[Paul1958]

You need to check the actual resistance of the sender with a known quantity of fuel in the tank. Anything less than full is difficult to know exactly how much unless you drain/refill with a know quantity. It should check pretty close to 10 ohms full - start there. If not then you will need to pull it out and see if the float has failed or the arm is bent wrong or the variable resistor is failing, etc. Truth is that many of these senders on the market today are poorly manufactured and are junk.

Good luck
Paul

 

[Woodchuck]

A couple potential issues. Since you know that the scale of resistance is good, the problem must be that the arm is in the wrong place at different fuel levels. One possibility can be a partially sunk float...one that has a bit of a split and has partially filled with fuel, or a composite one that has become "fuellogged". I don't think this is your issue, though, as it would have been fine for a while and then changed.

What I think needs to be done is removal of the sender and a little persuasion of the arm, bending it toward the bottom of the tank to make the gauge read higher. One way to "fine tune" this would be to attach a long hose clamp to the float with the "tail" pointing down and then bending a "foot" at the end so that when it touches the bottom of the tank it keeps the center of the float about 2" above the bottom of the tank. Then you can insert, check, bend and repeat until your gauge reads on the "E" line at that point, remove the clamp and install for good. This should ensure that when your gauge hits "E" that you'll still have a couple gallons of gas in the tank.

 

[Harleydave]

I took mine out of the tank, extended the wire along with a temporary ground long enough to reach the front seat. Then I sat in the drivers seat with the ignition on and went through the range (F to E). It came out much better than when I started. But you have to remember these were not that accurate when they were new.

 

[tony's67mustang]

I replaced my tank and sending unit with the same product and have exactly the same issue.

 

[70M]

my setup was new as well (including dash gauge and tank) but not as far off as yours. (read just over 3/4 when full)

new probe in tank was fine, same readings as yours. i had to tweak gauge to match actual fuel qt.

adjusted Full - full (obviously ended up being my major adjustment)

drained 5 gallons. (my tank has drain) - verified 3/4

drained 5 more = 1/2, another minor adjustment on mine.

drained 5 more = 1/4 (close enough)

drained 5 more = empty but must have had a couple gallons left over as its a 22 gallon tank.

My boat was then refueled nicely!

bottom line: long day as that gauge was a pain to tweak. carried a spare can of gas and ran it down to E reading twice before trusting it. dislike the whole 70 gauge system, but it all works now. (with modern backup oil & temp gauges in console, screw trusting original gauges on those two readings)

 

[SA70coupe]

I am chasing an inaccurate fuel gauge issue. A couple of years ago I bought this kit from CJ Pony: fuel-tank-kit-premium-with-drain-plug-1965-1967 which comes with this sending unit: scott-drake-fuel-tank-sending-unit-3-8in-with-brass-float-1965-1968.

I tested the new sending unit and it read 0 - 73 ohms before I installed it, but the fuel gauge is reading 3/4 at full and empty by about 8 gallons (should be reading 1/2 full).

Are there any ideas what's going on here? I would love to get an accurate reading from the stock fuel gauge.

take out the sender and test it for accuracy. they are notorious for being off. If it is not accurate , then no matter what else you do you will not fix this problem
Here is a post for a simple tool to bench test your sender, for full, half and empty. A few bucks and you will be sure.
Make a tool to test your gauges - easy - 1969-70 Technical Forum - 69stang.com and 1969stang.com The 1969 and 1970 Mustang Supersite

If your sender is good, move on to the gauge. Unfortunately , both the sender and the gauge are items that are almost impossible to repair yourself. Test them and be sure of what to replace. it is a pity these items are so poorly made

 

[Pmustang]

I got so sick and tired of crap senders that I went overboard this last time

Now I have a "tanks" tank so my sender bolts in.

I spent $150 for a custom built to spec issapro sender. And $100 for a NOS gauge. Finally after decades of poor operational gauges I have one that is very accurate with low chance of failure due to the construction of the sender.

Peter

 

[70M]

I certainly like the potentiometer idea to tweak all the gauges. still wouldn't put my engine's life on the line with original temp/oil gauges though, modern style only for me.

but I do disagree with the idea everything has to be perfect. the gauge has the two adjustments for tweaking. just getting gauge/sender matched together will give accurate readings. No idea if my adjustments were making up for a gauge or a sender deficiency, but gauge is designed for minor adjustments and my system works as advertised now. ...if it wasn't for the cheesy gauge design itself, felt like a bull in a china shop working on it.

 

[Flade]

If your gauge is reading consistently low, then you have a couple potential issues. 1st of course I'd the sender. Use a meter to check the restance reading at the tank (between the sender post and the tank). If it reads correctly, you'd need to check your wiring between the gauge and sender. And excess resistance will cause a low reading. Make sure your tank is well grounded as well. It grounds through the mounting screws. Check resistance between the tank body and neg battery terminal. If you confirm you have good wiring and a good sender then consider replacing the gage itself.
One other potential issue is sender installation. If it is not installed correct.y it will read off.

 

[Franks68coupe]

My gauge is so far off, when it works at all. It reads full at full, and has 5 gallons left when it reads E, through keeping track of how much I put in and knowing the size of the tank. I know when it reads E... I have some wiggle room, but not to venture out too far.

This is among my long list of things yet to do.

 

[Dx2ry]

Is there a procedure you used to make the adjustment?

my setup was new as well (including dash gauge and tank) but not as far off as yours. (read just over 3/4 when full)

new probe in tank was fine, same readings as yours. i had to tweak gauge to match actual fuel qt.

adjusted Full - full (obviously ended up being my major adjustment)

drained 5 gallons. (my tank has drain) - verified 3/4

drained 5 more = 1/2, another minor adjustment on mine.

drained 5 more = 1/4 (close enough)

drained 5 more = empty but must have had a couple gallons left over as its a 22 gallon tank.

