'67 Fairlane gas gage reading wrong

Roger V

Roger V

Well-known member
My gage didn't read at all. I grounded the wire to the sending unit and the gage went from empty to full. I bought a new sending unit and it functioned outside of the tank as it should on the gage. After installing the sending unit, the gage reads half full when the tank is actually full and empty when the tank is half full. Comparing the sending unit to the original it duplicates it in dimensions, travel, etc. A true mystery. While I know it's half full when reading empty, it's too far off for comfort. Any solutions?
 
Is the gas tank getting a good ground? I would try adding a ground wire to the tank or better yet, try adding a ground wire to the sender itself.
 
If Lowriders suggestion doesn't work, then, maybe this. The span in Ohms resistance of the sending unit must be similar to what your gas gauge reads. Too much difference in readouts of sending unit and gauge will result sometimes in your problem. Need to determine your cars gauge specs and also the sending units output.
 
streetdreams said:
If Lowriders suggestion doesn't work, then, maybe this. The span in Ohms resistance of the sending unit must be similar to what your gas gauge reads. Too much difference in readouts of sending unit and gauge will result sometimes in your problem. Need to determine your cars gauge specs and also the sending units output.
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Probably, I'm not electrically savvy enough to do that. It seems that the proportion is right but its just half off no matter the level.
 
Roger V said:
My gage didn't read at all. I grounded the wire to the sending unit and the gage went from empty to full. I bought a new sending unit and it functioned outside of the tank as it should on the gage. After installing the sending unit, the gage reads half full when the tank is actually full and empty when the tank is half full. Comparing the sending unit to the original it duplicates it in dimensions, travel, etc. A true mystery. While I know it's half full when reading empty, it's too far off for comfort. Any solutions?
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I'm still leaning toward the 'bad connection' theory. When you tested the sender before installing it (and it worked correctly) did you connect it to the sender wire at the gas tank, or somewhere else? If you connected it at the tank end of the wire while it was not in the tank how did you ground the sender for your test?
As LOWRIDER suggested, give putting a better ground wire directly from the sender to the body another try. As much like when it worked correctly as you can.
If your 'good' test was done with the sender near the dashboard and a pair of jumper wires to connect it, that doesn't check the factory/original sender wire from the dash to the tank (which may have a connection/corrosion problem) or the ground connection from the sender top, thru it's mounting screws to the gas tank then to the car body. Poor contact in any part of the electrical path could throw the gauge reading off. Don't assume the gas tank has a good electrical connection to the car body.
.
 
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LOOK IN THE SHOP MANUAL OF YOUR FAIRLANE AND IT SHOULD GIVE THE RESISTANCE SPAN OF YOUR GAS GUAGE. THE SAME SHOULD BE AVAILABLE FROM THE PAPERWORK OR MANUFACTURER OF YOUR SENDING UNIT. if YOU CAN GET THAT DATA AND PUT IT HERE IN THE FORUM, THAT POSSIBLE SOLUTION CAN BE VERIFIED OR DISPELLED BY THIS GROUP.SORRY FOR THE CAPS. FAT FINGERED THE CONTROLS AND DIDN'T WANT TO RETYPE.
 
dmsfrr said:
i'm still leaning toward the 'bad connection' theory. When you tested the sender before installing it (and it worked correctly) did you connect it to the sender wire at the gas tank(yes), or somewhere else? If you connected it at the tank end of the wire while it was not in the tank how did you ground the sender for your test?(jumper wire from the sending unit to a good body ground)
as lowrider suggested, give putting a better ground wire directly from the sender to the body another try. As much like when it worked correctly as you can. (did that)
if your 'good' test was done with the sender near the dashboard and a pair of jumper wires to connect it, that doesn't check the factory/original sender wire from the dash to the tank (which may have a connection/corrosion problem) or the ground connection from the sender top, thru it's mounting screws to the gas tank then to the car body. Poor contact in any part of the electrical path could throw the gauge reading off. Don't assume the gas tank has a good electrical connection to the car body. (agree)
.
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streetdreams said:
look in the shop manual of your fairlane and it should give the resistance span of your gas guage. The same should be available from the paperwork or manufacturer of your sending unit. If you can get that data and put it here in the forum, that possible solution can be verified or dispelled by this group.sorry for the caps. Fat fingered the controls and didn't want to retype.
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(i'll see if i can find that data)
 
Your new sending unit float arm may need to be bent to get the travel adjusted properly with the fuel tank height.
 
