How Ford Gauge Unit Work

The gauges don´t care about voltage !!
What you need to keep down is the current rush (amperage).
If you let the current rush freely the gauge will be jumping around keeping it low will give you a slower movement.
So the voltage drop will give you less current flow since resitance in the circuit is constant.
A step down converter ready built is starting at about $1 today...you want them to deliver over 3-4amps they start to get expensive.
 
rotorwrench said:
I've heard both sides of the coin on the rheostat variable resistance types. Most resistance based units use a specific Ohm resistance range and they vary between manufactures of the different units. Many claim that the repos work OK and then many claim they don't indicate accurate quantities. I think if you play with the adjustments you can gain some accuracy within a narrow band of meter movement but you can never get them to be accurate through out the range of movement. The resistance levels are more minute for working a current based indicator and the rheostats just can't attain the accuracy in those small levels of resistance change but they will give you some idea whether you are getting close to empty which is likely the most important use of a quantity indicating system. You may not know how much is left but that won't matter as much as the system causing you to run out of gas on the side of the road somewhere.
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I can't really argue with anything you've said here, except to add that virtually all gauges are really just ammeters calibrated to read whatever we're measuring. Therefore, there is a range of resistance that will calibrate correctly - whether or not the commonly available aftermarket sending units are precise enough is another debate.


And all I can add is if you've got a 1940 Car/Truck with a BATT gauge instead of an Ammeter, all it does is replace the sending unit with a 60ohm resistance built-in. Therefore, when converting to 12V, adding another 60ohm resistor in series with the gauge will produce a pretty good 12V BATT gauge.
 
Flat Ernie said:
I can't really argue with anything you've said here, except to add that virtually all gauges are really just ammeters calibrated to read whatever we're measuring. Therefore, there is a range of resistance that will calibrate correctly - whether or not the commonly available aftermarket sending units are precise enough is another debate.


And all I can add is if you've got a 1940 Car/Truck with a BATT gauge instead of an Ammeter, all it does is replace the sending unit with a 60ohm resistance built-in. Therefore, when converting to 12V, adding another 60ohm resistor in series with the gauge will produce a pretty good 12V BATT gauge.
Click to expand...

The problem is that with fords ohm range being very low its hard to use a resistive sender.
Since the sender is wirewound it will go in steps changing from one wire to another and with a low ohm range each step will make a bigger change in the reading.
And the sender can´t be linear in it´s winding or only empty and full reading will be accurate.
This is one the reason modern instruments went to higher ranges like 120/240 ohm.
 
An ammeter is a completely different type of meter than the various pressure, temperature, and quantity units made by King Seeley. Ammeters have a d'arsonval type meter movement that measures current difference between two sides of a circuit through either a shunt or a shunt wire arrangement. Ford's use of ammeters was spotty since they found that an induction type meter worked more reliably for the purpose of a charge or discharge meter since that is basically all they are. This way they didn't have to worry about a shunt and it was more simple plus less expensive to produce.

King Seeley units were more of a variable amperage device with the device on the other end mirroring it's position. Since it uses temperature sensitive bimetallic switches, it could be easily adapted for both temperature and pressure sensitive reading devices. On the fuel quantity it was a pressure sensitive device with the float arm providing different pressures depending on the level in the tank. A very simple but ingenious device.

Ohmic type rheostats start wearing out the second you start to rub the brush on the wire wound resistor. It's just a matter of time before the brush wears down or the wire wears through making an open circuit. The more wear they have the less reliable they become due to wear and crud build up.
 
flatheadmurre said:
The problem is that with fords ohm range being very low its hard to use a resistive sender.
Since the sender is wirewound it will go in steps changing from one wire to another and with a low ohm range each step will make a bigger change in the reading.
And the sender can´t be linear in it´s winding or only empty and full reading will be accurate.
This is one the reason modern instruments went to higher ranges like 120/240 ohm.
Click to expand...
Not sure what you're getting at - the current aftermarket sending units do work reasonably well and are a reasonably close match to work pretty well with original gauges.

That said, I only replace the original senders if they're broken and I can't find another...but it's nice to know the aftermarket senders can be made to work reasonably well.

rotorwrench said:
An ammeter is a completely different type of meter than the various pressure, temperature, and quantity units made by King Seeley. Ammeters have a d'arsonval type meter movement that measures current difference between two sides of a circuit through either a shunt or a shunt wire arrangement. Ford's use of ammeters was spotty since they found that an induction type meter worked more reliably for the purpose of a charge or discharge meter since that is basically all they are. This way they didn't have to worry about a shunt and it was more simple plus less expensive to produce.
Click to expand...

