How Ford Gauge Unit Work

[Flat Ernie]

Originally Posted by Paul Bennett

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.

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...