How Ford Gauge Unit Work

[rotorwrench]

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.

[Flat Ernie]

Originally Posted by flatheadmurre

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.

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.

Originally Posted by rotorwrench

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.

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).

Originally Posted by rotorwrench

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.

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.