Fuel gauge issues and adjustments

[Dean 1]

Hi, for what it is worth I had an issue with the fuel gauge on my 1970 convertible.  It was constantly showing empty and after trying various things including grounds and wire connectivity, I replaced the sender unit in my fuel tank.

My understanding is that ford gauges work off a variable resistance (approx between 0 - 78 ohms) from the sender unit.  (~0 ohms = full & ~78 ohms = empty).

Once I replaced the sender unit, the gauge did start working but a full tank of gas sent the needle a fair way past the f mark on the gauge.  This was annoying me (I also don't like picture frames hanging on walls at an angle) so I manually added a small amount of resistance (cut into the sender wire) until the gauge needle sat exactly on the F (full) mark.

This trick has worked well and it can assist in adjusting the gauge reading based on the sometimes pretty poor and inaccurate fuel sender units presently available. 

[Dave R. 85]

Did this make the gauge accurate at 3/4, 1/2, 1/4 and EMPTY?      Dave R. 

[bigmal 225]

Hi Dean,

 I had similar issues. I bent the float arm so it rea E when the tank was empty and it pretty good for the other figures. FYI, I'm on my third float (all metal) as the others sank when they filled with fuel. they don't seam to like our unleaded. 

[RPM 1,190]

2 hours ago, Dave R. said:

Did this make the gauge accurate at 3/4, 1/2, 1/4 and EMPTY?      Dave R. 

Ditto. The E mark is the only important mark. I had the adjust my float arm to get is close. On an empty tank I started adding fuel till it moved off of E. I have 2+ gallons of fuel when it hits empty. It won't hit the full mark, but half a tank on the gauge is pretty close to half a tank of gas. Ymmv.

[Midlife 814]

You can also adjust the gauge reading by adjusting the potentiometer on the back of the constant voltage regulator.  Sometimes, it is potted in epoxy.  This small screw or post adjusts how long the bimetallic strip spends between 12V and 0V, adjusting the output voltage.  I adjust things so that when I am sure I have a quarter tank of gas, the gas gauge reads 1/4 full.  This is probably the most important reading on the gas gauge.

By the way, the range of resistance is 13 to 79 ohms or thereabouts. not 0 - 79.

[dream car 12]

Can you do this to oil pressure gauge?  my replacement meter don't match the sender unit. It read 100+ when idling.

[Mach1 Driver 560]

If it were me, I would get rid of the original gauge constant voltage regulator, and replace it with a modern electronic one. The old ones try to provide around 5v by pulsing 12v on about 42% of the time and off about 58% of the time. Since the gauges are dampened it doesn't affect the readings. The new electronic regulators give a rock solid 5v.

Just for some background information; Ford intentionally developed gauges that take time to react- they call this dampening. This is so the gauge doesn't bounce all over when you take a curve or do a jump like the General Lee. All the gauges are reportedly the same except for the scale. Even though the oil sender measures pressure and the water sender measures temperature, and the fuel measures fluid level, all three sensors are just variable resistors that read from 78 ohms on Low to 10 ohms on Hi. The gauges have high and low adjustment screws, and an internet search turned up this information- hopefully it is accurate. What he is saying below is that the top screw adjusts the low end of the scale and the bottom screw adjusts the high end of the scale. Within some (unknown) "range" they are all adjustable.

2007 jonsee on allfordmustangs

Yes there are two adjustments on the back of the three gauges, oil, temp, and fuel - the adjustment on the top is to adjust the zero at 70 -73 ohms and the bottom adjustment is to adjust the 100% reading (better known as the span) at 10 ohms - with 5 -6 volt supply, an increase in resistance decreases current flow through the gauge (better known as indicator) which decreases the reading and a decrease in resistance, increases current flow through the gauge increasing the reading.

[Midlife 814]

Huh.  I'll have to examine some of my gauges.  Usually, all one sees are the 2 posts sticking up through the dash cluster where you attach the wires (before 69) or the nuts that hold the circuit card on. 

[Mach1 Driver 560]

11 hours ago, Midlife said:

Huh.  I'll have to examine some of my gauges.  Usually, all one sees are the 2 posts sticking up through the dash cluster where you attach the wires (before 69) or the nuts that hold the circuit card on. 

How do my 69 gauges differ from previous years Mid? I haven't taken mine apart to look for adjustment screws (yet).

[Midlife 814]

The gauges really don't change in their basic design; only how they fasten to the dash cluster housing. 

Well I'll be damned!  I looked at a 69 fuel gauge, and sure enough, there are two small holes, and barely sticking out below them is a white cogged wheel.  It does take a fair amount of force to move the cog.

[aslanefe 333]

2 hours ago, Midlife said:

The gauges really don't change in their basic design; only how they fasten to the dash cluster housing. 

Well I'll be damned!  I looked at a 69 fuel gauge, and sure enough, there are two small holes, and barely sticking out below them is a white cogged wheel.  It does take a fair amount of force to move the cog.

Randy, I couldn't find the fuel gauge I have from an 80's Ford truck to check, can you remove the back cover on that gauge and see if the cogged wheels only adjust the high and low stops of the needle or do something else?

[Midlife 814]

Ummm...no.  Unless the gauge is toast (which this one isn't), I'm not breaking into the gauge itself.  Those in my inventory that are toast are very rusty and probably don't want to be broken into without a lot a swear words... 

 

Another way of saying things: I don't have any gauges that are disposable!

[aslanefe 333]

No worries, I'll bump into the truck gauge I have one day and open it up if it has the holes and cogs.

[Mach1 Driver 560]

I just saw this great picture courtesy of Jharcinske on VMF. Ford developed a gauge that doesn't use the typical D'Arsonal meter movement. It is a bi-metal device with a heater to make it move. That's why these things have so much dampening- its inherent in the design, and probably costs much less than a typical meter movement. A little more explanation is probably in order here: a bi-metal is two sheets of dissimilar metal formed together in layers like a sandwich. They use materials that have greatly different expansion rates. When heat is applied they bend, because one side expands faster than the other. The bending is very predictable and repeatable.

In the same thread (about gauges) I got a response from TheRktmn who owns a gauge repair shop. I asked him about the the two adjustments and he responds:

They set the high and low points for the gauge. You will need a face on there to do it, but with a 73 ohm resistor for low and a 10 ohm resistor for high and a 5VDC power supply you can set the gauge back to factory specs.
Use a small screwdriver in through that hole to engage the teeth and move the arm. You will see the pointer move.
Once you change one you need to check the other since it will change some too.

[Midlife 814]

Nice!

[aslanefe 333]

That is a good picture you found Mach1 Driver. Looks like the adjustments change the sweep angle of the needle, and the needle position at full and empty.

[RPM 1,190]

Nice work on your end Terry for being a big part in finding an answer to our fuel gauge fiasco. Bravo!