Not sure if you were replying to my post about all gauges essentially being ammeters. You are technically correct about ammeters and the various types. My point was more practical - all we're really measuring on the gauge is the total current flow (amps) of the circuit, hence, the gauge is effectively an ammeter calibrated to the application (oil, temp, fuel).


rotorwrench said:
Ohmic type rheostats start wearing out the second you start to rub the brush on the wire wound resistor. It's just a matter of time before the brush wears down or the wire wears through making an open circuit. The more wear they have the less reliable they become due to wear and crud build up.
Click to expand...
Again, technically correct, however, in these applications, the wear is relatively minimal and insignificant in the grand scheme of things. As I mentioned above, however, I will always reuse the original sender if it's in good shape and working properly, even with a 12V conversion.




Something else no one has addressed is the adjustment of the early gauges - on most of these gauges, there is an adjustment to change the bias of the bimetallic spring to add or reduce the at-rest tension.
 
All this "technically correct' is NOT technically correct. Ammeters do NOT measure flow but measure voltage across a fixed resistance calibrated (or not) in Amps using a d'Arsenval movement.

Ford applications of 'Gas Level' 'Oil Pressure' and 'Temperature' do NOT use d'Arsenval movement gauges but THERMOSTATIC gauge which, as JSeary has tried to point out, is different.

Resistance senders were considered hazardous early on because the armature must transition from one coil to the next. So Ted Smulski invented and patented the thermostatic gauge system, which is odd considering the sender must open and close a set of terminals in the flammable fluid.

Ted's patent used the term 'thermostatic gauge' - it was assigned to Anderson Corp who contracted King-Seeley to manufacture gauges/senders. The patent claims was compact, convenient, duragle and not sensitive to voltage.

Senders - resistance vs thermostatic. Actually, were a d'Arsonval meter be used with a thermostatic sender, the gauge would pulse hi-lo-hi-lo but a thermostatic meter evens out the peaks and presents a true ratio of on-off periods.

Ironically, a year after the Smulski patent, Joe Zublaty was granted a patent on a resistance gauge because the flamability nature of gasoline was found not a danger with a resistance sender. A galvanometer or ohm meter was used for the readout.

King-Seeley filed for a resistance gauge patent in 1954 obviously with knowlege of Ford's conversion to 12v. I've heard Ford switched to resitance fuel gauge in 1955 for one year switching back until 1980 but can't confirm it.
 
It would seem most posters have their own way of thinking how electricity works and how gauges fit into the equation. I could quibble but won't. What it boils down to is interesting and I do hope JSeary is able to recover from his seemingly futile attempt to educate the troops but not so futile, really.

Ford gauges boil down to politics and economics to which we are not privy. Some of the history we do know.

Ford applications of 'Gas Level' 'Oil Pressure' and 'Temperature' do NOT use d'Arsenval movement gauges but THERMOSTATIC gauge which, as JSeary has tried to point out, is different. This undoubtly stems from:

Resistance senders were considered hazardous early on because the armature must transition from one coil to the next. So Ted Smulski invented and patented the thermostatic gauge system, which is odd considering the sender must open and close a set of terminals in the flammable fluid.

Ted's patent used the term 'thermostatic gauge' - it was assigned to Anderson Corp who contracted King-Seeley to manufacture gauges/senders. The patent claims was 'compact, convenient, duragle and not sensitive to voltage'. The Oil Pressure and Temperature gauge applications tagged along soon afterwards.

Ironically, a year after the Smulski patent, Joe Zublaty was granted a patent on a resistance gauge because the flamability nature of gasoline was found not a danger with a resistance sender. A galvanometer or ohm meter was used for the readout.

King-Seeley filed for a resistance gauge patent in 1954 obviously with knowlege of Ford's conversion to 12v. I've heard Ford switched to resitance fuel gauge in 1955 for one year switching back until 1980 but can't confirm it.

.
 
Paul Bennett said:
All this "technically correct' is NOT technically correct. Ammeters do NOT measure flow but measure voltage across a fixed resistance calibrated (or not) in Amps using a d'Arsenval movement.

Ford applications of 'Gas Level' 'Oil Pressure' and 'Temperature' do NOT use d'Arsenval movement gauges but THERMOSTATIC gauge which, as JSeary has tried to point out, is different.