My boat was then refueled nicely!

bottom line: long day as that gauge was a pain to tweak. carried a spare can of gas and ran it down to E reading twice before trusting it. dislike the whole 70 gauge system, but it all works now. (with modern backup oil & temp gauges in console, screw trusting original gauges on those two readings)

 

[FordsForever]

Take the sender out so you can test and make adjustments. Drop the arm all the way down to the stop. it should measure 73 -78 ohms. If not adjust the down stop. if that doesn't do it bend the arm so it hits the stop and measures in the 73-78 ohm range. Then bend the arm so the float would be exactly at the bottom of the tank equal to the level of the end of the pickup screen.

Then raise the arm to the top and it should measure about 10 ohms at the upper stop. If not adjust the stop to get that reading. Once these adjustments are made the sender should be correct as long as the gauge and the CVR is working correctly.

If the other gauges are working correctly then the CVR is probably ok. Once the sender is adjusted correctly the only thing left as a problem would be the gauge. Make sure you have a good ground on the sender and the tank for consistant function. A bad ground to the sender can cause very erratic readings.
Ron

 

[70M]

Is there a procedure you used to make the adjustment?

a lot of forum threads and sites out there showing pics. this is just one pic randomly chosen.

inside of a temperature, oil pressure, and fuel gauge. Two elements contribute to gauge function-a bimetallic strip and heating element (resistor). As current flows through the resistor (heating element), it gets warm. As it warms, it acts on the bimetallic strip attached to the needle (by expanding), causing the needle to move to the right. The geared elements are the instrument's calibration points.

two basic adjustments. you can see the two holes and keyed plates that slide. it's "delicate" if you have big hands like mine, but not hard


my temp and oil work fine as stated above, but looking at a digital showing exactly 180 or 205 is much different than guessing how much the needle swayed and if its at 220/225/230. I spend allot of time idling in traffic and have a big investment under the hood. so I only rely on the fuel gauge where "close" is plenty good enough. ...temp and oil swing back and forth, I just pay no attention. but they look ...marvelous....

 

[22GT]

my setup was new as well (including dash gauge and tank) but not as far off as yours. (read just over 3/4 when full)

new probe in tank was fine, same readings as yours. i had to tweak gauge to match actual fuel qt.

adjusted Full - full (obviously ended up being my major adjustment)

drained 5 gallons. (my tank has drain) - verified 3/4

drained 5 more = 1/2, another minor adjustment on mine.

drained 5 more = 1/4 (close enough)

drained 5 more = empty but must have had a couple gallons left over as its a 22 gallon tank.

My boat was then refueled nicely!

bottom line: long day as that gauge was a pain to tweak. carried a spare can of gas and ran it down to E reading twice before trusting it. dislike the whole 70 gauge system, but it all works now. (with modern backup oil & temp gauges in console, screw trusting original gauges on those two readings)

First, you need an OEM sender, or one of the few on the market that are accurate, most senders are hopeless. Then do as described above to calibrate the gauge to the sender. Better-than-new accuracy can be achieved.

 

[Benny]

My gauge is so far off, when it works at all. It reads full at full, and has 5 gallons left when it reads E, through keeping track of how much I put in and knowing the size of the tank. I know when it reads E... I have some wiggle room

Mine does the same....full at full, then empty when there is plenty of fuel left.

But, at least mine does work all the time!

 

[Woodchuck]

First, you need an OEM sender, or one of the few on the market that are accurate, most senders are hopeless. Then do as described above to calibrate the gauge to the sender. Better-than-new accuracy can be achieved.

The aftermarket senders may be accurate at the full and empty positions, when the arm is properly adjusted, but will be less accurate the farther from full they get. This is because the original variable resistor used in the sender from Ford was non-linear, meaning that the resistance decreased at a progressively faster rate the farther the arm went down, to compensate for the shape of the fuel tank. The new, replacement senders contain a linear variable resistor which reduces resistance at a constant rate from one limit to the other.

 

[22GT]

The aftermarket senders may be accurate at the full and empty positions, when the arm is properly adjusted, but will be less accurate the farther from full they get. This is because the original variable resistor used in the sender from Ford was non-linear, meaning that the resistance decreased at a progressively faster rate the farther the arm went down, to compensate for the shape of the fuel tank. The new, replacement senders contain a linear variable resistor which reduces resistance at a constant rate from one limit to the other.

Not all of them. I'm not referring to bending the float arm. That only gives a correct reading at full or empty.

There is a better source for senders. A unit offered by the Mustang Barn, made expressly to their specification, has the OEM correct Ω value both at full and empty. Should work well out of the box. If you take the additional time to adjust the fuel gauge, which typically is 40+ years old, and you will have the accuracy of an aviation gauge.

 

[George.Galo]

Not all of them. I'm not referring to bending the float arm. That only gives a correct reading at full or empty.

There is a better source for senders. A unit offered by the Mustang Barn, made expressly to their specification, has the OEM correct Ω value both at full and empty. Should work well out of the box. If you take the additional time to adjust the fuel gauge, which typically is 40+ years old, and you will have the accuracy of an aviation gauge.

How do you adjust the gauge? Mine pegs past full, stays there for what seems longer than it should. I never let it go lower than a quarter tank, and it takes about 12 gallons there, so the low end seems at least kind of accurate. If I adjust for full, will I screw up the lower end?

 

[22GT]

No. There are two small holes on the back, with the edge of the adjusting gear showing. These allow precise adjustment of the E and F readings. You might argue that the 1/2 point is not perfect, but who cares, if the E and F are perfect.