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Erratic Fuel Sending Unit
- Thread starter TimK
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Occasionally the sliding wire contact in the sending unit gets erratic.
Loose wiring connections always cause problems.
The most common problem is the lack of a case ground wire on the dash unit and/or lack of a dedicated ground wire on the tank unit. Neither of these grounds were present on original cars. A poor ground at the dash can cause the fuel level reading to be off by 1/3 tank or more.
All original gages are subject to the float bouncing around as the fuel level sloshes.
Original & many replacement tanks have internal baffle plates to minimize side to side sloshing, but they all bounce when fuel sloshes from front to rear or vs.
With a sending unit in good condition & all grounds working, the gage gives a somewhat consistent indication of fuel level when the car is stopped on a level surface. Don't throw the broom handle away though.
D
Loose wiring connections always cause problems.
The most common problem is the lack of a case ground wire on the dash unit and/or lack of a dedicated ground wire on the tank unit. Neither of these grounds were present on original cars. A poor ground at the dash can cause the fuel level reading to be off by 1/3 tank or more.
All original gages are subject to the float bouncing around as the fuel level sloshes.
Original & many replacement tanks have internal baffle plates to minimize side to side sloshing, but they all bounce when fuel sloshes from front to rear or vs.
With a sending unit in good condition & all grounds working, the gage gives a somewhat consistent indication of fuel level when the car is stopped on a level surface. Don't throw the broom handle away though.
D
OP
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Thanks Dave, I'm not concerned about sloshing or bouncing of the gauge while driving; it is the inability to get a good reading when parked. The gauge either is dead empty or pinned on full. I suspect a bad ground and will try to add one now that you have pointed out there is no dedicated ground wire.
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Try a couple of tests. Disconnect the wire from sender to gage, AT the sender. Ground the wire coming from the gage. The gage should read empty or a little below. With the wire ungrounded the gage should read above full.If not, the gage or the wiring are defective. You can isolate the back of the gage & do the same tests to find if it is the gage or the wiring.
Measure the resistance from the isolated sender terminal to ground. with the tank full there should be around 70 to 80 ohms. With the tank empty the reading should be around zero to 7 ohms.
It may be easier to remove the sender for tests. Be prepared to replace the cork seal around the sender & there should be copper washers under the sender securing screw heads. Permatex "Hylomar HPF" is a good sealer to use on the gasket & screws.
Obviously there should be no fuel in the brass float. If there is, drill a small hole in it & drain. Solder the hole & submerge the float in hot water. Air bubbles will come out wherever the leak is. Solder the leak & retest.
As you move the float arm from empty position to full the ohmeter connected from sender terminal to ground should change smoothly from around zero to 70 ohms. Sometimes the arm can be making a poor ground contact on the sender body & cleaning or tightening the arm pivot can help. There is a small resistance wire coil that the arm travels over which gives the resistance changes with arm motion. It is pretty hard to repair the tiny wire but it can sometimes be done.
If you get a new sender (Moss is as good as any) do the same tests before installing things.
D
Measure the resistance from the isolated sender terminal to ground. with the tank full there should be around 70 to 80 ohms. With the tank empty the reading should be around zero to 7 ohms.
It may be easier to remove the sender for tests. Be prepared to replace the cork seal around the sender & there should be copper washers under the sender securing screw heads. Permatex "Hylomar HPF" is a good sealer to use on the gasket & screws.
Obviously there should be no fuel in the brass float. If there is, drill a small hole in it & drain. Solder the hole & submerge the float in hot water. Air bubbles will come out wherever the leak is. Solder the leak & retest.
As you move the float arm from empty position to full the ohmeter connected from sender terminal to ground should change smoothly from around zero to 70 ohms. Sometimes the arm can be making a poor ground contact on the sender body & cleaning or tightening the arm pivot can help. There is a small resistance wire coil that the arm travels over which gives the resistance changes with arm motion. It is pretty hard to repair the tiny wire but it can sometimes be done.
If you get a new sender (Moss is as good as any) do the same tests before installing things.
