There's also a 90 ohm resistor across the fuel gauge. It appears that this resistor is open. These resistors make up a voltage divider circuit. When the tank is full (sending unit at 90 ohm) this circuit provides 6 volts to the gauge that equals Full. When the tank is empty, the sending unit outputs "ground" and the gauge moves to empty. Now I'm just waiting for the "new" assembly from Eckler's. I didn't think the original sending unit was bad, I just didn't know about the other resistor. As I don't want to open my console right now, I'm going to add a 90 ohm resistor to 12V to create a new circuit. I tested it out and the gauge works correctly.You may have brought up one subject that I didn't cover when I did the manual and testing on the fuel gauges! The subject being: Will a gauge with a bad resistor still peg full when the input ohms is removed! I'm pretty sure it will but I will test this tomorrow and post back.
Former Vendor
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From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
Ok.. Tested a fuel gauge today and then thought... boy what a bone head. Of coarse it will pass the deflection test.
So everyone knows, when the gauge has a blown resistor the only way to be 100 percent sure the issue is the resistor is to power up the gauge (in or out of the car) and input ohm's to the gauge. Just like I stated in the write up I did a few years back, the gauge will only read slightly past 1/4 tank. The gauge will function back and forth but it will not go past the gauge.
In most cases you'll find the resistor to be the green one that is right at 90 ohms, but every now and then you'll get one with a orange resistor which is 88 ohms.
So everyone knows, when the gauge has a blown resistor the only way to be 100 percent sure the issue is the resistor is to power up the gauge (in or out of the car) and input ohm's to the gauge. Just like I stated in the write up I did a few years back, the gauge will only read slightly past 1/4 tank. The gauge will function back and forth but it will not go past the gauge.
In most cases you'll find the resistor to be the green one that is right at 90 ohms, but every now and then you'll get one with a orange resistor which is 88 ohms.
Last edited by Willcox Corvette; Jan 13, 2012 at 03:24 PM.
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From: Platteville WI
I'm following with great interest becasue I have the same issue with my 76. I've done all of the testing in the Wilcox write-up. The only thing left, I thought, was a collapsed liner inside the tank, interfering with the float.
By the way, I also found it convenient to swap resisters with one of the other gauges as a means of double checking the resister.
Incidentally, exactly where does one purchase a replacement resister? Radio shack just laughed at me.
By the way, I also found it convenient to swap resisters with one of the other gauges as a means of double checking the resister.
Incidentally, exactly where does one purchase a replacement resister? Radio shack just laughed at me.
Former Vendor
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From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
They are not available from GM or in reproduction.. But your guy at radio shack could have just helped you.. All you need is a standard resistor and put loops on the wire ends. As long as you can make 90ohms you'd be ok..