Daves55Sedan said:
Your new sending unit float arm may need to be bent to get the travel adjusted properly with the fuel tank height.
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Perhaps, but it matched the original dimensions and travel. Hard to know how much to bend though a guess might be equivalent to half the travel.
 
streetdreams said:
LOOK IN THE SHOP MANUAL OF YOUR FAIRLANE AND IT SHOULD GIVE THE RESISTANCE SPAN OF YOUR GAS GUAGE. THE SAME SHOULD BE AVAILABLE FROM THE PAPERWORK OR MANUFACTURER OF YOUR SENDING UNIT. if YOU CAN GET THAT DATA AND PUT IT HERE IN THE FORUM, THAT POSSIBLE SOLUTION CAN BE VERIFIED OR DISPELLED BY THIS GROUP.SORRY FOR THE CAPS. FAT FINGERED THE CONTROLS AND DIDN'T WANT TO RETYPE.
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As an update, I am unable to find this information.
 
I agree with Dave to check float travel. If tank is empty, tie a piece of string to float arm and with it installed in tank, check guage reading while pulling up on string _don't have mounting screws too tight. Guage should read full when float hits top of tank. Will also check that float arm isn't hitting something on it's travel.
 
CrownVic55 said:
I agree with Dave to check float travel. If tank is empty, tie a piece of string to float arm and with it installed in tank, check guage reading while pulling up on string _don't have mounting screws too tight. Guage should read full when float hits top of tank. Will also check that float arm isn't hitting something on it's travel.
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While the principle seems good, the tank is full but due to the relatively horizontal fill tube to the license plate, I don't think it can be pulled-up to the top so it would be unknown as to where the float is.

It seems if there was an obstruction preventing the float from reaching the top of the tank knowing it is full, that when it "floats" down to half, it would read a half. When the tank is actually or approximately at a half, it reads empty.

Logically, this is beyond strange. On the Fairlane forum, another owner has the same problem and not-good-to-hear says he's been chasing this for four years! In his case, another strange thing is he says, if he took every tank mounting bolt out, it reads correctly. With just one even started, it returns to reading wrong! This would seem to be just the opposite relative to grounding. And the beat goes on..............
 
Hank said:
Check the instrument panel voltage regulator to make sure it is in proper working order.
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Not sure how to do that but will check the shop manual. BTW, Tom Petit who lives a mile from you actually installed the sending unit. He and I did the outside-the-tank check of the sending unit which then worked correctly.
 
Your sending unit should be a 78-10 ohm unit. If you checked the calibration out of the tank, then it reads 1/2 and E all the time upon installation, you have a potential wiring issue. Instrument voltage regultors tend to fail and give full voltage or no voltage. If your other gauges are working, the IVR is fine.
www.morrisgauge.com
 
Roger V said:
In his case, another strange thing is he says, if he took every tank mounting bolt out, it reads correctly. With just one even started, it returns to reading wrong! This would seem to be just the opposite relative to grounding.
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This comment reminded me of something possibly similar.
At night have you ever noticed an older car with 4 headlights, with one headlamp on and the other 3 are glowing orange? It took me a while to figure that out. It's caused by a bad/missing ground for the headlamp bulbs on the 'both orange' side of the car. The power to the one headlamp on that side has to travel thru the common filament connections of 3 bulbs to get to a good ground, behind the one bulb that's working correctly.