D
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Tim,
What some folks have done is this:
1) solder a jumper lead between the float swing arm and the base--can't remember if they tapped a hole in the base for a screw.
2) run a lead from a screw on the rectangular top to a good ground on the chassis. Use Hylomar to seal around the screw on top.
What some folks have done is this:
1) solder a jumper lead between the float swing arm and the base--can't remember if they tapped a hole in the base for a screw.
2) run a lead from a screw on the rectangular top to a good ground on the chassis. Use Hylomar to seal around the screw on top.
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Hi Steve,
As you say, a flexible copper braid running from the movable arm to the sender body will work but usually isn't needed.
The copper braid found at electronics stores & used as "Solder -Wick" comes in several thicknesses & works well for this purpose.
D
As you say, a flexible copper braid running from the movable arm to the sender body will work but usually isn't needed.
The copper braid found at electronics stores & used as "Solder -Wick" comes in several thicknesses & works well for this purpose.
D
OP
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I checked again and found the gauge is pinned at full. I just received Norman Nock's book of tech tips and noted that he said that grounding of the green w/black stripe wire (through chafing of the insulation) will peg the fuel gauge when the ignition is on. I'm going to work on this and your other suggestions. Thanks.
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Don't think so, if you mean pegged at full - A grounded green/black wire would give an empty reading. An open circuit would give a full gage reading. If you meant pegged at empty then yes.
The strange thing about this is that I "think" you said that the gage randomly goes from full to empty & vs. This would require two separate problems.
The only way this could happen is if the green/black wire is sometimes being grounded & at other times open circuiting.
Could possibly be a loose connection at the gage or inside that is sometimes losing connection & at other times grounding.
A remote possibility is the the green/black wire is sometimes grounding & also at other times the tank is losing it's ground connection.
Try the simple tests first. Lift the green/black wire at the tank unit. The gage should go to full. Ground this same wire & the gage should go to empty.
If there are no intermittent connections or grounds on the green/black wire, the problem is somewhere in the sender.
For more than you want to know about this gage, go here. Although it is written around an MGA gage, they are nearly identical to your Healey gage.
https://www.mgaguru.com/mgtech/electric/fg_05.htm
D
The strange thing about this is that I "think" you said that the gage randomly goes from full to empty & vs. This would require two separate problems.
The only way this could happen is if the green/black wire is sometimes being grounded & at other times open circuiting.
Could possibly be a loose connection at the gage or inside that is sometimes losing connection & at other times grounding.
A remote possibility is the the green/black wire is sometimes grounding & also at other times the tank is losing it's ground connection.
Try the simple tests first. Lift the green/black wire at the tank unit. The gage should go to full. Ground this same wire & the gage should go to empty.
If there are no intermittent connections or grounds on the green/black wire, the problem is somewhere in the sender.
For more than you want to know about this gage, go here. Although it is written around an MGA gage, they are nearly identical to your Healey gage.
https://www.mgaguru.com/mgtech/electric/fg_05.htm
D
After frequent erratic gauge performance problems followed by repeated cleaning I soldered a flexible braid to the swinging arm and looped it around one of the cover screws. Corrosion was affecting the electrical connection between the pivot pin and body. Pennsylvania humidity, temperature swings and condensation were at work. For many years now the gauge has been accurate and problem free. You have to be quick with the soldering because the solder wicks up the fexible braid in a hurry.
BJ8Healeys
Jedi Warrior
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If you have a replacement fuel tank sending unit from Moss with the plastic float, then remove the top from the sending unit and look at the rheostat coil underneath.
I got a replacement unit from Moss that was erratic, as you describe. I found the rheostat coil very poorly/sloppily wound. Instead of being smooth, tight turns like the original unit, the coil had a few turns here, and a few turns there, with gaps all over the place.
I got a replacement unit from Moss that was erratic, as you describe. I found the rheostat coil very poorly/sloppily wound. Instead of being smooth, tight turns like the original unit, the coil had a few turns here, and a few turns there, with gaps all over the place.