Resistance senders were considered hazardous early on because the armature must transition from one coil to the next. So Ted Smulski invented and patented the thermostatic gauge system, which is odd considering the sender must open and close a set of terminals in the flammable fluid.

Ted's patent used the term 'thermostatic gauge' - it was assigned to Anderson Corp who contracted King-Seeley to manufacture gauges/senders. The patent claims was compact, convenient, duragle and not sensitive to voltage.

Senders - resistance vs thermostatic. Actually, were a d'Arsonval meter be used with a thermostatic sender, the gauge would pulse hi-lo-hi-lo but a thermostatic meter evens out the peaks and presents a true ratio of on-off periods.

Ironically, a year after the Smulski patent, Joe Zublaty was granted a patent on a resistance gauge because the flamability nature of gasoline was found not a danger with a resistance sender. A galvanometer or ohm meter was used for the readout.

King-Seeley filed for a resistance gauge patent in 1954 obviously with knowlege of Ford's conversion to 12v. I've heard Ford switched to resitance fuel gauge in 1955 for one year switching back until 1980 but can't confirm it.
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You're making it far more complicated than it needs to be, my friend.

From a practical perspective, all gauges are measuring current flow in a closed circuit between the sender and the gauge - there is no other way to measure anything electrically (leaving capacitance type gauges out of the equation because they're generally not used in automotive applications).

In a 'modern' resistance type set up, a gas gauge float moves an arm on a potentiometer which changes resistance, which, in turn, changes the total current flowing through the gauge. The gauge is reading this change in current...therefore, it is an ammeter insofar as it is reacting (measuring) the total current in the circuit. It just happens to be calibrated to show you how much resistance = full/half/empty.

The same is true of the heated bimetallic strip bending and making/breaking a set of contact points. It opens/closes allowing intermittent full current to go through the gauge circuit...which has its own bimetallic arm moving a gear train and then a needle...it is reacting to the average current flowing through it...and it's an ammeter calibrated to reflect oil/gas/temp.

There is more than one way to measure total current in any circuit.

At the end of the day, the electrical theorists (myself included) do those with a little less electrical knowledge a great disservice by complicating things that don't need to be complicated...
 
Voltage and amperes work hand in hand. Voltage is the electromotive force that allows for flow and amperes are the electrical force that flows. The variable resistor or rheostat is a variable load device. The voltage is altered by the change in electrical load so it also changes how much amperage can flow. Ohms law is used to calculate the variables between the two. If there is no load in circuit (from the rheostat) then there is less resistance to flow but the voltage is still 6 to 7-volts. As the load increases, the resistance to flow increases. This is what the meter reads. You can call it an ammeter or a voltmeter but one is a bit more accurate than the other.
 
On my 39 Ford coupe, when I converted to 12 volts, all I did was add resisters to two of the three gauges. Everything worked fine. Temperature was not considered, as it was mechanical.
JSeery
Thanks so much for your very informative posts.
I appreciate them all
Jim
 
This is a great thread, Thanks to all of you who contributed. You guys seem to be at the electrical engineer level. Way over my head but has explained alot. I have a f100 that is a hodge podge of a 55 and 56. It uses the 55 tank and the 56 speedo head/gauges ( long story) Anyway I will be dealing with the gas gauge later. You can imagine the problems I have ran into with the voltage change between 55 and 56, and the 56 only heater/ defroster assembly. I will be studying out this thread when I get to that point. Thanks again, Greg
 
Remember that when Ford switched to 12v the gauges remained 6v. Ford used a voltage reducer for the gauges.
 
I just have a simple question, my gauge reads full when tank is full but when it reads about E I only have room for 10 gal. in a 17 gal. tank. Do I need to bend the float arm down or shorten it. Al
 
And I have a question too - sorry!
Running 36 fuel tank, original sender, original dash unit, 12 volt conversion, [alternator], runtz on gas gauge, another on oil pressure gauge.
Problem is that upon turning on ignition, needle in fuel gauge will climb and indicate [approximate] fuel level, but then when I start the engine and start driving, the needle goes down below empty and stays there.
Why would this be?
 
grumppyoldman said:
I just have a simple question, my gauge reads full when tank is full but when it reads about E I only have room for 10 gal. in a 17 gal. tank. Do I need to bend the float arm down or shorten it. Al
Click to expand...
I think you want to bend it up.it takes patience and more than one shot.
 
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