Check for a good clean connections on the ground cable from the body/firewall to the engine block and negative battery cable.
The positive battery cable is connected to the starter solenoid, then thru other wires to the fuses, switches and various accessories. The negative battery cable usually bolts to the engine block because that's the shortest ground path for the (high current) starter motor.

In order for any body mounted electrical accessory, gauge or light to function, each of those items has to have a good connection to the body and the body has to have a good connection to the engine block and/or negative battery cable.

It's possible that the reason the fuel gauge goes up, or reads incorrectly, when the sender is grounded is because some other voltage source (another dash gauge?) is using it to find a path to ground.
.
 
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The easy way to make sure what is going on is to remove the sender from the tank. Connect the feed from the gauge to the sender and connect an earth to the mounting plate of the sender unit. Turn the ignition on and move the sender through its travel to see what the gauge is doing. If it works correctly it is an earth problem on the tank which you can fix by connecting an earth to one of the sender mounting screws and screw it to a clean metal surface in the boot. It could also indicate that your float is being restricted in travel.
If it still behaves the same you have the wrong sender.
The problem someone else has mentioned about needing to remove all the screws for the gauge to work correctly suggests he has a sender unit of the incorrect value. Wit the unit unearthed they will find a path back to the battery however they can. What this guy is doing is increasing the resistance in the circuit.
Your problem sounds like your sender resistance is too high.
You may just simply have to bend the arm of the sender so it hits the bottom of the tank.
 
dmsfrr said:
This comment reminded me of something possibly similar.
At night have you ever noticed an older car with 4 headlights, with one headlamp on and the other 3 are glowing orange? It took me a while to figure that out. It's caused by a bad/missing ground for the headlamp bulbs on the 'both orange' side of the car. The power to the one headlamp on that side has to travel thru the common filament connections of 3 bulbs to get to a good ground, behind the one bulb that's working correctly.

Check for a good clean connections on the ground cable from the body/firewall to the engine block and negative battery cable.
The positive battery cable is connected to the starter solenoid, then thru other wires to the fuses, switches and various accessories. The negative battery cable usually bolts to the engine block because that's the shortest ground path for the (high current) starter motor.

In order for any body mounted electrical accessory, gauge or light to function, each of those items has to have a good connection to the body and the body has to have a good connection to the engine block and/or negative battery cable.

It's possible that the reason the fuel gauge goes up, or reads incorrectly, when the sender is grounded is because some other voltage source (another dash gauge?) is using it to find a path to ground.
.
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I had read somewhere before regarding a good ground from engine to body. I tried jumping from engine to body with no change. I "didn't" check the battery ground cable to the engine though it cranks well. I'll check that next. If it's grounding through a gage, etc., I don't know if I'll ever find it.
 
rick55 said:
The easy way to make sure what is going on is to remove the sender from the tank. Connect the feed from the gauge to the sender and connect an earth to the mounting plate of the sender unit. Turn the ignition on and move the sender through its travel to see what the gauge is doing. If it works correctly it is an earth problem on the tank which you can fix by connecting an earth to one of the sender mounting screws and screw it to a clean metal surface in the boot. It could also indicate that your float is being restricted in travel.
If it still behaves the same you have the wrong sender.
The problem someone else has mentioned about needing to remove all the screws for the gauge to work correctly suggests he has a sender unit of the incorrect value. Wit the unit unearthed they will find a path back to the battery however they can. What this guy is doing is increasing the resistance in the circuit.
Your problem sounds like your sender resistance is too high.
You may just simply have to bend the arm of the sender so it hits the bottom of the tank.
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As mentioned in the above posts, it works correctly out of the tank. In the tank the gage reads half when full and empty with the tank about half full. I installed a second ground wire from the sending unit to the body to no avail. The one thing that was not checked was ohm's but it read correctly out of the tank and grounded. When the tank gets low enough to drain, I'm going to remove the sending unit, try to see if any obstruction and reluctantly bend the float wire which will be a guess. I still come back to my earlier statement, that if it's obstructed from reaching the top of the tank, it should read correctly below the obstruction. I'll check the engine/body, etc. grounds first.
 
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